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Biophysical Foundations of Human Movement
by Bruce Abernethy, Vaughan Kippers, Stephanie J. Hanrahan, Marcus G. Pandy, Ali McManus and Laurel Mackinnon
408 Pages
Biophysical Foundations of Human Movement, Third Edition, introduces readers to key concepts concerning the anatomical, mechanical, physiological, neural, and psychological bases of human movement. The text provides undergraduate students with a broad foundation for more detailed study of the subdisciplines of human movement and for cross-disciplinary studies. Readers will learn the multi-dimensional changes in movement and movement potential that occur throughout the life span as well as those changes that occur as adaptations to training, practice, and other lifestyle factors.
This third edition includes the latest research and improved presentation to address areas of growth and change in the fields of human movement. The following are important updates to this edition:
- A new chapter on historical origins of human movement science provides students with an appreciation of the development of the field as well as its future directions.
- Content regarding exercise physiology has been reorganized to provide more discrete coverage of key concepts in nutrition.
- A new concluding section focuses on applications in the areas of prevention and management of chronic disease, prevention and management of injury, and performance enhancement in sport and the workplace, as well as the benefits of sport and exercise science to work, sport, and everyday living.
- Ancillary materials support instructors in teaching across disciplines as they assist students in understanding the breadth of content in this comprehensive text.
Using a modular approach to teaching sport and exercise science, Biophysical Foundations of Human Movement, Third Edition, offers students a structured understanding of how the subdisciplines work independently and in tandem. Following a general introduction to the field of human movement studies, readers are introduced to basic concepts, life-span changes, and adaptations arising in response to training in each of the five major biophysical subdisciplines of human movement. Each subdiscipline is given a brief introduction, including the definition and historical development of the subdiscipline, the typical issues and problems it addresses, the levels of analysis it uses, and relevant professional training and organizations. Multi-disciplinary and cross-disciplinary approaches to human movement are also discussed along with contemporary applications. By studying the integration of knowledge from a number of the biophysical subdisciplines, students will be better prepared for advanced study and careers reliant on the integration of knowledge from various disciplines and perspectives.
The third edition offers tools for retaining the material, including learning objectives and summaries in each chapter, a glossary, and lists of web-based resources. Throughout the text, special “In Focus” features highlight key organizations, individuals, and studies from around the world that have contributed to the current understanding of human movement. These features help readers appreciate the evolution of the field so that they may better understand its direction. Students interested in further study will find specialized texts for each of the subdisciplines listed in the Further Reading and References section of each chapter along with updated lists of websites.
The third edition of Biophysical FFoundations of Human Movement offers a comprehensive introduction for students, scientists, and practitioners involved in the many professions grounded in or related to human movement, kinesiology, and sport and exercise science. By considering the effect of adaptations in each of the biophysical subdisciplines of human movement, Biophysical Foundations of Human Movement also illustrates the important role physical activity plays in the maintenance of health throughout the life span.
Part I: Introduction to Human Movement Studies
Chapter 1. Human Movement Studies as a Discipline and a Profession
What is Human Movement Studies and Why is it Important?
Disciplines and Professions
Is Human Movement Studies a Discipline?
Structure of a Discipline of Human Movement Studies
What Should the Discipline of Human Movement Studies Be Called?
Professions Based on Human Movement Studies
Professional Organisations
Relationships Between the Discipline and the Professions
Summary
Further Reading and References
Chapter 2. Historical Origins of the Academic Study of Human Movement
Scholarly Writings on Human Movement From Ancient Civilisations (ca. 1000 BC-350 AD)
The Middle Ages as a Period of Suppression of the Study of Human Movement (ca. 350-1350 AD)
Scholarly Works on Human Movement From the Renaissance and Reformation Periods (ca. 1350-1650 AD)
Scholarly Works on Human Movement During the Period 1650-1885
Professionalization of Physical Education During the Period 1885-1929
Organisation of Research Efforts in Physical Education During the Period 1930-1959
Beginnings of a Discipline of Human Movement Studies During the Period 1960-1970
Emergence of Subdisciplines and Specialisations, 1970-Present
Future Directions, Challenges, and Opportunities
Summary
Further Reading
Part II: Anatomical Bases of Human Movement: Functional Anatomy
Chapter 3. Basic Concepts of the Musculoskeletal System
Tools for Measurement
Skeletal System
Articular System
Muscular System
Summary
Further Reading
Chapter 4. Basic Concepts of Anthropometry
Definition of Anthropometry
Tools for Measurement
Body Size
Determination of Body Shape
Tissues Composing the Body
Somatotyping as a Description of Body Build
Human Variation
Summary
Further Reading and References
Chapter 5. Musculoskeletal Changes Across the Life Span
Definitions of Auxology and Gerontology
Tools for Measurement
Physical Growth, Maturation, and Ageing
Age-Related Changes in the Skeletal and Articular Systems
Age-Related Changes in the Muscular System
Changes in Body Dimensions Across the Life Span
Methods of Determining Age
Summary
Further Reading
Chapter 6. Musculoskeletal Adaptations to Training
Effects of Physical Activity on Bone
Effects of Physical Activity on Joint Structure and Ranges of Motion
Effects of Physical Activity on Muscle–Tendon Units
Effects of Physical Activity on Body Size, Shape, and Composition
Summary
Further Reading and References
Part III: Mechanical Bases of Human Movement: Biomechanics
Chapter 7. Basic Concepts of Kinematics and Kinetics
Vectors
Motion
Generalized Coordinates and Degrees of Freedom
Force
Moment of Force
Force Analyses
Equations of Motion
Computer Modeling of Movement
Summary
Further Reading
Chapter 8. Basic Concepts of Energetics
Kinetic Energy
Potential Energy
Total Mechanical Energy
Power
Elastic Strain Energy
Metabolic Energy Consumption
Efficiency of Movement
Summary
Further Reading
Chapter 9. Biomechanics Across the Life Span
Biomechanics of Normal Gait
Changes in Muscle Strength with Age
Gait Development in Children
Gait Changes in Older Adults
Summary
Further Reading
Chapter 10. Biomechanical Adaptations to Training
Muscular Adaptations to Training
Neuromuscular Adaptations to Training
Training to Prevent Anterior Cruciate Ligament Injury
Biomechanical Adaptations to Injury
Dependence of Motor Performance on Changes in Muscle Properties
Using Computer Modelling to Study Vertical Jumping Performance
Insights Into the Effects of Training Provided by Computer Models
Summary
Further Reading
Part IV: Physiological Bases of Human Movement: Exercise Physiology
Chapter 11. Basic Concepts of Exercise Metabolism
Production of Energy for Exercise
Oxygen Supply During Sustained Exercise
VO2max as an Indicator of Endurance-Exercise Capacity
Measurement of Exercise Capacity
Human Skeletal Muscle Cells
Summary
Further Reading
Chapter 12. Basic Concepts of Nutrition and Exercise
Energy Requirements of Exercise
Nutrients for Exercise
Fluid Requirements During Exercise
Summary
Further Reading
Chapter 13. Physiological Capacity Across the Life Span
Responses to Exercise in Children
Exercise in Older Adult Life
Summary
Further Reading and References
Chapter 14. Physiological Adaptations to Training
Training-Induced Metabolic Adaptations
Immediate and Anaerobic-System Changes After High-Intensity Sprint and Strength Training
Changes in Aerobic Metabolism After Endurance Training
Endurance Training-Induced Changes in the Cardiorespiratory System
Endurance Training-Induced Respiratory Changes
Endurance Training-Induced Changes in Lactate Threshold
Changes in the Muscular System After Strength Training
Basic Principles of Training
Continuous and Interval Training
Training for Cardiovascular Endurance
Methods of Strength Training
Causes of Muscle Soreness
Summary
Further Reading
Part V: Neural Bases of Human Movement: Motor Control
Chapter 15. Basic Concepts of Motor Control: Neuroscience Perspectives
Nervous System as an Elaborate Communications Network
Components of the Nervous System
Neurons and Synapses as the Building Blocks of the Nervous System
Sensory Receptor Systems for Movement
Effector Systems for Movement
Motor Control Functions of the Spinal Cord
Motor Control Functions of the Brain
Integrative Brain Mechanisms for Movement
Summary
Further Reading
Chapter 16. Basic Concepts of Motor Control: Cognitive Science Perspectives
Using Models to Study Motor Control
Key Properties to be Explained by Models of Motor Control
Information-Processing Models of Motor Control
Some Alternative Models of Motor Control
Summary
Further Reading
Chapter 17. Motor Control Changes Throughout the Life Span
Changes in Observable Motor Performance
Changes at the Neurophysiological Level
Changes in Information-Processing Capabilities
Summary
Further Reading
Chapter 18. Motor Control Adaptations to Training
Changes in Observable Motor Performance
Changes at the Neurophysiological Level
Changes in Information-Processing Capabilities
Factors Affecting the Learning of Motor Skills
Summary
Further Reading
Part VI: Psychological Bases of Human Movement: Sport and Exercise Psychology
Chapter 19. Basic Concepts in Sport Psychology
Personality
Motivation in Sport
Self-Determination Theory
Arousal, Anxiety, and Sport Performance
The Practice of Applied Sport Psychology
Imagery: An Example of Psychological Skill
Summary
Further Reading
Chapter 20. Basic Concepts in Exercise Psychology
Effects of Psychological Factors on Exercise
Effects of Exercise on Psychological Factors
Summary
Further Reading
Chapter 21. Physical Activity and Psychological Factors Across the Life Span
Changes in Personality
Psychosocial Development Through Sport Participation
Exercise in the Aged
Termination of Athletic Careers
Summary
Further Reading
Chapter 22. Psychological Adaptations to Training
Aerobic Fitness and the Response to Psychological Stress
Changes in Personality
Changes in Motivation: Staleness, Overtraining, and Burnout
Changes in Mental Skills
Summary
Further Reading
Part VII: Multi- and Cross-Disciplinary Applications to Human Movement Science
Chapter 23. Applications to Health in Chronic-Disease Prevention and Management
Major Causes of Disease and Death Globally
Cost of Physical Inactivity
Measuring Physical Activity and Sedentary Behavior
Levels of Physical Activity in Adults and Children
Recommendations for Physical Activity
Summary
Further Reading
Chapter 24. Applications to Health in Injury Prevention and Management
Preventing Manual-Lifting Injuries in the Workplace
Preventing and Managing Overuse Injuries in Sport
Preventing Injuries Related to Osteoporosis
Summary
Further Reading
Chapter 25. Applications to Performance Enhancement in Sport and the Workplace
Talent Identification
Performance Optimization
Summary
Further Reading and References
Bruce Abernethy, PhD, is professor of human movement science in the School of Human Movement Studies and deputy executive dean and associate dean (research) in the faculty of health sciences at the University of Queensland, Brisbane, Australia. He also holds a visiting professor appointment at the University of Hong Kong, where he was previously the inaugural chair professor and director of the Institute of Human Performance. He is also coeditor of Creative Side of Experimentation.
Abernethy earned his PhD from the University of Otago. He is an international fellow of the National Academy of Kinesiology (USA), a fellow of Sports Medicine Australia, and a fellow of Exercise and Sports Science Australia.
Stephanie J. Hanrahan, PhD, is a registered sport psychologist and an associate professor in the Schools of Human Movement Studies and Psychology at the University of Queensland, Brisbane, Australia. Hanrahan has over 20 years of experience in teaching human movement studies at the undergraduate level. She is a recipient of the University of Queensland's Excellence in Teaching Award. In addition to being part of the author team for the first two editions of Biophysical Foundations of Human Movement, Hanrahan has authored or edited nine other books.
Hanrahan is a fellow of the Australian Sports Medicine Federation and a fellow of the Association for Applied Sport Psychology, for which she is chair of the organization’s International Relations Division. Hanrahan serves on the national executive committee of the College of Sport and Exercise Psychologists in the Australian Psychological Society.
Hanrahan earned her doctorate in sport psychology in 1990 from the University of Western Australia. She resides in Moorooka, Queensland, and enjoys traveling, Latin dancing, and kayaking.
Vaughan Kippers, PhD, is a senior lecturer in the School of Biomedical Sciences at the University of Queensland. He coordinates anatomy courses for students enrolled in medicine, physiotherapy, and occupational therapy programs. His major research involves the use of electromyography, in which the electrical signals produced by muscles as they contract are analyzed to determine muscular control of human movement.
Kippers is a fellow of the International Association of Medical Science Educators and is on the board of directors of that association. He is also secretary of the Australian and New Zealand Association of Clinical Anatomists.
Cycling and photography are Kippers’ main interests. He commutes on a bicycle daily and regularly participates in long rides on weekend. He is a former president of Audax Queensland, an international long-distance cycling association.
Marcus G. Pandy, PhD, is a professor of mechanical and biomedical engineering in the department of mechanical engineering at the University of Melbourne, Parkville, Victoria, Australia. Pandy earned his PhD in mechanical engineering at Ohio State University in Columbus and then completed a postdoctoral fellowship in mechanical engineering at Stanford University. Before joining the University of Melbourne, he held the Joe J. King professorship in engineering at the University of Texas at Austin.
Pandy is an associate editor for the Journal of Biomechanics and a fellow of the Institute of Engineers Australia, the American Institute of Medical and Biological Engineering, and the American Society of Mechanical Engineers.
Ali McManus, PhD, is an associate professor and assistant director of the Institute of Human Performance at the University of Hong Kong. Her research focuses on the role exercise and free-living physical activity play in the health and well-being of children, the development of population measures of obesity and its associated health risks, and the provision of a more comprehensive understanding of the complex metabolic bases of exercise and physical activity in obese children.
McManus earned her PhD from the University of Exeter, UK. She lives in Clearwater Bay, Hong Kong, and enjoys going to the gym, horse riding, playing tennis, and spending time with her children, Tash and Bella, and husband, John.
Laurel T. Mackinnon, PhD, is a science writer and editor based in Brisbane, Queensland, Australia. She is also a former associate professor and now adjunct associate professor in the School of Human Movement Studies at the University of Queensland, Brisbane, Australia.
Mackinnon conducted research on the immune response to exercise in the 1980s and 1990s and is internationally recognized for her work on overtraining and immune function in athletes. She is the author of 6 books and 12 book chapters, including Exercise and Immunology (Human Kinetics, 1992), the first book to explore the intriguing relationship between exercise and immune response. She has published over 65 peer-reviewed articles in international journals.
Mackinnon has worked since 2000 as a science writer and editor. She is editing team manager for OnLine English, an Internet-based service that specializes in editing academic, research, and industry communications written by non-native speakers of English wishing to publish in English-language scientific journals.
Mackinnon is a fellow of the American College of Sports Medicine and a member of the Australasian Medical Writers Association. She is a former board member of the International Society of Exercise and Immunology (ISEI) and the Australian Association for Exercise and Sports Science. Mackinnon earned her PhD in exercise science from the University of Michigan.
She enjoys exercising, reading, and listening to classical and jazz music. Mackinnon resides in Brisbane, Queensland.
"The book is made up of standalone sections that make reading easy and understandable. The authors are well-respected scientists in the field, and the information they provide throughout originates from evidence-based research."
--Doody's Book Review (5 Star Review)
Lifespan changes in skeletal system can lead to injury
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages.
Biophysical Foundations of Human Movement, Third Edition.
Age-Related Changes in the Skeletal and Articular Systems
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages. During the early fetal growth period there is rapid multiplication of cells (hyperplasia), whereas during the pubertal period growth is caused mainly by development and enlargement of existing cells (hypertrophy). Development can also occur via the replacement of one type of tissue by another.
Stages in the Development of Bone
Adult human bones appear in a variety of shapes and sizes. Their basic shape is genetically determined, but their final shape is influenced greatly by the environment in which they develop. Environmental influences include mechanical factors, such as muscle forces acting on the developing bone, and metabolic factors, which include the supply of nutrients.
Typical long bones, such as the humerus in the arm and the femur in the thigh, develop via a process of endochondral ossification in which a cartilaginous model precedes the bone formation. The cartilage is eventually replaced by bone. The cartilage model initially is small, and its replacement by bone begins at an early stage of development. The primary ossification centres appear near the middle of the shaft of the future long bone at about 8 wk after fertilisation, when the embryo is about 35 mm (1.4 in.) long (see figure 5.1).
The cartilage model of the future bone grows before it is replaced by bone. This replacement occurs in a series of stages. Initially, the cartilage model grows by two processes: an increase in the number of cartilage cells and then an increase in the size of each cell. Next, the gel-like matrix surrounding the cartilage cells is calcified. This hardening of the tissue surrounding the cells effectively cuts off their supply of nutrients and prevents the removal of metabolic waste products. These imprisoned cartilage cells eventually die so that the calcified cartilage has a honeycomb appearance. Invading blood vessels bring nutrients and bone-forming cells (osteoblasts) to the calcified cartilage to lay down new bone. This new bone has a disorganised appearance. If ossification were the final stage, a long bone would not develop its hollow structure. What happens instead is that some of the newly laid bone is removed from internal sites by special bone-eroding cells. These osteoclasts work with osteoblasts during remodelling of the bone, causing the cortex to drift away from the central axis of the shaft during growth. Changes in shaft diameter and compact bone thickness can occur at any age by a remodelling process. Bone can respond to changes in mechanical stimuli by changing its structure.
One type of remodelling that occurs is the replacement of the originally laid bone by compact bone that has an organised structure (figure 3.1). Most primary centres of ossification (in the developing shaft) appear before birth, whereas most secondary centres of ossification (in the developing ends of the bone) appear after birth. The times of appearance of these secondary centres of ossification vary widely. The centre in the femur near the knee is normally present at birth, but the centre in the clavicle (collarbone) closest to the sternum (breastbone) does not appear until about 18 yr of age.
Growth in Length and Width of Bone
When both the shaft and ends of a long bone are developing they are separated by a growth plate, or epiphyseal plate. In this region, all the stages of bone development can be seen under a microscope (see figure 5.2). Each zone in figure 5.2 represents a stage in the growth of bone and then replacement of cartilage by bone; that is, the zones illustrated do not migrate downward, but rather retain their position and change their nature. This effectively increases the distance between the growth plates at each end of the bone. The process illustrated continues for almost 20 yr. Growth in length of the long bones ceases when the process stops and epiphyseal plates fuse and disappear. Actually, the growth stops because the cartilage cells no longer respond to hormonal influence.
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Although growth in height ceases at a certain age, changes in thickness of the compact bone of the shaft and the density of spongy bone can occur at any time of life (see chapter 6). Growth in the thickness and diameter of long bones occurs by appositional growth. In this process, bone is normally added on the outside of the shaft and removed from the inside of the shaft.
Skeletal Composition Changes Across the Life Span
The relative proportions of the inorganic salt crystals and the organic collagen change throughout the life span. In a child, the flexibility of the bone is related to the large proportion of collagen. It is thought that the rapid growth of children's bones and the consequent amount of remodelling results in less-than-optimal mineralisation. In a newborn baby, the skeleton accounts for about 13% of the total body weight, but about two-thirds of the skeleton is cartilaginous.
In bone of a young adult the inorganic material is close to the optimal two-thirds mineralisation, so the bone is strong and tough. Though the proportion of skeletal material in the mature adult is also about 13% of total body weight, just over 10% of the skeleton is cartilaginous. Of the bone, four-fifths is compact and one-fifth is spongy.
Eventually, aged bone becomes brittle. This change relates partly to the increased proportion of the inorganic salts, but of more significance is the reduced total mass of bone material in the aged musculoskeletal system. Bone mass decreases mainly because of a decrease in the number of trabeculae in spongy bone (see “In Focus: Why Trunk Height Decreases With Ageing”) combined with resorption and consequent thinning of the trabeculae. In compact bone, the size of the Haversian canals (see figure 3.1) increases and the cross-sectional area of the bone surrounding each Haversian canal decreases. This increased porosity of bone, in turn, causes decreased bone density (mass per unit volume of tissue). In an 80-yr-old male the density of bone in the spine is often 55% of the value found in a 20-yr-old; in females who are elderly, it is often only 40% of its peak value. With ageing, bone also becomes stiffer, partly because of the collagen cross-linking. A consequence of these mechanical changes is that aged bone can absorb less energy before it fractures, so it is more liable to fail when subjected to large forces.
Osteoporosis
For most tissues in the body, their mass at any time is determined by the balance between the simultaneous formation and destruction of the tissue. Osteopenia (“bone poverty”) is reduced bone density, particularly in women during the postmenopausal years, when the dramatic decrease in the female hormone estrogen allows the rate of synthesis to be lower than the rate of resorption. The initial effects are mainly to the spongy bone, but later the compact bone in the shafts of long bones also decreases in thickness.
According to the World Health Organization, osteoporosis (“bone porosity”) is present when the bone density is more than 2.5 standard deviations below the average bone mass of a young adult woman. In general, the porosity of bone in the human femur doubles from 40 to 80 yr of age and people with osteoporosis show an even greater change. Thus, osteoporotic bone is even less able to absorb energy before it fails than is normally aged bone. Moreover, even though a chemical analysis of osteoporotic bone material may indicate slightly increased levels of calcium per unit mass of bone, the overall mass of bone decreases so markedly that there is less calcium stored in the skeleton overall.
During maturation, the density and calcium content of bones is increased due to genetic predisposition aided by the interaction between dietary calcium and exercise. It is now thought that peak bone mass occurs at about 16 to 20 yr of age in women, so some health authorities claim osteoporosis is a pediatric disease and have promoted the idea of “bone banks” to help prevent osteoporosis. The idea is that if a person's lifestyle during maturation and early adulthood promoted maximum deposition of bone material, then later in life, when withdrawals of bone material are inevitable, the structural integrity of the bones will be maintained longer. Financial advisers tell everybody to start saving as young as possible, and this concept is even more important for prevention of osteoporosis.
Osteoporosis is commonly associated with women, but the prevalence among men is now the same as it was in women about 25 yr ago. This trend has been attributed to lifestyle changes, which include inadequate intake of calcium and less-than-optimal levels of physical activity. The personal lifestyle changes due to osteoporosis and the public health care costs of osteoporosis are so great that they are considered further in chapter 23.
Bone Failure in Relation to Bone Development, Age, or Activity
The modern automotive industry attempts to protect passengers by enclosing them in a cage that is lined by material that cushions the forces involved in a collision. In similar fashion, the soft tissues of the human body, including fat and muscle, perform the role of a crumple zone. However, despite this protection, sometimes the energy involved in a mishap is enough to cause a fracture anyway. Bone failure may be related to bone development, age, or activity.
Certain types of fractures are associated with particular stages of development. The flexible bones of a child tend to splinter in a manner similar to the broken branch of a growing tree; this type of fracture is called a greenstick fracture. Late in life the fracture is more likely to be of the brittle type because of the increased stiffness of old bones. The bones of adults who are elderly are also more likely to fail because of the decreased bone mass and density in osteoporosis.
Specific fractures are related to certain ages. For example, fractures of the neck of the femur (commonly called hip fractures) are common in women in the elderly population. Incidence of forearm fracture increases near the peak of the pubescent growth spurt. This is associated with increased porosity of compact bone as the remodelling space increases to provide calcium to the rapid-growth regions. A second increase in wrist fractures is seen in women who are elderly; this is related to the combination of osteoporosis and increased incidence of falls. In developing children, the cartilaginous growth plate may also fail; this most often occurs at the zone of maturation, or hypertrophy (see figure 5.2).
The development of bone has implications for potential injury to the musculoskeletal system. For example, injury to the cartilaginous growth plates can be produced by single excessive forces causing trauma or by repetitive compressive forces, or tensile forces produced by muscle-tendon pull on an area of developing bone.
In sports medicine, specific injuries have been attributed to certain activities. One of the best examples is Little League elbow, described in more detail in chapter 24. Other examples of the relationships between activity and types of growth-plate problems are wrist injuries in gymnasts and tibial tuberosity avulsions in sports involving sprinting and jumping. Pain over the tibial tuberosity (just below the knee cap) in a prepubescent child is indicative of Osgood-Schlatter's condition.
Effects of Various Factors on Range of Motion
Range of motion at joints is affected by joint structure and the mechanical properties of the tissues associated with the joints. A general perception is that joint range of motion decreases during life. Although this is the general trend, the rate of loss is not constant. Ranges of joint motion are very large in a newborn baby. As an example, the range of ankle dorsiflexion in a newborn is limited only by the contact of the top of the foot against the shin. Try this movement yourself to indicate how much flexibility you have lost since birth.
Between the ages of 6 and 15 yr, there is a general trend for joint range of motion to decrease in boys, whereas for girls the effects are variable and joint dependent. Girls are generally more flexible than boys during childhood and adolescence. Changes in joint ranges of motion between adolescence and young adulthood are variable and, as discussed in chapter 6, appear to be related to physical activity. Similarly, the general trend for joint flexibility to decrease during ageing may not be completely explained by biological ageing processes.
Ageing people may suffer arthritis (“inflammation of joints”), which markedly restricts range of motion. Rheumatoid arthritis involves inflammation of the synovial membrane, while osteoarthritis involves degeneration of the articular cartilage (see figure 3.6). In the total adult population, about three times as many women have osteoarthritis as do men. Thus there is some genetic determination of osteoarthritis, to which are added the effects of environmental factors. It would appear that the risk factors for rheumatoid arthritis are mainly related to a family history of the condition.
Age-Related Changes in the Muscular System
Mesoderm, which forms toward the end of the third week of development, eventually forms the muscle and connective tissues of the body. Some of the mesoderm is segmented such that it is in lumps on either side of the midline, and these lumps form the muscles of the trunk. Other mesoderm forms the muscles of the limbs; this tissue migrates into the limbs when they first appear. Limb muscles migrate toward their final position during further development.
Certain precursor cells differentiate into myoblasts (muscle-forming cells) that fuse longitudinally to form the long muscles and muscle fibres we can observe. The number of muscle fibres appears to be genetically determined so that large people often have more muscle fibres than persons who are smaller in stature. As muscles grow in length, the number of sarcomeres increases. Although direct evidence is sparse, many believe that the stimulus for growth of muscle is the bone growth that pulls on the attached ends of the muscle. Developmental growth occurs in both muscle length and cross-sectional area, and these factors may also be affected by activity. The effects of physical training on muscle is summarised in chapters 6, 10, and 13.
In an infant, muscle tissue accounts for about 25% of total body weight, but in a young adult the proportion of muscle is more than 40%. In terms of growth, muscle increases from about 850 g (1.9 lb) at birth to about 30 kg (66 lb) in a young adult male weighing 70 kg (154 lb).
Strength, which may be defined as the capacity to produce force against an external resistance, is related to the cross-sectional areas of the muscles producing the force. Muscle volume peaks at about 30 yr of age and then gradually decreases. The atrophy of skeletal muscle associated with disuse is much faster than any ageing-related atrophy. Muscle elasticity also decreases during ageing so that muscles are stiffer and less extensible. This is one contributing factor to the loss of joint range of motion described earlier.
Read more from Biophysical Foundations of Human Movement, Third Edition by Bruce Abernethy, Vaughan Kippers, Stephanie Hanrahan, Marcus Pandy, Ali McManus, and Laurel Mackinnon.
Training can increase both muscle strength and fiber contraction speed
Adaptations are common following injury and surgical treatment.
Biophysical Foundations of Human Movement, Third Edition.
Biomechanical Adaptations to Injury
Adaptations are common following injury and surgical treatment. One interesting example found in the orthopaedics literature is a phenomenon known as quadriceps avoidance gait. This particular adaptation occurs in ACL-deficient (ACLD) patients; that is, patients who have suffered a complete rupture of the ACL. ACLD patients usually have difficulty with movements involving either lateral thrusts (e.g., cutting from side to side) or twisting (i.e., rotation of the femur relative to the tibia in the transverse plane).
Gait analysis has been done on ACLD patients during various activities of daily living, including level walking, jogging, walking up and down stairs, and even running, cutting, and pivoting. Kinematic and force-plate data were used to estimate the net muscle moments exerted at the knee in all three planes of movement: flexion-extension, abduction-
adduction, and internal-external rotation. Interestingly, the greatest functional changes between ACLD patients and healthy subjects occurred while walking at preferred speeds on level ground. During the midstance portion of the gait cycle, the ACLD patients exhibited a net extensor moment that was significantly lower than normal (figure 10.9, solid line). One interpretation of these findings is that the ACLD patients walk by reducing the demand on their quadriceps during stance (i.e., the quadriceps muscles are activated less in the ACL-deficient patients, giving rise to a quadriceps-avoidance gait pattern).
It is important to realise that a lower net extensor moment at the knee does not necessarily mean that the moment exerted by quadriceps is lower. Because the net moment at the knee is a combination of quadriceps and hamstrings muscle action, a lower extensor moment could result from an increase in the flexor moment applied by hamstrings. However, muscle electromyography results tend to support the interpretation that quadriceps moment is lower because the activation level of the quadriceps is lower in the ACLD patients.
Gait analysis results have also shown that the ACLD patients reduce their net extensor moment by 25% during jogging compared with a 100% reduction in walking. However, the net extensor moment in stair climbing was the same in ACLD patients and normal subjects. When taken together, these results suggest that the observed quadriceps avoidance in ACLD patients is not a result of quadriceps weakness but rather is due to a change in neuromuscular control.
Dependence of Motor Performance on Changes in Muscle Properties
Strength and quickness are critical determinants of performance in explosive movements such as jumping, throwing, and sprinting. Together, these two factors determine the amount of power developed instantaneously during a task (i.e., muscle power is given by the product of muscle force and muscle contraction speed; see “In Focus: At What Speed Must a Muscle Shorten to Develop Maximum Power?”). As discussed earlier in this chapter, resistance strength training can alter both muscle force and fibre contraction speed. This section describes the effects that changes in the contractile properties of muscle have on motor performance, specifically, how biomechanical performance depends on muscle strength and muscle fibre contraction speed.
Vertical jumping is one of the most heavily studied motor tasks. Many scientists have studied jumping with the aim of learning more about the biomechanics and control of this task and, specifically, how performance may be influenced by training. In a recent jump-training program that incorporated stretching, plyometric exercises, and weight lifting, female high school volleyball players demonstrated a mean increase of 4 cm (1.5 in.) in jump height after training. This represented an almost 10% increase in jump height as a result of the 6 wk period of training. Even larger increases in vertical jumping performance have been documented in the literature. One extreme example is the 1984 United States Olympic gold medal volleyball team, which showed a 10 cm (4 in.) increase in jump height after 2 yr of jump training.
Using Computer Modelling to Study Vertical Jumping Performance
It is gratifying to learn that jumping performance can be increased significantly by strength and neuromuscular training. However, it is difficult, if not impossible, to explain why these increases occur. Noninvasive measurements of biomechanical performance cannot pinpoint the factor or factors responsible for the increase in jump height. The reason is that several properties of the neuromuscular and musculoskeletal systems change simultaneously during the training regimen. Alternatively, computer models may be used to study the relationships between training effects and performance; that is, a model of the neuromusculoskeletal system may be used to predict how changes to specific parameters affect the performance of a motor
task.
Models of the body similar to the one shown in figure 8.7 have been used to study how changes in muscle strength, muscle contraction speed, and motor unit recruitment affect jump height. The values of each of these parameters in the model were increased by amounts consistent with results obtained from strength-training programs. For example, the peak isometric strength and maximum shortening velocity of each muscle in the model were increased by 20% and the activation level of each muscle was increased by 10%. The three training effects were first applied to all muscles simultaneously and then to the ankle plantarflexors, knee extensors, and hip extensors separately. In this way, the modelling results were used to determine whether it is better to train all the leg muscles simultaneously or to isolate specific muscle groups such as the quadriceps (knee extensors) and gluteus maximus (hip
extensors).
Insights Into the Effects of Training Provided by Computer Models
Increasing the peak isometric force of all the leg muscles in the model by 20% produced the largest increase in jump height. In this case, jump height increased by 7 cm, or 5% of that obtained before the simulated training effect. Increasing the maximum shortening velocity of each muscle by 20% or the activation level of each muscle by 10% increased jump height by only 4 cm, or 3% of that obtained for the nominal (untrained) model. When all three training effects were introduced simultaneously, jumping performance increased by nearly 17 cm, or 12% of that calculated for the untrained model. Thus, training programs that increase strength, fibre contraction speed, and motor recruitment (i.e., activation level) of all the leg muscles simultaneously are most beneficial to overall jumping performance.
The modelling results also suggest that training the knee extensors is better than training either the ankle plantarflexors or the hip extensors. When peak isometric force and maximum shortening velocity of the quadriceps were increased by 20% and quadriceps activation was simultaneously increased by 10%, jump height increased by almost 10 cm, or 7% of the value calculated for the untrained model. The same changes made to either the ankle plantarflexors or the hip extensors produced increases in jump height of only 3 cm, or 2% of the value obtained for the untrained model.
The latter result is a little puzzling because the model calculations had previously shown that the quadriceps and gluteus maximus are the major energy producers—the prime movers—of the body in vertical jumping (see “In Focus: Which Muscles Are Most Important to Vertical Jumping Performance?” in chapter 8). It should be noted here that each time a change was introduced to the model, a new optimal pattern of muscle activations was found by re-solving the optimisation problem for a maximum-height jump. Thus, one interpretation of the result is that what matters most in terms of performance is the quadriceps:gluteus maximus muscle strength ratio and not the absolute strength of these muscles. In other words, jumping performance is most sensitive to a change in the knee-extensor:hip-extensor muscle strength ratio. Increasing quadriceps strength by 20% increases the knee-extensor:hip-extensor muscle strength ratio in the model, whereas increasing gluteus maximus muscle strength decreases this ratio. The same line of reasoning may be used to explain why increasing ankle plantarflexor muscle strength by 20% leads to an increase in jump height of just 3 cm in the model.
Training for strength is also better than training for quickness or speed. Figure 10.10 shows the effects of increasing muscle strength and muscle contraction speed on vertical jump height as predicted by the four-segment, eight-muscle, sagittal-plane model of the body described in chapter 8 (see figure 8.7). An increase in muscle strength was simulated in the model by simultaneously increasing body weight because muscle strength increases in proportion to muscle mass. Thus, changes in body strength:weight ratio are represented in figure 10.10 rather than changes in muscle strength alone. Also, jumping performance is normalised in these results by dividing by the value of jump height calculated for the untrained model. Similarly, body strength:weight ratio and muscle contraction speed are each normalised by dividing by the value of body strength:weight ratio and muscle contraction speed in the untrained model, respectively.
The simulation results show that the slope of the line predicted for changes in body strength:weight ratio is twice as large as that obtained when changes in muscle fibre contraction speed are made (compare solid and dashed lines in figure 10.10). Thus, muscle strength has a greater effect on vertical jump height than muscle fibre contraction speed, even when the accompanying increase in body mass is taken into account.
Another important lesson learned from the modelling studies is that musculoskeletal changes must be accompanied by appropriate changes in neuromuscular control; otherwise, the expected improvement in motor performance will not be seen. In vertical jumping, if the pattern of muscle activations remains unchanged subsequent to strength training, jump height actually decreases relative to the untrained state. Figure 10.11 shows a simulated jump in which the strength of the knee-extensor muscles has been increased by 20% but the control exerted over the joints is the same as that calculated for the untrained model before the training effect was introduced. When the pattern of muscle activations was not optimised to match the changes introduced to the neuromusculoskeletal model, the body left the ground prematurely (i.e., the centre of mass was at a lower height at liftoff than is optimal for a maximal jump).
One consequence of not optimising the pattern of muscle activations (i.e., the controls) is that a larger fraction of the total work produced by the muscles goes into rotating the body segments rather than accelerating the centre of mass upward. In a maximum-height jump, approximately 90% of the total work done by the leg muscles is used to propel the centre of mass upward. This number is closer to 80% when muscle coordination (i.e., the sequence and timing of muscle activations) is not optimal. So, even though jumping performance depends heavily on muscle strength, and to a lesser extent on muscle fibre contraction speed, optimal performance is also intimately related to neuromuscular control. For this reason, and as discussed at the beginning of this chapter, jump-training programs now focus on maneuvers that blend muscle strength with neural control.
Summary
Training can change both muscle strength and fibre contraction speed. Strength may be increased by increasing motor unit recruitment (net neural drive to the muscle) or by increasing muscle fibre size. Contraction speed may be altered by changing the shape of a muscle's force-velocity curve or by changing the value of its intrinsic maximum shortening velocity. Early in a training program (2-8 wk), increases in muscle strength are brought by neural adaptations rather than by increases in muscle size. Training can also change the way muscle action is coordinated during activity (i.e., neuromuscular control). Neuromuscular training programs are usually designed to improve stability (balance and coordination) and proprioception (joint position sense) in addition to muscle strength. Training is vital for improving biomechanical performance and for preventing injuries during sport.
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The three components of motivation affect exercise adherence
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
Effects of Psychological Factors on Exercise
The three components of motivation—direction, intensity, and persistence—mentioned in chapter 19 in relation to sport apply to exercise motivation as well. Direction clearly relates to whether an individual chooses the direction of the gym or the couch, the stairs or the elevator, or the pool or the bath. Once the direction of exercise has been chosen, intensity and persistence become important. For example, as a New Year's resolution two individuals may have both decided to join a gym. They have both chosen the direction of exercise. During their first day at the gym, one rides the bicycle for 30 min, tries the stepper and the rowing machine, completes multiple sets on the different weights available, and finishes off with an aerobics class. The other individual begins with just a few minutes on the bicycle and then completes just one set of each of the different weights exercises using light weights. The first person in this example exhibits high intensity and the second person exhibits low intensity. If we stop at this point in the example, you may conclude that the first person is more motivated than the second. Your opinion may change when the third component of motivation—persistence—is considered. Two days later the first individual is home in bed, too sore to move (thinking that exercise is painful and should be avoided whenever possible). The second individual, however, is once again at the gym adding a couple of minutes to his time on the bicycle and sticking to his light-weights workout.
Exercise Participation Motivation
Exercise participation motivation refers primarily to the direction component of motivation. Exercise participation motivation is the initiation of exercise. A variety of factors influence whether individuals initiate an exercise program.
Knowledge, attitudes, and beliefs about exercise influence motivation toward exercise participation. Individuals who understand the importance and value of regular exercise are more likely to initiate an exercise program than those who do not. Similarly, if people have positive attitudes about the value and importance of regular exercise they will have greater motivation to participate in exercise than will people with negative attitudes.
Valuing the importance of exercise, however, is not the only determinant of exercise participation. Beliefs about ourselves influence motivation as well. Even if individuals understand that exercise is important, they will be unlikely to begin an exercise program if they believe that they cannot succeed at it. If they believe that the exercise program is too difficult or that it requires more fitness, strength, coordination, or time than they have, it is doubtful that they will join the program. This confidence in one's ability to succeed at an exercise program is called exercise self-efficacy. It is logical that self-efficacy would influence behaviour. How likely would people be to do something they were convinced they could not do, particularly if they had to pay to do it? How likely would they be to invest any energy in pursuing that activity? Most people in this situation would not attempt the activity. People who have such feelings are described as having low self-efficacy. As self-efficacy, or one's belief in one's ability to succeed at a particular task, increases, so does the likelihood of undertaking that task.
How can we enhance exercise participation motivation? Educating people about the importance and value of exercise can be a valuable first step because individuals who understand the merit of exercise are more likely to adopt an exercise program. Unfortunately, imparting knowledge is not enough. A lot of us do things that we know are not good for us and, similarly, do not do things that we know are good for us. Enhancing the exercise self-efficacy of individuals will increase their motivation. Demonstrating how individuals can control their own activity is useful. Some people have low self-efficacy about exercise because they believe that they are too unfit to begin exercising. They may equate exercise with young, thin, Lycra-clad gym enthusiasts whom they see in the media. Programs that emphasise choice of activities, illustrate exercisers similar in age and fitness level to the potential exercisers, and reveal that exercising can be enjoyable may increase exercise participation motivation. Exercise programs that begin with activities that the individuals already know they are capable of doing, such as walking and climbing stairs, may also increase self-efficacy and thereby increase motivation. Imagery may also help individuals enhance their self-efficacy beliefs (see “Imagery, Exercise, and Self-Efficacy”).
Exercise Adherence Motivation
Although many people get motivated to begin an exercise program, many of the people who begin fail to continue. Approximately 50% of individuals who begin a regular physical-activity program drop out within the first 6 mo. These people had exercise participation motivation, but they lack exercise adherence motivation, the persistence component of motivation.
Biological, psychological, sensory, and situational factors all interact to influence exercise adherence. Biologically, body composition, aerobic fitness, and the presence of disease influence adherence. Unfortunately, it is usually the people who could gain the most from exercise who are the least likely to adhere. Overweight or obese, unfit, or chronically ill people are less likely to adhere to an exercise program than are thinner, fitter, and healthier people.
As is the case with exercise participation motivation, attitudes and beliefs influence exercise adherence motivation. Attitudes and beliefs about the importance of exercise play a role in adherence, but so too do individuals' expectations about the effects that exercise is having on them personally. For example, if individuals believe that major changes in fitness and body composition should occur after 6 wk of regular exercise and they do not perceive major improvement in their own bodies after 6 wk, they may believe that exercise does not do what it should and therefore quit. Even though it is unrealistic to believe that 6 wk of exercise can make up for 6 yr of inactivity, it is individuals' beliefs, not reality, that influence behaviour. Therefore, when introducing newcomers to exercise it is important that they have realistic expectations about the time and effort required and the anticipated effects of the proposed program.
In addition to attitudes and beliefs, other psychological factors influence exercise adherence motivation. Extroverts (people who are social and outgoing) tend to adhere to exercise programs better than do introverts. Exercise programs that are executed in the presence of other people are probably more comfortable for extroverts than for introverts. Extroverts tend to enjoy the interaction with class members and exercise partners, possibly encouraging their adherence. Introverts, on the other hand, may adhere better to individual, home-based exercise programs. The bulk of the research on exercise adherence has involved programs that take place on site at fitness facilities with other people. This setting may have led to the conclusion that extroverts are better adherers than introverts. If the research had been done on independent, home-based exercise programs, it might have been found that introverts were better adherers. Clearly efforts need to be made to match the social environment of the exercise program to the personality of the exerciser.
Individuals with high levels of self-motivation are more likely to adhere to exercise programs than are individuals with low levels of self-motivation. It is logical that highly self-motivated people have better adherence rates. The challenge is to help those individuals with low levels of self-motivation. One of the most effective methods of helping these individuals is to encourage their involvement in the goal-setting process.
Goal Setting
Setting goals can help enhance motivation for a number of reasons. Goal setting addresses all three components of motivation. Goals give direction by providing a target. Intensity and effort also can be enhanced because goals provide reasons for participating in the activity. We are all more likely to put in effort when we feel there is a reason for doing so. If individuals are given two jobs to do at work, one that has a particular target and objective and one that seems vague and purposeless, into which job are they more likely to put their effort? Having goals helps focus attention and effort. In addition, goals can augment persistence by fostering new strategies. If individuals have a goal to which they are committed and initial tactics appear unsuccessful, they will search for alternative strategies to achieve their aim. If the goal had not been set in the first place, instead of persisting with different plans of action, they would likely give up.
Goals are also beneficial because they reflect improvement. Too often people make short-term comparisons regarding their strength, fitness, flexibility, or weight. Because the positive effects of exercise take time to emerge, improvements being made are often not noticed when people use a short timeframe for comparison. If a goal is achieved, evidence of improvement exists.
Goal setting involves a number of steps:
- setting the goal,
- setting a target date by which to achieve the goal,
- determining strategies to achieve the goal, and
- evaluating the goal on a regular basis.
If a target date is not set (e.g., “One of these days I'll ride the exercise bike continuously for an hour”), the goal is really just a dream. For this idea to be a goal, the individual needs to set a specific date by which to achieve the behaviour. Goal setting usually involves both long-term and short-term goals. The long-term goal provides direction; the short-term goals provide the increase in intensity and effort. People often err by setting only long-term goals. They begin to work toward achieving the goal, but success seems so far away that they give up before they get there. If someone had decided to ride a bike for 1 h and was currently having trouble lasting 10 min, 1 h would seem virtually impossible. Achieving short-term goals along the way to the long-term goal boosts confidence and motivation because it is obvious that the effort is worthwhile because improvement is being made. Target dates are set for each short-term goal in a progressive order until the long-term goal is achieved. This pattern of goal setting can be considered as a staircase, where each short-term goal is a step on the way to the long-term goal (see figure 20.1).
For goals to be effective, however, they need to be properly set. Goals can be considered to be good if they meet certain criteria (table 20.1). Goals need to be challenging but realistic. If goals are not challenging, they probably are not requiring any real change in behaviour and therefore will have little effect. However, if goals are so challenging that they are unrealistic, people are setting themselves up for failure. Continued failure leads to lowered confidence and less motivation.
Goals also need to be specific and measurable. Saying “I want to be fitter” or “My goal is to be stronger” does not provide any way of knowing when success has been achieved. What is “fitter”? How strong is “stronger”? There needs to be some way of knowing whether the goal has been achieved when the target date arrives. The easiest way of making goals specific is to make goals numerical. Numbers can easily be used to quantify time spent exercising, distance travelled, repetitions accomplished, weight lifted, or exercise sessions attended.
In addition to being specific and measurable, goals need to be positive. If an exerciser makes it her goal not to recline and rest during the abdominals section of her aerobics class, she will be thinking about reclining and resting. If instead she makes it her goal to complete first 1 min and then 2 min of the abdominal exercises, she will be thinking about doing the exercises, thus increasing her chances of doing them. If we tell you not to think about pink elephants, what is the first thing you think about? Similarly, if you set a goal of not letting your back arch off the bench when you do bench presses, you will be thinking about your back arching. When you think about your back arching, your brain may be sending messages to the muscles that make your back arch (just like in the psychoneuromuscular theory of imagery discussed in chapter 19). By making your goal “not arching,” you may actually be increasing the likelihood of arching your back. A goal of keeping your back pushed flat against the bench would be much more effective. Goals should stipulate the desired behaviour. Positively worded goals help you think about, plan for, and prepare to do what it is you want to do.
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Lifespan changes in skeletal system can lead to injury
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages.
Biophysical Foundations of Human Movement, Third Edition.
Age-Related Changes in the Skeletal and Articular Systems
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages. During the early fetal growth period there is rapid multiplication of cells (hyperplasia), whereas during the pubertal period growth is caused mainly by development and enlargement of existing cells (hypertrophy). Development can also occur via the replacement of one type of tissue by another.
Stages in the Development of Bone
Adult human bones appear in a variety of shapes and sizes. Their basic shape is genetically determined, but their final shape is influenced greatly by the environment in which they develop. Environmental influences include mechanical factors, such as muscle forces acting on the developing bone, and metabolic factors, which include the supply of nutrients.
Typical long bones, such as the humerus in the arm and the femur in the thigh, develop via a process of endochondral ossification in which a cartilaginous model precedes the bone formation. The cartilage is eventually replaced by bone. The cartilage model initially is small, and its replacement by bone begins at an early stage of development. The primary ossification centres appear near the middle of the shaft of the future long bone at about 8 wk after fertilisation, when the embryo is about 35 mm (1.4 in.) long (see figure 5.1).
The cartilage model of the future bone grows before it is replaced by bone. This replacement occurs in a series of stages. Initially, the cartilage model grows by two processes: an increase in the number of cartilage cells and then an increase in the size of each cell. Next, the gel-like matrix surrounding the cartilage cells is calcified. This hardening of the tissue surrounding the cells effectively cuts off their supply of nutrients and prevents the removal of metabolic waste products. These imprisoned cartilage cells eventually die so that the calcified cartilage has a honeycomb appearance. Invading blood vessels bring nutrients and bone-forming cells (osteoblasts) to the calcified cartilage to lay down new bone. This new bone has a disorganised appearance. If ossification were the final stage, a long bone would not develop its hollow structure. What happens instead is that some of the newly laid bone is removed from internal sites by special bone-eroding cells. These osteoclasts work with osteoblasts during remodelling of the bone, causing the cortex to drift away from the central axis of the shaft during growth. Changes in shaft diameter and compact bone thickness can occur at any age by a remodelling process. Bone can respond to changes in mechanical stimuli by changing its structure.
One type of remodelling that occurs is the replacement of the originally laid bone by compact bone that has an organised structure (figure 3.1). Most primary centres of ossification (in the developing shaft) appear before birth, whereas most secondary centres of ossification (in the developing ends of the bone) appear after birth. The times of appearance of these secondary centres of ossification vary widely. The centre in the femur near the knee is normally present at birth, but the centre in the clavicle (collarbone) closest to the sternum (breastbone) does not appear until about 18 yr of age.
Growth in Length and Width of Bone
When both the shaft and ends of a long bone are developing they are separated by a growth plate, or epiphyseal plate. In this region, all the stages of bone development can be seen under a microscope (see figure 5.2). Each zone in figure 5.2 represents a stage in the growth of bone and then replacement of cartilage by bone; that is, the zones illustrated do not migrate downward, but rather retain their position and change their nature. This effectively increases the distance between the growth plates at each end of the bone. The process illustrated continues for almost 20 yr. Growth in length of the long bones ceases when the process stops and epiphyseal plates fuse and disappear. Actually, the growth stops because the cartilage cells no longer respond to hormonal influence.
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Although growth in height ceases at a certain age, changes in thickness of the compact bone of the shaft and the density of spongy bone can occur at any time of life (see chapter 6). Growth in the thickness and diameter of long bones occurs by appositional growth. In this process, bone is normally added on the outside of the shaft and removed from the inside of the shaft.
Skeletal Composition Changes Across the Life Span
The relative proportions of the inorganic salt crystals and the organic collagen change throughout the life span. In a child, the flexibility of the bone is related to the large proportion of collagen. It is thought that the rapid growth of children's bones and the consequent amount of remodelling results in less-than-optimal mineralisation. In a newborn baby, the skeleton accounts for about 13% of the total body weight, but about two-thirds of the skeleton is cartilaginous.
In bone of a young adult the inorganic material is close to the optimal two-thirds mineralisation, so the bone is strong and tough. Though the proportion of skeletal material in the mature adult is also about 13% of total body weight, just over 10% of the skeleton is cartilaginous. Of the bone, four-fifths is compact and one-fifth is spongy.
Eventually, aged bone becomes brittle. This change relates partly to the increased proportion of the inorganic salts, but of more significance is the reduced total mass of bone material in the aged musculoskeletal system. Bone mass decreases mainly because of a decrease in the number of trabeculae in spongy bone (see “In Focus: Why Trunk Height Decreases With Ageing”) combined with resorption and consequent thinning of the trabeculae. In compact bone, the size of the Haversian canals (see figure 3.1) increases and the cross-sectional area of the bone surrounding each Haversian canal decreases. This increased porosity of bone, in turn, causes decreased bone density (mass per unit volume of tissue). In an 80-yr-old male the density of bone in the spine is often 55% of the value found in a 20-yr-old; in females who are elderly, it is often only 40% of its peak value. With ageing, bone also becomes stiffer, partly because of the collagen cross-linking. A consequence of these mechanical changes is that aged bone can absorb less energy before it fractures, so it is more liable to fail when subjected to large forces.
Osteoporosis
For most tissues in the body, their mass at any time is determined by the balance between the simultaneous formation and destruction of the tissue. Osteopenia (“bone poverty”) is reduced bone density, particularly in women during the postmenopausal years, when the dramatic decrease in the female hormone estrogen allows the rate of synthesis to be lower than the rate of resorption. The initial effects are mainly to the spongy bone, but later the compact bone in the shafts of long bones also decreases in thickness.
According to the World Health Organization, osteoporosis (“bone porosity”) is present when the bone density is more than 2.5 standard deviations below the average bone mass of a young adult woman. In general, the porosity of bone in the human femur doubles from 40 to 80 yr of age and people with osteoporosis show an even greater change. Thus, osteoporotic bone is even less able to absorb energy before it fails than is normally aged bone. Moreover, even though a chemical analysis of osteoporotic bone material may indicate slightly increased levels of calcium per unit mass of bone, the overall mass of bone decreases so markedly that there is less calcium stored in the skeleton overall.
During maturation, the density and calcium content of bones is increased due to genetic predisposition aided by the interaction between dietary calcium and exercise. It is now thought that peak bone mass occurs at about 16 to 20 yr of age in women, so some health authorities claim osteoporosis is a pediatric disease and have promoted the idea of “bone banks” to help prevent osteoporosis. The idea is that if a person's lifestyle during maturation and early adulthood promoted maximum deposition of bone material, then later in life, when withdrawals of bone material are inevitable, the structural integrity of the bones will be maintained longer. Financial advisers tell everybody to start saving as young as possible, and this concept is even more important for prevention of osteoporosis.
Osteoporosis is commonly associated with women, but the prevalence among men is now the same as it was in women about 25 yr ago. This trend has been attributed to lifestyle changes, which include inadequate intake of calcium and less-than-optimal levels of physical activity. The personal lifestyle changes due to osteoporosis and the public health care costs of osteoporosis are so great that they are considered further in chapter 23.
Bone Failure in Relation to Bone Development, Age, or Activity
The modern automotive industry attempts to protect passengers by enclosing them in a cage that is lined by material that cushions the forces involved in a collision. In similar fashion, the soft tissues of the human body, including fat and muscle, perform the role of a crumple zone. However, despite this protection, sometimes the energy involved in a mishap is enough to cause a fracture anyway. Bone failure may be related to bone development, age, or activity.
Certain types of fractures are associated with particular stages of development. The flexible bones of a child tend to splinter in a manner similar to the broken branch of a growing tree; this type of fracture is called a greenstick fracture. Late in life the fracture is more likely to be of the brittle type because of the increased stiffness of old bones. The bones of adults who are elderly are also more likely to fail because of the decreased bone mass and density in osteoporosis.
Specific fractures are related to certain ages. For example, fractures of the neck of the femur (commonly called hip fractures) are common in women in the elderly population. Incidence of forearm fracture increases near the peak of the pubescent growth spurt. This is associated with increased porosity of compact bone as the remodelling space increases to provide calcium to the rapid-growth regions. A second increase in wrist fractures is seen in women who are elderly; this is related to the combination of osteoporosis and increased incidence of falls. In developing children, the cartilaginous growth plate may also fail; this most often occurs at the zone of maturation, or hypertrophy (see figure 5.2).
The development of bone has implications for potential injury to the musculoskeletal system. For example, injury to the cartilaginous growth plates can be produced by single excessive forces causing trauma or by repetitive compressive forces, or tensile forces produced by muscle-tendon pull on an area of developing bone.
In sports medicine, specific injuries have been attributed to certain activities. One of the best examples is Little League elbow, described in more detail in chapter 24. Other examples of the relationships between activity and types of growth-plate problems are wrist injuries in gymnasts and tibial tuberosity avulsions in sports involving sprinting and jumping. Pain over the tibial tuberosity (just below the knee cap) in a prepubescent child is indicative of Osgood-Schlatter's condition.
Effects of Various Factors on Range of Motion
Range of motion at joints is affected by joint structure and the mechanical properties of the tissues associated with the joints. A general perception is that joint range of motion decreases during life. Although this is the general trend, the rate of loss is not constant. Ranges of joint motion are very large in a newborn baby. As an example, the range of ankle dorsiflexion in a newborn is limited only by the contact of the top of the foot against the shin. Try this movement yourself to indicate how much flexibility you have lost since birth.
Between the ages of 6 and 15 yr, there is a general trend for joint range of motion to decrease in boys, whereas for girls the effects are variable and joint dependent. Girls are generally more flexible than boys during childhood and adolescence. Changes in joint ranges of motion between adolescence and young adulthood are variable and, as discussed in chapter 6, appear to be related to physical activity. Similarly, the general trend for joint flexibility to decrease during ageing may not be completely explained by biological ageing processes.
Ageing people may suffer arthritis (“inflammation of joints”), which markedly restricts range of motion. Rheumatoid arthritis involves inflammation of the synovial membrane, while osteoarthritis involves degeneration of the articular cartilage (see figure 3.6). In the total adult population, about three times as many women have osteoarthritis as do men. Thus there is some genetic determination of osteoarthritis, to which are added the effects of environmental factors. It would appear that the risk factors for rheumatoid arthritis are mainly related to a family history of the condition.
Age-Related Changes in the Muscular System
Mesoderm, which forms toward the end of the third week of development, eventually forms the muscle and connective tissues of the body. Some of the mesoderm is segmented such that it is in lumps on either side of the midline, and these lumps form the muscles of the trunk. Other mesoderm forms the muscles of the limbs; this tissue migrates into the limbs when they first appear. Limb muscles migrate toward their final position during further development.
Certain precursor cells differentiate into myoblasts (muscle-forming cells) that fuse longitudinally to form the long muscles and muscle fibres we can observe. The number of muscle fibres appears to be genetically determined so that large people often have more muscle fibres than persons who are smaller in stature. As muscles grow in length, the number of sarcomeres increases. Although direct evidence is sparse, many believe that the stimulus for growth of muscle is the bone growth that pulls on the attached ends of the muscle. Developmental growth occurs in both muscle length and cross-sectional area, and these factors may also be affected by activity. The effects of physical training on muscle is summarised in chapters 6, 10, and 13.
In an infant, muscle tissue accounts for about 25% of total body weight, but in a young adult the proportion of muscle is more than 40%. In terms of growth, muscle increases from about 850 g (1.9 lb) at birth to about 30 kg (66 lb) in a young adult male weighing 70 kg (154 lb).
Strength, which may be defined as the capacity to produce force against an external resistance, is related to the cross-sectional areas of the muscles producing the force. Muscle volume peaks at about 30 yr of age and then gradually decreases. The atrophy of skeletal muscle associated with disuse is much faster than any ageing-related atrophy. Muscle elasticity also decreases during ageing so that muscles are stiffer and less extensible. This is one contributing factor to the loss of joint range of motion described earlier.
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Training can increase both muscle strength and fiber contraction speed
Adaptations are common following injury and surgical treatment.
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Biomechanical Adaptations to Injury
Adaptations are common following injury and surgical treatment. One interesting example found in the orthopaedics literature is a phenomenon known as quadriceps avoidance gait. This particular adaptation occurs in ACL-deficient (ACLD) patients; that is, patients who have suffered a complete rupture of the ACL. ACLD patients usually have difficulty with movements involving either lateral thrusts (e.g., cutting from side to side) or twisting (i.e., rotation of the femur relative to the tibia in the transverse plane).
Gait analysis has been done on ACLD patients during various activities of daily living, including level walking, jogging, walking up and down stairs, and even running, cutting, and pivoting. Kinematic and force-plate data were used to estimate the net muscle moments exerted at the knee in all three planes of movement: flexion-extension, abduction-
adduction, and internal-external rotation. Interestingly, the greatest functional changes between ACLD patients and healthy subjects occurred while walking at preferred speeds on level ground. During the midstance portion of the gait cycle, the ACLD patients exhibited a net extensor moment that was significantly lower than normal (figure 10.9, solid line). One interpretation of these findings is that the ACLD patients walk by reducing the demand on their quadriceps during stance (i.e., the quadriceps muscles are activated less in the ACL-deficient patients, giving rise to a quadriceps-avoidance gait pattern).
It is important to realise that a lower net extensor moment at the knee does not necessarily mean that the moment exerted by quadriceps is lower. Because the net moment at the knee is a combination of quadriceps and hamstrings muscle action, a lower extensor moment could result from an increase in the flexor moment applied by hamstrings. However, muscle electromyography results tend to support the interpretation that quadriceps moment is lower because the activation level of the quadriceps is lower in the ACLD patients.
Gait analysis results have also shown that the ACLD patients reduce their net extensor moment by 25% during jogging compared with a 100% reduction in walking. However, the net extensor moment in stair climbing was the same in ACLD patients and normal subjects. When taken together, these results suggest that the observed quadriceps avoidance in ACLD patients is not a result of quadriceps weakness but rather is due to a change in neuromuscular control.
Dependence of Motor Performance on Changes in Muscle Properties
Strength and quickness are critical determinants of performance in explosive movements such as jumping, throwing, and sprinting. Together, these two factors determine the amount of power developed instantaneously during a task (i.e., muscle power is given by the product of muscle force and muscle contraction speed; see “In Focus: At What Speed Must a Muscle Shorten to Develop Maximum Power?”). As discussed earlier in this chapter, resistance strength training can alter both muscle force and fibre contraction speed. This section describes the effects that changes in the contractile properties of muscle have on motor performance, specifically, how biomechanical performance depends on muscle strength and muscle fibre contraction speed.
Vertical jumping is one of the most heavily studied motor tasks. Many scientists have studied jumping with the aim of learning more about the biomechanics and control of this task and, specifically, how performance may be influenced by training. In a recent jump-training program that incorporated stretching, plyometric exercises, and weight lifting, female high school volleyball players demonstrated a mean increase of 4 cm (1.5 in.) in jump height after training. This represented an almost 10% increase in jump height as a result of the 6 wk period of training. Even larger increases in vertical jumping performance have been documented in the literature. One extreme example is the 1984 United States Olympic gold medal volleyball team, which showed a 10 cm (4 in.) increase in jump height after 2 yr of jump training.
Using Computer Modelling to Study Vertical Jumping Performance
It is gratifying to learn that jumping performance can be increased significantly by strength and neuromuscular training. However, it is difficult, if not impossible, to explain why these increases occur. Noninvasive measurements of biomechanical performance cannot pinpoint the factor or factors responsible for the increase in jump height. The reason is that several properties of the neuromuscular and musculoskeletal systems change simultaneously during the training regimen. Alternatively, computer models may be used to study the relationships between training effects and performance; that is, a model of the neuromusculoskeletal system may be used to predict how changes to specific parameters affect the performance of a motor
task.
Models of the body similar to the one shown in figure 8.7 have been used to study how changes in muscle strength, muscle contraction speed, and motor unit recruitment affect jump height. The values of each of these parameters in the model were increased by amounts consistent with results obtained from strength-training programs. For example, the peak isometric strength and maximum shortening velocity of each muscle in the model were increased by 20% and the activation level of each muscle was increased by 10%. The three training effects were first applied to all muscles simultaneously and then to the ankle plantarflexors, knee extensors, and hip extensors separately. In this way, the modelling results were used to determine whether it is better to train all the leg muscles simultaneously or to isolate specific muscle groups such as the quadriceps (knee extensors) and gluteus maximus (hip
extensors).
Insights Into the Effects of Training Provided by Computer Models
Increasing the peak isometric force of all the leg muscles in the model by 20% produced the largest increase in jump height. In this case, jump height increased by 7 cm, or 5% of that obtained before the simulated training effect. Increasing the maximum shortening velocity of each muscle by 20% or the activation level of each muscle by 10% increased jump height by only 4 cm, or 3% of that obtained for the nominal (untrained) model. When all three training effects were introduced simultaneously, jumping performance increased by nearly 17 cm, or 12% of that calculated for the untrained model. Thus, training programs that increase strength, fibre contraction speed, and motor recruitment (i.e., activation level) of all the leg muscles simultaneously are most beneficial to overall jumping performance.
The modelling results also suggest that training the knee extensors is better than training either the ankle plantarflexors or the hip extensors. When peak isometric force and maximum shortening velocity of the quadriceps were increased by 20% and quadriceps activation was simultaneously increased by 10%, jump height increased by almost 10 cm, or 7% of the value calculated for the untrained model. The same changes made to either the ankle plantarflexors or the hip extensors produced increases in jump height of only 3 cm, or 2% of the value obtained for the untrained model.
The latter result is a little puzzling because the model calculations had previously shown that the quadriceps and gluteus maximus are the major energy producers—the prime movers—of the body in vertical jumping (see “In Focus: Which Muscles Are Most Important to Vertical Jumping Performance?” in chapter 8). It should be noted here that each time a change was introduced to the model, a new optimal pattern of muscle activations was found by re-solving the optimisation problem for a maximum-height jump. Thus, one interpretation of the result is that what matters most in terms of performance is the quadriceps:gluteus maximus muscle strength ratio and not the absolute strength of these muscles. In other words, jumping performance is most sensitive to a change in the knee-extensor:hip-extensor muscle strength ratio. Increasing quadriceps strength by 20% increases the knee-extensor:hip-extensor muscle strength ratio in the model, whereas increasing gluteus maximus muscle strength decreases this ratio. The same line of reasoning may be used to explain why increasing ankle plantarflexor muscle strength by 20% leads to an increase in jump height of just 3 cm in the model.
Training for strength is also better than training for quickness or speed. Figure 10.10 shows the effects of increasing muscle strength and muscle contraction speed on vertical jump height as predicted by the four-segment, eight-muscle, sagittal-plane model of the body described in chapter 8 (see figure 8.7). An increase in muscle strength was simulated in the model by simultaneously increasing body weight because muscle strength increases in proportion to muscle mass. Thus, changes in body strength:weight ratio are represented in figure 10.10 rather than changes in muscle strength alone. Also, jumping performance is normalised in these results by dividing by the value of jump height calculated for the untrained model. Similarly, body strength:weight ratio and muscle contraction speed are each normalised by dividing by the value of body strength:weight ratio and muscle contraction speed in the untrained model, respectively.
The simulation results show that the slope of the line predicted for changes in body strength:weight ratio is twice as large as that obtained when changes in muscle fibre contraction speed are made (compare solid and dashed lines in figure 10.10). Thus, muscle strength has a greater effect on vertical jump height than muscle fibre contraction speed, even when the accompanying increase in body mass is taken into account.
Another important lesson learned from the modelling studies is that musculoskeletal changes must be accompanied by appropriate changes in neuromuscular control; otherwise, the expected improvement in motor performance will not be seen. In vertical jumping, if the pattern of muscle activations remains unchanged subsequent to strength training, jump height actually decreases relative to the untrained state. Figure 10.11 shows a simulated jump in which the strength of the knee-extensor muscles has been increased by 20% but the control exerted over the joints is the same as that calculated for the untrained model before the training effect was introduced. When the pattern of muscle activations was not optimised to match the changes introduced to the neuromusculoskeletal model, the body left the ground prematurely (i.e., the centre of mass was at a lower height at liftoff than is optimal for a maximal jump).
One consequence of not optimising the pattern of muscle activations (i.e., the controls) is that a larger fraction of the total work produced by the muscles goes into rotating the body segments rather than accelerating the centre of mass upward. In a maximum-height jump, approximately 90% of the total work done by the leg muscles is used to propel the centre of mass upward. This number is closer to 80% when muscle coordination (i.e., the sequence and timing of muscle activations) is not optimal. So, even though jumping performance depends heavily on muscle strength, and to a lesser extent on muscle fibre contraction speed, optimal performance is also intimately related to neuromuscular control. For this reason, and as discussed at the beginning of this chapter, jump-training programs now focus on maneuvers that blend muscle strength with neural control.
Summary
Training can change both muscle strength and fibre contraction speed. Strength may be increased by increasing motor unit recruitment (net neural drive to the muscle) or by increasing muscle fibre size. Contraction speed may be altered by changing the shape of a muscle's force-velocity curve or by changing the value of its intrinsic maximum shortening velocity. Early in a training program (2-8 wk), increases in muscle strength are brought by neural adaptations rather than by increases in muscle size. Training can also change the way muscle action is coordinated during activity (i.e., neuromuscular control). Neuromuscular training programs are usually designed to improve stability (balance and coordination) and proprioception (joint position sense) in addition to muscle strength. Training is vital for improving biomechanical performance and for preventing injuries during sport.
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The three components of motivation affect exercise adherence
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
Effects of Psychological Factors on Exercise
The three components of motivation—direction, intensity, and persistence—mentioned in chapter 19 in relation to sport apply to exercise motivation as well. Direction clearly relates to whether an individual chooses the direction of the gym or the couch, the stairs or the elevator, or the pool or the bath. Once the direction of exercise has been chosen, intensity and persistence become important. For example, as a New Year's resolution two individuals may have both decided to join a gym. They have both chosen the direction of exercise. During their first day at the gym, one rides the bicycle for 30 min, tries the stepper and the rowing machine, completes multiple sets on the different weights available, and finishes off with an aerobics class. The other individual begins with just a few minutes on the bicycle and then completes just one set of each of the different weights exercises using light weights. The first person in this example exhibits high intensity and the second person exhibits low intensity. If we stop at this point in the example, you may conclude that the first person is more motivated than the second. Your opinion may change when the third component of motivation—persistence—is considered. Two days later the first individual is home in bed, too sore to move (thinking that exercise is painful and should be avoided whenever possible). The second individual, however, is once again at the gym adding a couple of minutes to his time on the bicycle and sticking to his light-weights workout.
Exercise Participation Motivation
Exercise participation motivation refers primarily to the direction component of motivation. Exercise participation motivation is the initiation of exercise. A variety of factors influence whether individuals initiate an exercise program.
Knowledge, attitudes, and beliefs about exercise influence motivation toward exercise participation. Individuals who understand the importance and value of regular exercise are more likely to initiate an exercise program than those who do not. Similarly, if people have positive attitudes about the value and importance of regular exercise they will have greater motivation to participate in exercise than will people with negative attitudes.
Valuing the importance of exercise, however, is not the only determinant of exercise participation. Beliefs about ourselves influence motivation as well. Even if individuals understand that exercise is important, they will be unlikely to begin an exercise program if they believe that they cannot succeed at it. If they believe that the exercise program is too difficult or that it requires more fitness, strength, coordination, or time than they have, it is doubtful that they will join the program. This confidence in one's ability to succeed at an exercise program is called exercise self-efficacy. It is logical that self-efficacy would influence behaviour. How likely would people be to do something they were convinced they could not do, particularly if they had to pay to do it? How likely would they be to invest any energy in pursuing that activity? Most people in this situation would not attempt the activity. People who have such feelings are described as having low self-efficacy. As self-efficacy, or one's belief in one's ability to succeed at a particular task, increases, so does the likelihood of undertaking that task.
How can we enhance exercise participation motivation? Educating people about the importance and value of exercise can be a valuable first step because individuals who understand the merit of exercise are more likely to adopt an exercise program. Unfortunately, imparting knowledge is not enough. A lot of us do things that we know are not good for us and, similarly, do not do things that we know are good for us. Enhancing the exercise self-efficacy of individuals will increase their motivation. Demonstrating how individuals can control their own activity is useful. Some people have low self-efficacy about exercise because they believe that they are too unfit to begin exercising. They may equate exercise with young, thin, Lycra-clad gym enthusiasts whom they see in the media. Programs that emphasise choice of activities, illustrate exercisers similar in age and fitness level to the potential exercisers, and reveal that exercising can be enjoyable may increase exercise participation motivation. Exercise programs that begin with activities that the individuals already know they are capable of doing, such as walking and climbing stairs, may also increase self-efficacy and thereby increase motivation. Imagery may also help individuals enhance their self-efficacy beliefs (see “Imagery, Exercise, and Self-Efficacy”).
Exercise Adherence Motivation
Although many people get motivated to begin an exercise program, many of the people who begin fail to continue. Approximately 50% of individuals who begin a regular physical-activity program drop out within the first 6 mo. These people had exercise participation motivation, but they lack exercise adherence motivation, the persistence component of motivation.
Biological, psychological, sensory, and situational factors all interact to influence exercise adherence. Biologically, body composition, aerobic fitness, and the presence of disease influence adherence. Unfortunately, it is usually the people who could gain the most from exercise who are the least likely to adhere. Overweight or obese, unfit, or chronically ill people are less likely to adhere to an exercise program than are thinner, fitter, and healthier people.
As is the case with exercise participation motivation, attitudes and beliefs influence exercise adherence motivation. Attitudes and beliefs about the importance of exercise play a role in adherence, but so too do individuals' expectations about the effects that exercise is having on them personally. For example, if individuals believe that major changes in fitness and body composition should occur after 6 wk of regular exercise and they do not perceive major improvement in their own bodies after 6 wk, they may believe that exercise does not do what it should and therefore quit. Even though it is unrealistic to believe that 6 wk of exercise can make up for 6 yr of inactivity, it is individuals' beliefs, not reality, that influence behaviour. Therefore, when introducing newcomers to exercise it is important that they have realistic expectations about the time and effort required and the anticipated effects of the proposed program.
In addition to attitudes and beliefs, other psychological factors influence exercise adherence motivation. Extroverts (people who are social and outgoing) tend to adhere to exercise programs better than do introverts. Exercise programs that are executed in the presence of other people are probably more comfortable for extroverts than for introverts. Extroverts tend to enjoy the interaction with class members and exercise partners, possibly encouraging their adherence. Introverts, on the other hand, may adhere better to individual, home-based exercise programs. The bulk of the research on exercise adherence has involved programs that take place on site at fitness facilities with other people. This setting may have led to the conclusion that extroverts are better adherers than introverts. If the research had been done on independent, home-based exercise programs, it might have been found that introverts were better adherers. Clearly efforts need to be made to match the social environment of the exercise program to the personality of the exerciser.
Individuals with high levels of self-motivation are more likely to adhere to exercise programs than are individuals with low levels of self-motivation. It is logical that highly self-motivated people have better adherence rates. The challenge is to help those individuals with low levels of self-motivation. One of the most effective methods of helping these individuals is to encourage their involvement in the goal-setting process.
Goal Setting
Setting goals can help enhance motivation for a number of reasons. Goal setting addresses all three components of motivation. Goals give direction by providing a target. Intensity and effort also can be enhanced because goals provide reasons for participating in the activity. We are all more likely to put in effort when we feel there is a reason for doing so. If individuals are given two jobs to do at work, one that has a particular target and objective and one that seems vague and purposeless, into which job are they more likely to put their effort? Having goals helps focus attention and effort. In addition, goals can augment persistence by fostering new strategies. If individuals have a goal to which they are committed and initial tactics appear unsuccessful, they will search for alternative strategies to achieve their aim. If the goal had not been set in the first place, instead of persisting with different plans of action, they would likely give up.
Goals are also beneficial because they reflect improvement. Too often people make short-term comparisons regarding their strength, fitness, flexibility, or weight. Because the positive effects of exercise take time to emerge, improvements being made are often not noticed when people use a short timeframe for comparison. If a goal is achieved, evidence of improvement exists.
Goal setting involves a number of steps:
- setting the goal,
- setting a target date by which to achieve the goal,
- determining strategies to achieve the goal, and
- evaluating the goal on a regular basis.
If a target date is not set (e.g., “One of these days I'll ride the exercise bike continuously for an hour”), the goal is really just a dream. For this idea to be a goal, the individual needs to set a specific date by which to achieve the behaviour. Goal setting usually involves both long-term and short-term goals. The long-term goal provides direction; the short-term goals provide the increase in intensity and effort. People often err by setting only long-term goals. They begin to work toward achieving the goal, but success seems so far away that they give up before they get there. If someone had decided to ride a bike for 1 h and was currently having trouble lasting 10 min, 1 h would seem virtually impossible. Achieving short-term goals along the way to the long-term goal boosts confidence and motivation because it is obvious that the effort is worthwhile because improvement is being made. Target dates are set for each short-term goal in a progressive order until the long-term goal is achieved. This pattern of goal setting can be considered as a staircase, where each short-term goal is a step on the way to the long-term goal (see figure 20.1).
For goals to be effective, however, they need to be properly set. Goals can be considered to be good if they meet certain criteria (table 20.1). Goals need to be challenging but realistic. If goals are not challenging, they probably are not requiring any real change in behaviour and therefore will have little effect. However, if goals are so challenging that they are unrealistic, people are setting themselves up for failure. Continued failure leads to lowered confidence and less motivation.
Goals also need to be specific and measurable. Saying “I want to be fitter” or “My goal is to be stronger” does not provide any way of knowing when success has been achieved. What is “fitter”? How strong is “stronger”? There needs to be some way of knowing whether the goal has been achieved when the target date arrives. The easiest way of making goals specific is to make goals numerical. Numbers can easily be used to quantify time spent exercising, distance travelled, repetitions accomplished, weight lifted, or exercise sessions attended.
In addition to being specific and measurable, goals need to be positive. If an exerciser makes it her goal not to recline and rest during the abdominals section of her aerobics class, she will be thinking about reclining and resting. If instead she makes it her goal to complete first 1 min and then 2 min of the abdominal exercises, she will be thinking about doing the exercises, thus increasing her chances of doing them. If we tell you not to think about pink elephants, what is the first thing you think about? Similarly, if you set a goal of not letting your back arch off the bench when you do bench presses, you will be thinking about your back arching. When you think about your back arching, your brain may be sending messages to the muscles that make your back arch (just like in the psychoneuromuscular theory of imagery discussed in chapter 19). By making your goal “not arching,” you may actually be increasing the likelihood of arching your back. A goal of keeping your back pushed flat against the bench would be much more effective. Goals should stipulate the desired behaviour. Positively worded goals help you think about, plan for, and prepare to do what it is you want to do.
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Lifespan changes in skeletal system can lead to injury
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages.
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Age-Related Changes in the Skeletal and Articular Systems
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages. During the early fetal growth period there is rapid multiplication of cells (hyperplasia), whereas during the pubertal period growth is caused mainly by development and enlargement of existing cells (hypertrophy). Development can also occur via the replacement of one type of tissue by another.
Stages in the Development of Bone
Adult human bones appear in a variety of shapes and sizes. Their basic shape is genetically determined, but their final shape is influenced greatly by the environment in which they develop. Environmental influences include mechanical factors, such as muscle forces acting on the developing bone, and metabolic factors, which include the supply of nutrients.
Typical long bones, such as the humerus in the arm and the femur in the thigh, develop via a process of endochondral ossification in which a cartilaginous model precedes the bone formation. The cartilage is eventually replaced by bone. The cartilage model initially is small, and its replacement by bone begins at an early stage of development. The primary ossification centres appear near the middle of the shaft of the future long bone at about 8 wk after fertilisation, when the embryo is about 35 mm (1.4 in.) long (see figure 5.1).
The cartilage model of the future bone grows before it is replaced by bone. This replacement occurs in a series of stages. Initially, the cartilage model grows by two processes: an increase in the number of cartilage cells and then an increase in the size of each cell. Next, the gel-like matrix surrounding the cartilage cells is calcified. This hardening of the tissue surrounding the cells effectively cuts off their supply of nutrients and prevents the removal of metabolic waste products. These imprisoned cartilage cells eventually die so that the calcified cartilage has a honeycomb appearance. Invading blood vessels bring nutrients and bone-forming cells (osteoblasts) to the calcified cartilage to lay down new bone. This new bone has a disorganised appearance. If ossification were the final stage, a long bone would not develop its hollow structure. What happens instead is that some of the newly laid bone is removed from internal sites by special bone-eroding cells. These osteoclasts work with osteoblasts during remodelling of the bone, causing the cortex to drift away from the central axis of the shaft during growth. Changes in shaft diameter and compact bone thickness can occur at any age by a remodelling process. Bone can respond to changes in mechanical stimuli by changing its structure.
One type of remodelling that occurs is the replacement of the originally laid bone by compact bone that has an organised structure (figure 3.1). Most primary centres of ossification (in the developing shaft) appear before birth, whereas most secondary centres of ossification (in the developing ends of the bone) appear after birth. The times of appearance of these secondary centres of ossification vary widely. The centre in the femur near the knee is normally present at birth, but the centre in the clavicle (collarbone) closest to the sternum (breastbone) does not appear until about 18 yr of age.
Growth in Length and Width of Bone
When both the shaft and ends of a long bone are developing they are separated by a growth plate, or epiphyseal plate. In this region, all the stages of bone development can be seen under a microscope (see figure 5.2). Each zone in figure 5.2 represents a stage in the growth of bone and then replacement of cartilage by bone; that is, the zones illustrated do not migrate downward, but rather retain their position and change their nature. This effectively increases the distance between the growth plates at each end of the bone. The process illustrated continues for almost 20 yr. Growth in length of the long bones ceases when the process stops and epiphyseal plates fuse and disappear. Actually, the growth stops because the cartilage cells no longer respond to hormonal influence.
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Although growth in height ceases at a certain age, changes in thickness of the compact bone of the shaft and the density of spongy bone can occur at any time of life (see chapter 6). Growth in the thickness and diameter of long bones occurs by appositional growth. In this process, bone is normally added on the outside of the shaft and removed from the inside of the shaft.
Skeletal Composition Changes Across the Life Span
The relative proportions of the inorganic salt crystals and the organic collagen change throughout the life span. In a child, the flexibility of the bone is related to the large proportion of collagen. It is thought that the rapid growth of children's bones and the consequent amount of remodelling results in less-than-optimal mineralisation. In a newborn baby, the skeleton accounts for about 13% of the total body weight, but about two-thirds of the skeleton is cartilaginous.
In bone of a young adult the inorganic material is close to the optimal two-thirds mineralisation, so the bone is strong and tough. Though the proportion of skeletal material in the mature adult is also about 13% of total body weight, just over 10% of the skeleton is cartilaginous. Of the bone, four-fifths is compact and one-fifth is spongy.
Eventually, aged bone becomes brittle. This change relates partly to the increased proportion of the inorganic salts, but of more significance is the reduced total mass of bone material in the aged musculoskeletal system. Bone mass decreases mainly because of a decrease in the number of trabeculae in spongy bone (see “In Focus: Why Trunk Height Decreases With Ageing”) combined with resorption and consequent thinning of the trabeculae. In compact bone, the size of the Haversian canals (see figure 3.1) increases and the cross-sectional area of the bone surrounding each Haversian canal decreases. This increased porosity of bone, in turn, causes decreased bone density (mass per unit volume of tissue). In an 80-yr-old male the density of bone in the spine is often 55% of the value found in a 20-yr-old; in females who are elderly, it is often only 40% of its peak value. With ageing, bone also becomes stiffer, partly because of the collagen cross-linking. A consequence of these mechanical changes is that aged bone can absorb less energy before it fractures, so it is more liable to fail when subjected to large forces.
Osteoporosis
For most tissues in the body, their mass at any time is determined by the balance between the simultaneous formation and destruction of the tissue. Osteopenia (“bone poverty”) is reduced bone density, particularly in women during the postmenopausal years, when the dramatic decrease in the female hormone estrogen allows the rate of synthesis to be lower than the rate of resorption. The initial effects are mainly to the spongy bone, but later the compact bone in the shafts of long bones also decreases in thickness.
According to the World Health Organization, osteoporosis (“bone porosity”) is present when the bone density is more than 2.5 standard deviations below the average bone mass of a young adult woman. In general, the porosity of bone in the human femur doubles from 40 to 80 yr of age and people with osteoporosis show an even greater change. Thus, osteoporotic bone is even less able to absorb energy before it fails than is normally aged bone. Moreover, even though a chemical analysis of osteoporotic bone material may indicate slightly increased levels of calcium per unit mass of bone, the overall mass of bone decreases so markedly that there is less calcium stored in the skeleton overall.
During maturation, the density and calcium content of bones is increased due to genetic predisposition aided by the interaction between dietary calcium and exercise. It is now thought that peak bone mass occurs at about 16 to 20 yr of age in women, so some health authorities claim osteoporosis is a pediatric disease and have promoted the idea of “bone banks” to help prevent osteoporosis. The idea is that if a person's lifestyle during maturation and early adulthood promoted maximum deposition of bone material, then later in life, when withdrawals of bone material are inevitable, the structural integrity of the bones will be maintained longer. Financial advisers tell everybody to start saving as young as possible, and this concept is even more important for prevention of osteoporosis.
Osteoporosis is commonly associated with women, but the prevalence among men is now the same as it was in women about 25 yr ago. This trend has been attributed to lifestyle changes, which include inadequate intake of calcium and less-than-optimal levels of physical activity. The personal lifestyle changes due to osteoporosis and the public health care costs of osteoporosis are so great that they are considered further in chapter 23.
Bone Failure in Relation to Bone Development, Age, or Activity
The modern automotive industry attempts to protect passengers by enclosing them in a cage that is lined by material that cushions the forces involved in a collision. In similar fashion, the soft tissues of the human body, including fat and muscle, perform the role of a crumple zone. However, despite this protection, sometimes the energy involved in a mishap is enough to cause a fracture anyway. Bone failure may be related to bone development, age, or activity.
Certain types of fractures are associated with particular stages of development. The flexible bones of a child tend to splinter in a manner similar to the broken branch of a growing tree; this type of fracture is called a greenstick fracture. Late in life the fracture is more likely to be of the brittle type because of the increased stiffness of old bones. The bones of adults who are elderly are also more likely to fail because of the decreased bone mass and density in osteoporosis.
Specific fractures are related to certain ages. For example, fractures of the neck of the femur (commonly called hip fractures) are common in women in the elderly population. Incidence of forearm fracture increases near the peak of the pubescent growth spurt. This is associated with increased porosity of compact bone as the remodelling space increases to provide calcium to the rapid-growth regions. A second increase in wrist fractures is seen in women who are elderly; this is related to the combination of osteoporosis and increased incidence of falls. In developing children, the cartilaginous growth plate may also fail; this most often occurs at the zone of maturation, or hypertrophy (see figure 5.2).
The development of bone has implications for potential injury to the musculoskeletal system. For example, injury to the cartilaginous growth plates can be produced by single excessive forces causing trauma or by repetitive compressive forces, or tensile forces produced by muscle-tendon pull on an area of developing bone.
In sports medicine, specific injuries have been attributed to certain activities. One of the best examples is Little League elbow, described in more detail in chapter 24. Other examples of the relationships between activity and types of growth-plate problems are wrist injuries in gymnasts and tibial tuberosity avulsions in sports involving sprinting and jumping. Pain over the tibial tuberosity (just below the knee cap) in a prepubescent child is indicative of Osgood-Schlatter's condition.
Effects of Various Factors on Range of Motion
Range of motion at joints is affected by joint structure and the mechanical properties of the tissues associated with the joints. A general perception is that joint range of motion decreases during life. Although this is the general trend, the rate of loss is not constant. Ranges of joint motion are very large in a newborn baby. As an example, the range of ankle dorsiflexion in a newborn is limited only by the contact of the top of the foot against the shin. Try this movement yourself to indicate how much flexibility you have lost since birth.
Between the ages of 6 and 15 yr, there is a general trend for joint range of motion to decrease in boys, whereas for girls the effects are variable and joint dependent. Girls are generally more flexible than boys during childhood and adolescence. Changes in joint ranges of motion between adolescence and young adulthood are variable and, as discussed in chapter 6, appear to be related to physical activity. Similarly, the general trend for joint flexibility to decrease during ageing may not be completely explained by biological ageing processes.
Ageing people may suffer arthritis (“inflammation of joints”), which markedly restricts range of motion. Rheumatoid arthritis involves inflammation of the synovial membrane, while osteoarthritis involves degeneration of the articular cartilage (see figure 3.6). In the total adult population, about three times as many women have osteoarthritis as do men. Thus there is some genetic determination of osteoarthritis, to which are added the effects of environmental factors. It would appear that the risk factors for rheumatoid arthritis are mainly related to a family history of the condition.
Age-Related Changes in the Muscular System
Mesoderm, which forms toward the end of the third week of development, eventually forms the muscle and connective tissues of the body. Some of the mesoderm is segmented such that it is in lumps on either side of the midline, and these lumps form the muscles of the trunk. Other mesoderm forms the muscles of the limbs; this tissue migrates into the limbs when they first appear. Limb muscles migrate toward their final position during further development.
Certain precursor cells differentiate into myoblasts (muscle-forming cells) that fuse longitudinally to form the long muscles and muscle fibres we can observe. The number of muscle fibres appears to be genetically determined so that large people often have more muscle fibres than persons who are smaller in stature. As muscles grow in length, the number of sarcomeres increases. Although direct evidence is sparse, many believe that the stimulus for growth of muscle is the bone growth that pulls on the attached ends of the muscle. Developmental growth occurs in both muscle length and cross-sectional area, and these factors may also be affected by activity. The effects of physical training on muscle is summarised in chapters 6, 10, and 13.
In an infant, muscle tissue accounts for about 25% of total body weight, but in a young adult the proportion of muscle is more than 40%. In terms of growth, muscle increases from about 850 g (1.9 lb) at birth to about 30 kg (66 lb) in a young adult male weighing 70 kg (154 lb).
Strength, which may be defined as the capacity to produce force against an external resistance, is related to the cross-sectional areas of the muscles producing the force. Muscle volume peaks at about 30 yr of age and then gradually decreases. The atrophy of skeletal muscle associated with disuse is much faster than any ageing-related atrophy. Muscle elasticity also decreases during ageing so that muscles are stiffer and less extensible. This is one contributing factor to the loss of joint range of motion described earlier.
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Training can increase both muscle strength and fiber contraction speed
Adaptations are common following injury and surgical treatment.
Biophysical Foundations of Human Movement, Third Edition.
Biomechanical Adaptations to Injury
Adaptations are common following injury and surgical treatment. One interesting example found in the orthopaedics literature is a phenomenon known as quadriceps avoidance gait. This particular adaptation occurs in ACL-deficient (ACLD) patients; that is, patients who have suffered a complete rupture of the ACL. ACLD patients usually have difficulty with movements involving either lateral thrusts (e.g., cutting from side to side) or twisting (i.e., rotation of the femur relative to the tibia in the transverse plane).
Gait analysis has been done on ACLD patients during various activities of daily living, including level walking, jogging, walking up and down stairs, and even running, cutting, and pivoting. Kinematic and force-plate data were used to estimate the net muscle moments exerted at the knee in all three planes of movement: flexion-extension, abduction-
adduction, and internal-external rotation. Interestingly, the greatest functional changes between ACLD patients and healthy subjects occurred while walking at preferred speeds on level ground. During the midstance portion of the gait cycle, the ACLD patients exhibited a net extensor moment that was significantly lower than normal (figure 10.9, solid line). One interpretation of these findings is that the ACLD patients walk by reducing the demand on their quadriceps during stance (i.e., the quadriceps muscles are activated less in the ACL-deficient patients, giving rise to a quadriceps-avoidance gait pattern).
It is important to realise that a lower net extensor moment at the knee does not necessarily mean that the moment exerted by quadriceps is lower. Because the net moment at the knee is a combination of quadriceps and hamstrings muscle action, a lower extensor moment could result from an increase in the flexor moment applied by hamstrings. However, muscle electromyography results tend to support the interpretation that quadriceps moment is lower because the activation level of the quadriceps is lower in the ACLD patients.
Gait analysis results have also shown that the ACLD patients reduce their net extensor moment by 25% during jogging compared with a 100% reduction in walking. However, the net extensor moment in stair climbing was the same in ACLD patients and normal subjects. When taken together, these results suggest that the observed quadriceps avoidance in ACLD patients is not a result of quadriceps weakness but rather is due to a change in neuromuscular control.
Dependence of Motor Performance on Changes in Muscle Properties
Strength and quickness are critical determinants of performance in explosive movements such as jumping, throwing, and sprinting. Together, these two factors determine the amount of power developed instantaneously during a task (i.e., muscle power is given by the product of muscle force and muscle contraction speed; see “In Focus: At What Speed Must a Muscle Shorten to Develop Maximum Power?”). As discussed earlier in this chapter, resistance strength training can alter both muscle force and fibre contraction speed. This section describes the effects that changes in the contractile properties of muscle have on motor performance, specifically, how biomechanical performance depends on muscle strength and muscle fibre contraction speed.
Vertical jumping is one of the most heavily studied motor tasks. Many scientists have studied jumping with the aim of learning more about the biomechanics and control of this task and, specifically, how performance may be influenced by training. In a recent jump-training program that incorporated stretching, plyometric exercises, and weight lifting, female high school volleyball players demonstrated a mean increase of 4 cm (1.5 in.) in jump height after training. This represented an almost 10% increase in jump height as a result of the 6 wk period of training. Even larger increases in vertical jumping performance have been documented in the literature. One extreme example is the 1984 United States Olympic gold medal volleyball team, which showed a 10 cm (4 in.) increase in jump height after 2 yr of jump training.
Using Computer Modelling to Study Vertical Jumping Performance
It is gratifying to learn that jumping performance can be increased significantly by strength and neuromuscular training. However, it is difficult, if not impossible, to explain why these increases occur. Noninvasive measurements of biomechanical performance cannot pinpoint the factor or factors responsible for the increase in jump height. The reason is that several properties of the neuromuscular and musculoskeletal systems change simultaneously during the training regimen. Alternatively, computer models may be used to study the relationships between training effects and performance; that is, a model of the neuromusculoskeletal system may be used to predict how changes to specific parameters affect the performance of a motor
task.
Models of the body similar to the one shown in figure 8.7 have been used to study how changes in muscle strength, muscle contraction speed, and motor unit recruitment affect jump height. The values of each of these parameters in the model were increased by amounts consistent with results obtained from strength-training programs. For example, the peak isometric strength and maximum shortening velocity of each muscle in the model were increased by 20% and the activation level of each muscle was increased by 10%. The three training effects were first applied to all muscles simultaneously and then to the ankle plantarflexors, knee extensors, and hip extensors separately. In this way, the modelling results were used to determine whether it is better to train all the leg muscles simultaneously or to isolate specific muscle groups such as the quadriceps (knee extensors) and gluteus maximus (hip
extensors).
Insights Into the Effects of Training Provided by Computer Models
Increasing the peak isometric force of all the leg muscles in the model by 20% produced the largest increase in jump height. In this case, jump height increased by 7 cm, or 5% of that obtained before the simulated training effect. Increasing the maximum shortening velocity of each muscle by 20% or the activation level of each muscle by 10% increased jump height by only 4 cm, or 3% of that obtained for the nominal (untrained) model. When all three training effects were introduced simultaneously, jumping performance increased by nearly 17 cm, or 12% of that calculated for the untrained model. Thus, training programs that increase strength, fibre contraction speed, and motor recruitment (i.e., activation level) of all the leg muscles simultaneously are most beneficial to overall jumping performance.
The modelling results also suggest that training the knee extensors is better than training either the ankle plantarflexors or the hip extensors. When peak isometric force and maximum shortening velocity of the quadriceps were increased by 20% and quadriceps activation was simultaneously increased by 10%, jump height increased by almost 10 cm, or 7% of the value calculated for the untrained model. The same changes made to either the ankle plantarflexors or the hip extensors produced increases in jump height of only 3 cm, or 2% of the value obtained for the untrained model.
The latter result is a little puzzling because the model calculations had previously shown that the quadriceps and gluteus maximus are the major energy producers—the prime movers—of the body in vertical jumping (see “In Focus: Which Muscles Are Most Important to Vertical Jumping Performance?” in chapter 8). It should be noted here that each time a change was introduced to the model, a new optimal pattern of muscle activations was found by re-solving the optimisation problem for a maximum-height jump. Thus, one interpretation of the result is that what matters most in terms of performance is the quadriceps:gluteus maximus muscle strength ratio and not the absolute strength of these muscles. In other words, jumping performance is most sensitive to a change in the knee-extensor:hip-extensor muscle strength ratio. Increasing quadriceps strength by 20% increases the knee-extensor:hip-extensor muscle strength ratio in the model, whereas increasing gluteus maximus muscle strength decreases this ratio. The same line of reasoning may be used to explain why increasing ankle plantarflexor muscle strength by 20% leads to an increase in jump height of just 3 cm in the model.
Training for strength is also better than training for quickness or speed. Figure 10.10 shows the effects of increasing muscle strength and muscle contraction speed on vertical jump height as predicted by the four-segment, eight-muscle, sagittal-plane model of the body described in chapter 8 (see figure 8.7). An increase in muscle strength was simulated in the model by simultaneously increasing body weight because muscle strength increases in proportion to muscle mass. Thus, changes in body strength:weight ratio are represented in figure 10.10 rather than changes in muscle strength alone. Also, jumping performance is normalised in these results by dividing by the value of jump height calculated for the untrained model. Similarly, body strength:weight ratio and muscle contraction speed are each normalised by dividing by the value of body strength:weight ratio and muscle contraction speed in the untrained model, respectively.
The simulation results show that the slope of the line predicted for changes in body strength:weight ratio is twice as large as that obtained when changes in muscle fibre contraction speed are made (compare solid and dashed lines in figure 10.10). Thus, muscle strength has a greater effect on vertical jump height than muscle fibre contraction speed, even when the accompanying increase in body mass is taken into account.
Another important lesson learned from the modelling studies is that musculoskeletal changes must be accompanied by appropriate changes in neuromuscular control; otherwise, the expected improvement in motor performance will not be seen. In vertical jumping, if the pattern of muscle activations remains unchanged subsequent to strength training, jump height actually decreases relative to the untrained state. Figure 10.11 shows a simulated jump in which the strength of the knee-extensor muscles has been increased by 20% but the control exerted over the joints is the same as that calculated for the untrained model before the training effect was introduced. When the pattern of muscle activations was not optimised to match the changes introduced to the neuromusculoskeletal model, the body left the ground prematurely (i.e., the centre of mass was at a lower height at liftoff than is optimal for a maximal jump).
One consequence of not optimising the pattern of muscle activations (i.e., the controls) is that a larger fraction of the total work produced by the muscles goes into rotating the body segments rather than accelerating the centre of mass upward. In a maximum-height jump, approximately 90% of the total work done by the leg muscles is used to propel the centre of mass upward. This number is closer to 80% when muscle coordination (i.e., the sequence and timing of muscle activations) is not optimal. So, even though jumping performance depends heavily on muscle strength, and to a lesser extent on muscle fibre contraction speed, optimal performance is also intimately related to neuromuscular control. For this reason, and as discussed at the beginning of this chapter, jump-training programs now focus on maneuvers that blend muscle strength with neural control.
Summary
Training can change both muscle strength and fibre contraction speed. Strength may be increased by increasing motor unit recruitment (net neural drive to the muscle) or by increasing muscle fibre size. Contraction speed may be altered by changing the shape of a muscle's force-velocity curve or by changing the value of its intrinsic maximum shortening velocity. Early in a training program (2-8 wk), increases in muscle strength are brought by neural adaptations rather than by increases in muscle size. Training can also change the way muscle action is coordinated during activity (i.e., neuromuscular control). Neuromuscular training programs are usually designed to improve stability (balance and coordination) and proprioception (joint position sense) in addition to muscle strength. Training is vital for improving biomechanical performance and for preventing injuries during sport.
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The three components of motivation affect exercise adherence
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
Effects of Psychological Factors on Exercise
The three components of motivation—direction, intensity, and persistence—mentioned in chapter 19 in relation to sport apply to exercise motivation as well. Direction clearly relates to whether an individual chooses the direction of the gym or the couch, the stairs or the elevator, or the pool or the bath. Once the direction of exercise has been chosen, intensity and persistence become important. For example, as a New Year's resolution two individuals may have both decided to join a gym. They have both chosen the direction of exercise. During their first day at the gym, one rides the bicycle for 30 min, tries the stepper and the rowing machine, completes multiple sets on the different weights available, and finishes off with an aerobics class. The other individual begins with just a few minutes on the bicycle and then completes just one set of each of the different weights exercises using light weights. The first person in this example exhibits high intensity and the second person exhibits low intensity. If we stop at this point in the example, you may conclude that the first person is more motivated than the second. Your opinion may change when the third component of motivation—persistence—is considered. Two days later the first individual is home in bed, too sore to move (thinking that exercise is painful and should be avoided whenever possible). The second individual, however, is once again at the gym adding a couple of minutes to his time on the bicycle and sticking to his light-weights workout.
Exercise Participation Motivation
Exercise participation motivation refers primarily to the direction component of motivation. Exercise participation motivation is the initiation of exercise. A variety of factors influence whether individuals initiate an exercise program.
Knowledge, attitudes, and beliefs about exercise influence motivation toward exercise participation. Individuals who understand the importance and value of regular exercise are more likely to initiate an exercise program than those who do not. Similarly, if people have positive attitudes about the value and importance of regular exercise they will have greater motivation to participate in exercise than will people with negative attitudes.
Valuing the importance of exercise, however, is not the only determinant of exercise participation. Beliefs about ourselves influence motivation as well. Even if individuals understand that exercise is important, they will be unlikely to begin an exercise program if they believe that they cannot succeed at it. If they believe that the exercise program is too difficult or that it requires more fitness, strength, coordination, or time than they have, it is doubtful that they will join the program. This confidence in one's ability to succeed at an exercise program is called exercise self-efficacy. It is logical that self-efficacy would influence behaviour. How likely would people be to do something they were convinced they could not do, particularly if they had to pay to do it? How likely would they be to invest any energy in pursuing that activity? Most people in this situation would not attempt the activity. People who have such feelings are described as having low self-efficacy. As self-efficacy, or one's belief in one's ability to succeed at a particular task, increases, so does the likelihood of undertaking that task.
How can we enhance exercise participation motivation? Educating people about the importance and value of exercise can be a valuable first step because individuals who understand the merit of exercise are more likely to adopt an exercise program. Unfortunately, imparting knowledge is not enough. A lot of us do things that we know are not good for us and, similarly, do not do things that we know are good for us. Enhancing the exercise self-efficacy of individuals will increase their motivation. Demonstrating how individuals can control their own activity is useful. Some people have low self-efficacy about exercise because they believe that they are too unfit to begin exercising. They may equate exercise with young, thin, Lycra-clad gym enthusiasts whom they see in the media. Programs that emphasise choice of activities, illustrate exercisers similar in age and fitness level to the potential exercisers, and reveal that exercising can be enjoyable may increase exercise participation motivation. Exercise programs that begin with activities that the individuals already know they are capable of doing, such as walking and climbing stairs, may also increase self-efficacy and thereby increase motivation. Imagery may also help individuals enhance their self-efficacy beliefs (see “Imagery, Exercise, and Self-Efficacy”).
Exercise Adherence Motivation
Although many people get motivated to begin an exercise program, many of the people who begin fail to continue. Approximately 50% of individuals who begin a regular physical-activity program drop out within the first 6 mo. These people had exercise participation motivation, but they lack exercise adherence motivation, the persistence component of motivation.
Biological, psychological, sensory, and situational factors all interact to influence exercise adherence. Biologically, body composition, aerobic fitness, and the presence of disease influence adherence. Unfortunately, it is usually the people who could gain the most from exercise who are the least likely to adhere. Overweight or obese, unfit, or chronically ill people are less likely to adhere to an exercise program than are thinner, fitter, and healthier people.
As is the case with exercise participation motivation, attitudes and beliefs influence exercise adherence motivation. Attitudes and beliefs about the importance of exercise play a role in adherence, but so too do individuals' expectations about the effects that exercise is having on them personally. For example, if individuals believe that major changes in fitness and body composition should occur after 6 wk of regular exercise and they do not perceive major improvement in their own bodies after 6 wk, they may believe that exercise does not do what it should and therefore quit. Even though it is unrealistic to believe that 6 wk of exercise can make up for 6 yr of inactivity, it is individuals' beliefs, not reality, that influence behaviour. Therefore, when introducing newcomers to exercise it is important that they have realistic expectations about the time and effort required and the anticipated effects of the proposed program.
In addition to attitudes and beliefs, other psychological factors influence exercise adherence motivation. Extroverts (people who are social and outgoing) tend to adhere to exercise programs better than do introverts. Exercise programs that are executed in the presence of other people are probably more comfortable for extroverts than for introverts. Extroverts tend to enjoy the interaction with class members and exercise partners, possibly encouraging their adherence. Introverts, on the other hand, may adhere better to individual, home-based exercise programs. The bulk of the research on exercise adherence has involved programs that take place on site at fitness facilities with other people. This setting may have led to the conclusion that extroverts are better adherers than introverts. If the research had been done on independent, home-based exercise programs, it might have been found that introverts were better adherers. Clearly efforts need to be made to match the social environment of the exercise program to the personality of the exerciser.
Individuals with high levels of self-motivation are more likely to adhere to exercise programs than are individuals with low levels of self-motivation. It is logical that highly self-motivated people have better adherence rates. The challenge is to help those individuals with low levels of self-motivation. One of the most effective methods of helping these individuals is to encourage their involvement in the goal-setting process.
Goal Setting
Setting goals can help enhance motivation for a number of reasons. Goal setting addresses all three components of motivation. Goals give direction by providing a target. Intensity and effort also can be enhanced because goals provide reasons for participating in the activity. We are all more likely to put in effort when we feel there is a reason for doing so. If individuals are given two jobs to do at work, one that has a particular target and objective and one that seems vague and purposeless, into which job are they more likely to put their effort? Having goals helps focus attention and effort. In addition, goals can augment persistence by fostering new strategies. If individuals have a goal to which they are committed and initial tactics appear unsuccessful, they will search for alternative strategies to achieve their aim. If the goal had not been set in the first place, instead of persisting with different plans of action, they would likely give up.
Goals are also beneficial because they reflect improvement. Too often people make short-term comparisons regarding their strength, fitness, flexibility, or weight. Because the positive effects of exercise take time to emerge, improvements being made are often not noticed when people use a short timeframe for comparison. If a goal is achieved, evidence of improvement exists.
Goal setting involves a number of steps:
- setting the goal,
- setting a target date by which to achieve the goal,
- determining strategies to achieve the goal, and
- evaluating the goal on a regular basis.
If a target date is not set (e.g., “One of these days I'll ride the exercise bike continuously for an hour”), the goal is really just a dream. For this idea to be a goal, the individual needs to set a specific date by which to achieve the behaviour. Goal setting usually involves both long-term and short-term goals. The long-term goal provides direction; the short-term goals provide the increase in intensity and effort. People often err by setting only long-term goals. They begin to work toward achieving the goal, but success seems so far away that they give up before they get there. If someone had decided to ride a bike for 1 h and was currently having trouble lasting 10 min, 1 h would seem virtually impossible. Achieving short-term goals along the way to the long-term goal boosts confidence and motivation because it is obvious that the effort is worthwhile because improvement is being made. Target dates are set for each short-term goal in a progressive order until the long-term goal is achieved. This pattern of goal setting can be considered as a staircase, where each short-term goal is a step on the way to the long-term goal (see figure 20.1).
For goals to be effective, however, they need to be properly set. Goals can be considered to be good if they meet certain criteria (table 20.1). Goals need to be challenging but realistic. If goals are not challenging, they probably are not requiring any real change in behaviour and therefore will have little effect. However, if goals are so challenging that they are unrealistic, people are setting themselves up for failure. Continued failure leads to lowered confidence and less motivation.
Goals also need to be specific and measurable. Saying “I want to be fitter” or “My goal is to be stronger” does not provide any way of knowing when success has been achieved. What is “fitter”? How strong is “stronger”? There needs to be some way of knowing whether the goal has been achieved when the target date arrives. The easiest way of making goals specific is to make goals numerical. Numbers can easily be used to quantify time spent exercising, distance travelled, repetitions accomplished, weight lifted, or exercise sessions attended.
In addition to being specific and measurable, goals need to be positive. If an exerciser makes it her goal not to recline and rest during the abdominals section of her aerobics class, she will be thinking about reclining and resting. If instead she makes it her goal to complete first 1 min and then 2 min of the abdominal exercises, she will be thinking about doing the exercises, thus increasing her chances of doing them. If we tell you not to think about pink elephants, what is the first thing you think about? Similarly, if you set a goal of not letting your back arch off the bench when you do bench presses, you will be thinking about your back arching. When you think about your back arching, your brain may be sending messages to the muscles that make your back arch (just like in the psychoneuromuscular theory of imagery discussed in chapter 19). By making your goal “not arching,” you may actually be increasing the likelihood of arching your back. A goal of keeping your back pushed flat against the bench would be much more effective. Goals should stipulate the desired behaviour. Positively worded goals help you think about, plan for, and prepare to do what it is you want to do.
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Lifespan changes in skeletal system can lead to injury
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages.
Biophysical Foundations of Human Movement, Third Edition.
Age-Related Changes in the Skeletal and Articular Systems
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages. During the early fetal growth period there is rapid multiplication of cells (hyperplasia), whereas during the pubertal period growth is caused mainly by development and enlargement of existing cells (hypertrophy). Development can also occur via the replacement of one type of tissue by another.
Stages in the Development of Bone
Adult human bones appear in a variety of shapes and sizes. Their basic shape is genetically determined, but their final shape is influenced greatly by the environment in which they develop. Environmental influences include mechanical factors, such as muscle forces acting on the developing bone, and metabolic factors, which include the supply of nutrients.
Typical long bones, such as the humerus in the arm and the femur in the thigh, develop via a process of endochondral ossification in which a cartilaginous model precedes the bone formation. The cartilage is eventually replaced by bone. The cartilage model initially is small, and its replacement by bone begins at an early stage of development. The primary ossification centres appear near the middle of the shaft of the future long bone at about 8 wk after fertilisation, when the embryo is about 35 mm (1.4 in.) long (see figure 5.1).
The cartilage model of the future bone grows before it is replaced by bone. This replacement occurs in a series of stages. Initially, the cartilage model grows by two processes: an increase in the number of cartilage cells and then an increase in the size of each cell. Next, the gel-like matrix surrounding the cartilage cells is calcified. This hardening of the tissue surrounding the cells effectively cuts off their supply of nutrients and prevents the removal of metabolic waste products. These imprisoned cartilage cells eventually die so that the calcified cartilage has a honeycomb appearance. Invading blood vessels bring nutrients and bone-forming cells (osteoblasts) to the calcified cartilage to lay down new bone. This new bone has a disorganised appearance. If ossification were the final stage, a long bone would not develop its hollow structure. What happens instead is that some of the newly laid bone is removed from internal sites by special bone-eroding cells. These osteoclasts work with osteoblasts during remodelling of the bone, causing the cortex to drift away from the central axis of the shaft during growth. Changes in shaft diameter and compact bone thickness can occur at any age by a remodelling process. Bone can respond to changes in mechanical stimuli by changing its structure.
One type of remodelling that occurs is the replacement of the originally laid bone by compact bone that has an organised structure (figure 3.1). Most primary centres of ossification (in the developing shaft) appear before birth, whereas most secondary centres of ossification (in the developing ends of the bone) appear after birth. The times of appearance of these secondary centres of ossification vary widely. The centre in the femur near the knee is normally present at birth, but the centre in the clavicle (collarbone) closest to the sternum (breastbone) does not appear until about 18 yr of age.
Growth in Length and Width of Bone
When both the shaft and ends of a long bone are developing they are separated by a growth plate, or epiphyseal plate. In this region, all the stages of bone development can be seen under a microscope (see figure 5.2). Each zone in figure 5.2 represents a stage in the growth of bone and then replacement of cartilage by bone; that is, the zones illustrated do not migrate downward, but rather retain their position and change their nature. This effectively increases the distance between the growth plates at each end of the bone. The process illustrated continues for almost 20 yr. Growth in length of the long bones ceases when the process stops and epiphyseal plates fuse and disappear. Actually, the growth stops because the cartilage cells no longer respond to hormonal influence.
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Although growth in height ceases at a certain age, changes in thickness of the compact bone of the shaft and the density of spongy bone can occur at any time of life (see chapter 6). Growth in the thickness and diameter of long bones occurs by appositional growth. In this process, bone is normally added on the outside of the shaft and removed from the inside of the shaft.
Skeletal Composition Changes Across the Life Span
The relative proportions of the inorganic salt crystals and the organic collagen change throughout the life span. In a child, the flexibility of the bone is related to the large proportion of collagen. It is thought that the rapid growth of children's bones and the consequent amount of remodelling results in less-than-optimal mineralisation. In a newborn baby, the skeleton accounts for about 13% of the total body weight, but about two-thirds of the skeleton is cartilaginous.
In bone of a young adult the inorganic material is close to the optimal two-thirds mineralisation, so the bone is strong and tough. Though the proportion of skeletal material in the mature adult is also about 13% of total body weight, just over 10% of the skeleton is cartilaginous. Of the bone, four-fifths is compact and one-fifth is spongy.
Eventually, aged bone becomes brittle. This change relates partly to the increased proportion of the inorganic salts, but of more significance is the reduced total mass of bone material in the aged musculoskeletal system. Bone mass decreases mainly because of a decrease in the number of trabeculae in spongy bone (see “In Focus: Why Trunk Height Decreases With Ageing”) combined with resorption and consequent thinning of the trabeculae. In compact bone, the size of the Haversian canals (see figure 3.1) increases and the cross-sectional area of the bone surrounding each Haversian canal decreases. This increased porosity of bone, in turn, causes decreased bone density (mass per unit volume of tissue). In an 80-yr-old male the density of bone in the spine is often 55% of the value found in a 20-yr-old; in females who are elderly, it is often only 40% of its peak value. With ageing, bone also becomes stiffer, partly because of the collagen cross-linking. A consequence of these mechanical changes is that aged bone can absorb less energy before it fractures, so it is more liable to fail when subjected to large forces.
Osteoporosis
For most tissues in the body, their mass at any time is determined by the balance between the simultaneous formation and destruction of the tissue. Osteopenia (“bone poverty”) is reduced bone density, particularly in women during the postmenopausal years, when the dramatic decrease in the female hormone estrogen allows the rate of synthesis to be lower than the rate of resorption. The initial effects are mainly to the spongy bone, but later the compact bone in the shafts of long bones also decreases in thickness.
According to the World Health Organization, osteoporosis (“bone porosity”) is present when the bone density is more than 2.5 standard deviations below the average bone mass of a young adult woman. In general, the porosity of bone in the human femur doubles from 40 to 80 yr of age and people with osteoporosis show an even greater change. Thus, osteoporotic bone is even less able to absorb energy before it fails than is normally aged bone. Moreover, even though a chemical analysis of osteoporotic bone material may indicate slightly increased levels of calcium per unit mass of bone, the overall mass of bone decreases so markedly that there is less calcium stored in the skeleton overall.
During maturation, the density and calcium content of bones is increased due to genetic predisposition aided by the interaction between dietary calcium and exercise. It is now thought that peak bone mass occurs at about 16 to 20 yr of age in women, so some health authorities claim osteoporosis is a pediatric disease and have promoted the idea of “bone banks” to help prevent osteoporosis. The idea is that if a person's lifestyle during maturation and early adulthood promoted maximum deposition of bone material, then later in life, when withdrawals of bone material are inevitable, the structural integrity of the bones will be maintained longer. Financial advisers tell everybody to start saving as young as possible, and this concept is even more important for prevention of osteoporosis.
Osteoporosis is commonly associated with women, but the prevalence among men is now the same as it was in women about 25 yr ago. This trend has been attributed to lifestyle changes, which include inadequate intake of calcium and less-than-optimal levels of physical activity. The personal lifestyle changes due to osteoporosis and the public health care costs of osteoporosis are so great that they are considered further in chapter 23.
Bone Failure in Relation to Bone Development, Age, or Activity
The modern automotive industry attempts to protect passengers by enclosing them in a cage that is lined by material that cushions the forces involved in a collision. In similar fashion, the soft tissues of the human body, including fat and muscle, perform the role of a crumple zone. However, despite this protection, sometimes the energy involved in a mishap is enough to cause a fracture anyway. Bone failure may be related to bone development, age, or activity.
Certain types of fractures are associated with particular stages of development. The flexible bones of a child tend to splinter in a manner similar to the broken branch of a growing tree; this type of fracture is called a greenstick fracture. Late in life the fracture is more likely to be of the brittle type because of the increased stiffness of old bones. The bones of adults who are elderly are also more likely to fail because of the decreased bone mass and density in osteoporosis.
Specific fractures are related to certain ages. For example, fractures of the neck of the femur (commonly called hip fractures) are common in women in the elderly population. Incidence of forearm fracture increases near the peak of the pubescent growth spurt. This is associated with increased porosity of compact bone as the remodelling space increases to provide calcium to the rapid-growth regions. A second increase in wrist fractures is seen in women who are elderly; this is related to the combination of osteoporosis and increased incidence of falls. In developing children, the cartilaginous growth plate may also fail; this most often occurs at the zone of maturation, or hypertrophy (see figure 5.2).
The development of bone has implications for potential injury to the musculoskeletal system. For example, injury to the cartilaginous growth plates can be produced by single excessive forces causing trauma or by repetitive compressive forces, or tensile forces produced by muscle-tendon pull on an area of developing bone.
In sports medicine, specific injuries have been attributed to certain activities. One of the best examples is Little League elbow, described in more detail in chapter 24. Other examples of the relationships between activity and types of growth-plate problems are wrist injuries in gymnasts and tibial tuberosity avulsions in sports involving sprinting and jumping. Pain over the tibial tuberosity (just below the knee cap) in a prepubescent child is indicative of Osgood-Schlatter's condition.
Effects of Various Factors on Range of Motion
Range of motion at joints is affected by joint structure and the mechanical properties of the tissues associated with the joints. A general perception is that joint range of motion decreases during life. Although this is the general trend, the rate of loss is not constant. Ranges of joint motion are very large in a newborn baby. As an example, the range of ankle dorsiflexion in a newborn is limited only by the contact of the top of the foot against the shin. Try this movement yourself to indicate how much flexibility you have lost since birth.
Between the ages of 6 and 15 yr, there is a general trend for joint range of motion to decrease in boys, whereas for girls the effects are variable and joint dependent. Girls are generally more flexible than boys during childhood and adolescence. Changes in joint ranges of motion between adolescence and young adulthood are variable and, as discussed in chapter 6, appear to be related to physical activity. Similarly, the general trend for joint flexibility to decrease during ageing may not be completely explained by biological ageing processes.
Ageing people may suffer arthritis (“inflammation of joints”), which markedly restricts range of motion. Rheumatoid arthritis involves inflammation of the synovial membrane, while osteoarthritis involves degeneration of the articular cartilage (see figure 3.6). In the total adult population, about three times as many women have osteoarthritis as do men. Thus there is some genetic determination of osteoarthritis, to which are added the effects of environmental factors. It would appear that the risk factors for rheumatoid arthritis are mainly related to a family history of the condition.
Age-Related Changes in the Muscular System
Mesoderm, which forms toward the end of the third week of development, eventually forms the muscle and connective tissues of the body. Some of the mesoderm is segmented such that it is in lumps on either side of the midline, and these lumps form the muscles of the trunk. Other mesoderm forms the muscles of the limbs; this tissue migrates into the limbs when they first appear. Limb muscles migrate toward their final position during further development.
Certain precursor cells differentiate into myoblasts (muscle-forming cells) that fuse longitudinally to form the long muscles and muscle fibres we can observe. The number of muscle fibres appears to be genetically determined so that large people often have more muscle fibres than persons who are smaller in stature. As muscles grow in length, the number of sarcomeres increases. Although direct evidence is sparse, many believe that the stimulus for growth of muscle is the bone growth that pulls on the attached ends of the muscle. Developmental growth occurs in both muscle length and cross-sectional area, and these factors may also be affected by activity. The effects of physical training on muscle is summarised in chapters 6, 10, and 13.
In an infant, muscle tissue accounts for about 25% of total body weight, but in a young adult the proportion of muscle is more than 40%. In terms of growth, muscle increases from about 850 g (1.9 lb) at birth to about 30 kg (66 lb) in a young adult male weighing 70 kg (154 lb).
Strength, which may be defined as the capacity to produce force against an external resistance, is related to the cross-sectional areas of the muscles producing the force. Muscle volume peaks at about 30 yr of age and then gradually decreases. The atrophy of skeletal muscle associated with disuse is much faster than any ageing-related atrophy. Muscle elasticity also decreases during ageing so that muscles are stiffer and less extensible. This is one contributing factor to the loss of joint range of motion described earlier.
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Training can increase both muscle strength and fiber contraction speed
Adaptations are common following injury and surgical treatment.
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Biomechanical Adaptations to Injury
Adaptations are common following injury and surgical treatment. One interesting example found in the orthopaedics literature is a phenomenon known as quadriceps avoidance gait. This particular adaptation occurs in ACL-deficient (ACLD) patients; that is, patients who have suffered a complete rupture of the ACL. ACLD patients usually have difficulty with movements involving either lateral thrusts (e.g., cutting from side to side) or twisting (i.e., rotation of the femur relative to the tibia in the transverse plane).
Gait analysis has been done on ACLD patients during various activities of daily living, including level walking, jogging, walking up and down stairs, and even running, cutting, and pivoting. Kinematic and force-plate data were used to estimate the net muscle moments exerted at the knee in all three planes of movement: flexion-extension, abduction-
adduction, and internal-external rotation. Interestingly, the greatest functional changes between ACLD patients and healthy subjects occurred while walking at preferred speeds on level ground. During the midstance portion of the gait cycle, the ACLD patients exhibited a net extensor moment that was significantly lower than normal (figure 10.9, solid line). One interpretation of these findings is that the ACLD patients walk by reducing the demand on their quadriceps during stance (i.e., the quadriceps muscles are activated less in the ACL-deficient patients, giving rise to a quadriceps-avoidance gait pattern).
It is important to realise that a lower net extensor moment at the knee does not necessarily mean that the moment exerted by quadriceps is lower. Because the net moment at the knee is a combination of quadriceps and hamstrings muscle action, a lower extensor moment could result from an increase in the flexor moment applied by hamstrings. However, muscle electromyography results tend to support the interpretation that quadriceps moment is lower because the activation level of the quadriceps is lower in the ACLD patients.
Gait analysis results have also shown that the ACLD patients reduce their net extensor moment by 25% during jogging compared with a 100% reduction in walking. However, the net extensor moment in stair climbing was the same in ACLD patients and normal subjects. When taken together, these results suggest that the observed quadriceps avoidance in ACLD patients is not a result of quadriceps weakness but rather is due to a change in neuromuscular control.
Dependence of Motor Performance on Changes in Muscle Properties
Strength and quickness are critical determinants of performance in explosive movements such as jumping, throwing, and sprinting. Together, these two factors determine the amount of power developed instantaneously during a task (i.e., muscle power is given by the product of muscle force and muscle contraction speed; see “In Focus: At What Speed Must a Muscle Shorten to Develop Maximum Power?”). As discussed earlier in this chapter, resistance strength training can alter both muscle force and fibre contraction speed. This section describes the effects that changes in the contractile properties of muscle have on motor performance, specifically, how biomechanical performance depends on muscle strength and muscle fibre contraction speed.
Vertical jumping is one of the most heavily studied motor tasks. Many scientists have studied jumping with the aim of learning more about the biomechanics and control of this task and, specifically, how performance may be influenced by training. In a recent jump-training program that incorporated stretching, plyometric exercises, and weight lifting, female high school volleyball players demonstrated a mean increase of 4 cm (1.5 in.) in jump height after training. This represented an almost 10% increase in jump height as a result of the 6 wk period of training. Even larger increases in vertical jumping performance have been documented in the literature. One extreme example is the 1984 United States Olympic gold medal volleyball team, which showed a 10 cm (4 in.) increase in jump height after 2 yr of jump training.
Using Computer Modelling to Study Vertical Jumping Performance
It is gratifying to learn that jumping performance can be increased significantly by strength and neuromuscular training. However, it is difficult, if not impossible, to explain why these increases occur. Noninvasive measurements of biomechanical performance cannot pinpoint the factor or factors responsible for the increase in jump height. The reason is that several properties of the neuromuscular and musculoskeletal systems change simultaneously during the training regimen. Alternatively, computer models may be used to study the relationships between training effects and performance; that is, a model of the neuromusculoskeletal system may be used to predict how changes to specific parameters affect the performance of a motor
task.
Models of the body similar to the one shown in figure 8.7 have been used to study how changes in muscle strength, muscle contraction speed, and motor unit recruitment affect jump height. The values of each of these parameters in the model were increased by amounts consistent with results obtained from strength-training programs. For example, the peak isometric strength and maximum shortening velocity of each muscle in the model were increased by 20% and the activation level of each muscle was increased by 10%. The three training effects were first applied to all muscles simultaneously and then to the ankle plantarflexors, knee extensors, and hip extensors separately. In this way, the modelling results were used to determine whether it is better to train all the leg muscles simultaneously or to isolate specific muscle groups such as the quadriceps (knee extensors) and gluteus maximus (hip
extensors).
Insights Into the Effects of Training Provided by Computer Models
Increasing the peak isometric force of all the leg muscles in the model by 20% produced the largest increase in jump height. In this case, jump height increased by 7 cm, or 5% of that obtained before the simulated training effect. Increasing the maximum shortening velocity of each muscle by 20% or the activation level of each muscle by 10% increased jump height by only 4 cm, or 3% of that obtained for the nominal (untrained) model. When all three training effects were introduced simultaneously, jumping performance increased by nearly 17 cm, or 12% of that calculated for the untrained model. Thus, training programs that increase strength, fibre contraction speed, and motor recruitment (i.e., activation level) of all the leg muscles simultaneously are most beneficial to overall jumping performance.
The modelling results also suggest that training the knee extensors is better than training either the ankle plantarflexors or the hip extensors. When peak isometric force and maximum shortening velocity of the quadriceps were increased by 20% and quadriceps activation was simultaneously increased by 10%, jump height increased by almost 10 cm, or 7% of the value calculated for the untrained model. The same changes made to either the ankle plantarflexors or the hip extensors produced increases in jump height of only 3 cm, or 2% of the value obtained for the untrained model.
The latter result is a little puzzling because the model calculations had previously shown that the quadriceps and gluteus maximus are the major energy producers—the prime movers—of the body in vertical jumping (see “In Focus: Which Muscles Are Most Important to Vertical Jumping Performance?” in chapter 8). It should be noted here that each time a change was introduced to the model, a new optimal pattern of muscle activations was found by re-solving the optimisation problem for a maximum-height jump. Thus, one interpretation of the result is that what matters most in terms of performance is the quadriceps:gluteus maximus muscle strength ratio and not the absolute strength of these muscles. In other words, jumping performance is most sensitive to a change in the knee-extensor:hip-extensor muscle strength ratio. Increasing quadriceps strength by 20% increases the knee-extensor:hip-extensor muscle strength ratio in the model, whereas increasing gluteus maximus muscle strength decreases this ratio. The same line of reasoning may be used to explain why increasing ankle plantarflexor muscle strength by 20% leads to an increase in jump height of just 3 cm in the model.
Training for strength is also better than training for quickness or speed. Figure 10.10 shows the effects of increasing muscle strength and muscle contraction speed on vertical jump height as predicted by the four-segment, eight-muscle, sagittal-plane model of the body described in chapter 8 (see figure 8.7). An increase in muscle strength was simulated in the model by simultaneously increasing body weight because muscle strength increases in proportion to muscle mass. Thus, changes in body strength:weight ratio are represented in figure 10.10 rather than changes in muscle strength alone. Also, jumping performance is normalised in these results by dividing by the value of jump height calculated for the untrained model. Similarly, body strength:weight ratio and muscle contraction speed are each normalised by dividing by the value of body strength:weight ratio and muscle contraction speed in the untrained model, respectively.
The simulation results show that the slope of the line predicted for changes in body strength:weight ratio is twice as large as that obtained when changes in muscle fibre contraction speed are made (compare solid and dashed lines in figure 10.10). Thus, muscle strength has a greater effect on vertical jump height than muscle fibre contraction speed, even when the accompanying increase in body mass is taken into account.
Another important lesson learned from the modelling studies is that musculoskeletal changes must be accompanied by appropriate changes in neuromuscular control; otherwise, the expected improvement in motor performance will not be seen. In vertical jumping, if the pattern of muscle activations remains unchanged subsequent to strength training, jump height actually decreases relative to the untrained state. Figure 10.11 shows a simulated jump in which the strength of the knee-extensor muscles has been increased by 20% but the control exerted over the joints is the same as that calculated for the untrained model before the training effect was introduced. When the pattern of muscle activations was not optimised to match the changes introduced to the neuromusculoskeletal model, the body left the ground prematurely (i.e., the centre of mass was at a lower height at liftoff than is optimal for a maximal jump).
One consequence of not optimising the pattern of muscle activations (i.e., the controls) is that a larger fraction of the total work produced by the muscles goes into rotating the body segments rather than accelerating the centre of mass upward. In a maximum-height jump, approximately 90% of the total work done by the leg muscles is used to propel the centre of mass upward. This number is closer to 80% when muscle coordination (i.e., the sequence and timing of muscle activations) is not optimal. So, even though jumping performance depends heavily on muscle strength, and to a lesser extent on muscle fibre contraction speed, optimal performance is also intimately related to neuromuscular control. For this reason, and as discussed at the beginning of this chapter, jump-training programs now focus on maneuvers that blend muscle strength with neural control.
Summary
Training can change both muscle strength and fibre contraction speed. Strength may be increased by increasing motor unit recruitment (net neural drive to the muscle) or by increasing muscle fibre size. Contraction speed may be altered by changing the shape of a muscle's force-velocity curve or by changing the value of its intrinsic maximum shortening velocity. Early in a training program (2-8 wk), increases in muscle strength are brought by neural adaptations rather than by increases in muscle size. Training can also change the way muscle action is coordinated during activity (i.e., neuromuscular control). Neuromuscular training programs are usually designed to improve stability (balance and coordination) and proprioception (joint position sense) in addition to muscle strength. Training is vital for improving biomechanical performance and for preventing injuries during sport.
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The three components of motivation affect exercise adherence
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
Effects of Psychological Factors on Exercise
The three components of motivation—direction, intensity, and persistence—mentioned in chapter 19 in relation to sport apply to exercise motivation as well. Direction clearly relates to whether an individual chooses the direction of the gym or the couch, the stairs or the elevator, or the pool or the bath. Once the direction of exercise has been chosen, intensity and persistence become important. For example, as a New Year's resolution two individuals may have both decided to join a gym. They have both chosen the direction of exercise. During their first day at the gym, one rides the bicycle for 30 min, tries the stepper and the rowing machine, completes multiple sets on the different weights available, and finishes off with an aerobics class. The other individual begins with just a few minutes on the bicycle and then completes just one set of each of the different weights exercises using light weights. The first person in this example exhibits high intensity and the second person exhibits low intensity. If we stop at this point in the example, you may conclude that the first person is more motivated than the second. Your opinion may change when the third component of motivation—persistence—is considered. Two days later the first individual is home in bed, too sore to move (thinking that exercise is painful and should be avoided whenever possible). The second individual, however, is once again at the gym adding a couple of minutes to his time on the bicycle and sticking to his light-weights workout.
Exercise Participation Motivation
Exercise participation motivation refers primarily to the direction component of motivation. Exercise participation motivation is the initiation of exercise. A variety of factors influence whether individuals initiate an exercise program.
Knowledge, attitudes, and beliefs about exercise influence motivation toward exercise participation. Individuals who understand the importance and value of regular exercise are more likely to initiate an exercise program than those who do not. Similarly, if people have positive attitudes about the value and importance of regular exercise they will have greater motivation to participate in exercise than will people with negative attitudes.
Valuing the importance of exercise, however, is not the only determinant of exercise participation. Beliefs about ourselves influence motivation as well. Even if individuals understand that exercise is important, they will be unlikely to begin an exercise program if they believe that they cannot succeed at it. If they believe that the exercise program is too difficult or that it requires more fitness, strength, coordination, or time than they have, it is doubtful that they will join the program. This confidence in one's ability to succeed at an exercise program is called exercise self-efficacy. It is logical that self-efficacy would influence behaviour. How likely would people be to do something they were convinced they could not do, particularly if they had to pay to do it? How likely would they be to invest any energy in pursuing that activity? Most people in this situation would not attempt the activity. People who have such feelings are described as having low self-efficacy. As self-efficacy, or one's belief in one's ability to succeed at a particular task, increases, so does the likelihood of undertaking that task.
How can we enhance exercise participation motivation? Educating people about the importance and value of exercise can be a valuable first step because individuals who understand the merit of exercise are more likely to adopt an exercise program. Unfortunately, imparting knowledge is not enough. A lot of us do things that we know are not good for us and, similarly, do not do things that we know are good for us. Enhancing the exercise self-efficacy of individuals will increase their motivation. Demonstrating how individuals can control their own activity is useful. Some people have low self-efficacy about exercise because they believe that they are too unfit to begin exercising. They may equate exercise with young, thin, Lycra-clad gym enthusiasts whom they see in the media. Programs that emphasise choice of activities, illustrate exercisers similar in age and fitness level to the potential exercisers, and reveal that exercising can be enjoyable may increase exercise participation motivation. Exercise programs that begin with activities that the individuals already know they are capable of doing, such as walking and climbing stairs, may also increase self-efficacy and thereby increase motivation. Imagery may also help individuals enhance their self-efficacy beliefs (see “Imagery, Exercise, and Self-Efficacy”).
Exercise Adherence Motivation
Although many people get motivated to begin an exercise program, many of the people who begin fail to continue. Approximately 50% of individuals who begin a regular physical-activity program drop out within the first 6 mo. These people had exercise participation motivation, but they lack exercise adherence motivation, the persistence component of motivation.
Biological, psychological, sensory, and situational factors all interact to influence exercise adherence. Biologically, body composition, aerobic fitness, and the presence of disease influence adherence. Unfortunately, it is usually the people who could gain the most from exercise who are the least likely to adhere. Overweight or obese, unfit, or chronically ill people are less likely to adhere to an exercise program than are thinner, fitter, and healthier people.
As is the case with exercise participation motivation, attitudes and beliefs influence exercise adherence motivation. Attitudes and beliefs about the importance of exercise play a role in adherence, but so too do individuals' expectations about the effects that exercise is having on them personally. For example, if individuals believe that major changes in fitness and body composition should occur after 6 wk of regular exercise and they do not perceive major improvement in their own bodies after 6 wk, they may believe that exercise does not do what it should and therefore quit. Even though it is unrealistic to believe that 6 wk of exercise can make up for 6 yr of inactivity, it is individuals' beliefs, not reality, that influence behaviour. Therefore, when introducing newcomers to exercise it is important that they have realistic expectations about the time and effort required and the anticipated effects of the proposed program.
In addition to attitudes and beliefs, other psychological factors influence exercise adherence motivation. Extroverts (people who are social and outgoing) tend to adhere to exercise programs better than do introverts. Exercise programs that are executed in the presence of other people are probably more comfortable for extroverts than for introverts. Extroverts tend to enjoy the interaction with class members and exercise partners, possibly encouraging their adherence. Introverts, on the other hand, may adhere better to individual, home-based exercise programs. The bulk of the research on exercise adherence has involved programs that take place on site at fitness facilities with other people. This setting may have led to the conclusion that extroverts are better adherers than introverts. If the research had been done on independent, home-based exercise programs, it might have been found that introverts were better adherers. Clearly efforts need to be made to match the social environment of the exercise program to the personality of the exerciser.
Individuals with high levels of self-motivation are more likely to adhere to exercise programs than are individuals with low levels of self-motivation. It is logical that highly self-motivated people have better adherence rates. The challenge is to help those individuals with low levels of self-motivation. One of the most effective methods of helping these individuals is to encourage their involvement in the goal-setting process.
Goal Setting
Setting goals can help enhance motivation for a number of reasons. Goal setting addresses all three components of motivation. Goals give direction by providing a target. Intensity and effort also can be enhanced because goals provide reasons for participating in the activity. We are all more likely to put in effort when we feel there is a reason for doing so. If individuals are given two jobs to do at work, one that has a particular target and objective and one that seems vague and purposeless, into which job are they more likely to put their effort? Having goals helps focus attention and effort. In addition, goals can augment persistence by fostering new strategies. If individuals have a goal to which they are committed and initial tactics appear unsuccessful, they will search for alternative strategies to achieve their aim. If the goal had not been set in the first place, instead of persisting with different plans of action, they would likely give up.
Goals are also beneficial because they reflect improvement. Too often people make short-term comparisons regarding their strength, fitness, flexibility, or weight. Because the positive effects of exercise take time to emerge, improvements being made are often not noticed when people use a short timeframe for comparison. If a goal is achieved, evidence of improvement exists.
Goal setting involves a number of steps:
- setting the goal,
- setting a target date by which to achieve the goal,
- determining strategies to achieve the goal, and
- evaluating the goal on a regular basis.
If a target date is not set (e.g., “One of these days I'll ride the exercise bike continuously for an hour”), the goal is really just a dream. For this idea to be a goal, the individual needs to set a specific date by which to achieve the behaviour. Goal setting usually involves both long-term and short-term goals. The long-term goal provides direction; the short-term goals provide the increase in intensity and effort. People often err by setting only long-term goals. They begin to work toward achieving the goal, but success seems so far away that they give up before they get there. If someone had decided to ride a bike for 1 h and was currently having trouble lasting 10 min, 1 h would seem virtually impossible. Achieving short-term goals along the way to the long-term goal boosts confidence and motivation because it is obvious that the effort is worthwhile because improvement is being made. Target dates are set for each short-term goal in a progressive order until the long-term goal is achieved. This pattern of goal setting can be considered as a staircase, where each short-term goal is a step on the way to the long-term goal (see figure 20.1).
For goals to be effective, however, they need to be properly set. Goals can be considered to be good if they meet certain criteria (table 20.1). Goals need to be challenging but realistic. If goals are not challenging, they probably are not requiring any real change in behaviour and therefore will have little effect. However, if goals are so challenging that they are unrealistic, people are setting themselves up for failure. Continued failure leads to lowered confidence and less motivation.
Goals also need to be specific and measurable. Saying “I want to be fitter” or “My goal is to be stronger” does not provide any way of knowing when success has been achieved. What is “fitter”? How strong is “stronger”? There needs to be some way of knowing whether the goal has been achieved when the target date arrives. The easiest way of making goals specific is to make goals numerical. Numbers can easily be used to quantify time spent exercising, distance travelled, repetitions accomplished, weight lifted, or exercise sessions attended.
In addition to being specific and measurable, goals need to be positive. If an exerciser makes it her goal not to recline and rest during the abdominals section of her aerobics class, she will be thinking about reclining and resting. If instead she makes it her goal to complete first 1 min and then 2 min of the abdominal exercises, she will be thinking about doing the exercises, thus increasing her chances of doing them. If we tell you not to think about pink elephants, what is the first thing you think about? Similarly, if you set a goal of not letting your back arch off the bench when you do bench presses, you will be thinking about your back arching. When you think about your back arching, your brain may be sending messages to the muscles that make your back arch (just like in the psychoneuromuscular theory of imagery discussed in chapter 19). By making your goal “not arching,” you may actually be increasing the likelihood of arching your back. A goal of keeping your back pushed flat against the bench would be much more effective. Goals should stipulate the desired behaviour. Positively worded goals help you think about, plan for, and prepare to do what it is you want to do.
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Lifespan changes in skeletal system can lead to injury
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages.
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Age-Related Changes in the Skeletal and Articular Systems
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages. During the early fetal growth period there is rapid multiplication of cells (hyperplasia), whereas during the pubertal period growth is caused mainly by development and enlargement of existing cells (hypertrophy). Development can also occur via the replacement of one type of tissue by another.
Stages in the Development of Bone
Adult human bones appear in a variety of shapes and sizes. Their basic shape is genetically determined, but their final shape is influenced greatly by the environment in which they develop. Environmental influences include mechanical factors, such as muscle forces acting on the developing bone, and metabolic factors, which include the supply of nutrients.
Typical long bones, such as the humerus in the arm and the femur in the thigh, develop via a process of endochondral ossification in which a cartilaginous model precedes the bone formation. The cartilage is eventually replaced by bone. The cartilage model initially is small, and its replacement by bone begins at an early stage of development. The primary ossification centres appear near the middle of the shaft of the future long bone at about 8 wk after fertilisation, when the embryo is about 35 mm (1.4 in.) long (see figure 5.1).
The cartilage model of the future bone grows before it is replaced by bone. This replacement occurs in a series of stages. Initially, the cartilage model grows by two processes: an increase in the number of cartilage cells and then an increase in the size of each cell. Next, the gel-like matrix surrounding the cartilage cells is calcified. This hardening of the tissue surrounding the cells effectively cuts off their supply of nutrients and prevents the removal of metabolic waste products. These imprisoned cartilage cells eventually die so that the calcified cartilage has a honeycomb appearance. Invading blood vessels bring nutrients and bone-forming cells (osteoblasts) to the calcified cartilage to lay down new bone. This new bone has a disorganised appearance. If ossification were the final stage, a long bone would not develop its hollow structure. What happens instead is that some of the newly laid bone is removed from internal sites by special bone-eroding cells. These osteoclasts work with osteoblasts during remodelling of the bone, causing the cortex to drift away from the central axis of the shaft during growth. Changes in shaft diameter and compact bone thickness can occur at any age by a remodelling process. Bone can respond to changes in mechanical stimuli by changing its structure.
One type of remodelling that occurs is the replacement of the originally laid bone by compact bone that has an organised structure (figure 3.1). Most primary centres of ossification (in the developing shaft) appear before birth, whereas most secondary centres of ossification (in the developing ends of the bone) appear after birth. The times of appearance of these secondary centres of ossification vary widely. The centre in the femur near the knee is normally present at birth, but the centre in the clavicle (collarbone) closest to the sternum (breastbone) does not appear until about 18 yr of age.
Growth in Length and Width of Bone
When both the shaft and ends of a long bone are developing they are separated by a growth plate, or epiphyseal plate. In this region, all the stages of bone development can be seen under a microscope (see figure 5.2). Each zone in figure 5.2 represents a stage in the growth of bone and then replacement of cartilage by bone; that is, the zones illustrated do not migrate downward, but rather retain their position and change their nature. This effectively increases the distance between the growth plates at each end of the bone. The process illustrated continues for almost 20 yr. Growth in length of the long bones ceases when the process stops and epiphyseal plates fuse and disappear. Actually, the growth stops because the cartilage cells no longer respond to hormonal influence.
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Although growth in height ceases at a certain age, changes in thickness of the compact bone of the shaft and the density of spongy bone can occur at any time of life (see chapter 6). Growth in the thickness and diameter of long bones occurs by appositional growth. In this process, bone is normally added on the outside of the shaft and removed from the inside of the shaft.
Skeletal Composition Changes Across the Life Span
The relative proportions of the inorganic salt crystals and the organic collagen change throughout the life span. In a child, the flexibility of the bone is related to the large proportion of collagen. It is thought that the rapid growth of children's bones and the consequent amount of remodelling results in less-than-optimal mineralisation. In a newborn baby, the skeleton accounts for about 13% of the total body weight, but about two-thirds of the skeleton is cartilaginous.
In bone of a young adult the inorganic material is close to the optimal two-thirds mineralisation, so the bone is strong and tough. Though the proportion of skeletal material in the mature adult is also about 13% of total body weight, just over 10% of the skeleton is cartilaginous. Of the bone, four-fifths is compact and one-fifth is spongy.
Eventually, aged bone becomes brittle. This change relates partly to the increased proportion of the inorganic salts, but of more significance is the reduced total mass of bone material in the aged musculoskeletal system. Bone mass decreases mainly because of a decrease in the number of trabeculae in spongy bone (see “In Focus: Why Trunk Height Decreases With Ageing”) combined with resorption and consequent thinning of the trabeculae. In compact bone, the size of the Haversian canals (see figure 3.1) increases and the cross-sectional area of the bone surrounding each Haversian canal decreases. This increased porosity of bone, in turn, causes decreased bone density (mass per unit volume of tissue). In an 80-yr-old male the density of bone in the spine is often 55% of the value found in a 20-yr-old; in females who are elderly, it is often only 40% of its peak value. With ageing, bone also becomes stiffer, partly because of the collagen cross-linking. A consequence of these mechanical changes is that aged bone can absorb less energy before it fractures, so it is more liable to fail when subjected to large forces.
Osteoporosis
For most tissues in the body, their mass at any time is determined by the balance between the simultaneous formation and destruction of the tissue. Osteopenia (“bone poverty”) is reduced bone density, particularly in women during the postmenopausal years, when the dramatic decrease in the female hormone estrogen allows the rate of synthesis to be lower than the rate of resorption. The initial effects are mainly to the spongy bone, but later the compact bone in the shafts of long bones also decreases in thickness.
According to the World Health Organization, osteoporosis (“bone porosity”) is present when the bone density is more than 2.5 standard deviations below the average bone mass of a young adult woman. In general, the porosity of bone in the human femur doubles from 40 to 80 yr of age and people with osteoporosis show an even greater change. Thus, osteoporotic bone is even less able to absorb energy before it fails than is normally aged bone. Moreover, even though a chemical analysis of osteoporotic bone material may indicate slightly increased levels of calcium per unit mass of bone, the overall mass of bone decreases so markedly that there is less calcium stored in the skeleton overall.
During maturation, the density and calcium content of bones is increased due to genetic predisposition aided by the interaction between dietary calcium and exercise. It is now thought that peak bone mass occurs at about 16 to 20 yr of age in women, so some health authorities claim osteoporosis is a pediatric disease and have promoted the idea of “bone banks” to help prevent osteoporosis. The idea is that if a person's lifestyle during maturation and early adulthood promoted maximum deposition of bone material, then later in life, when withdrawals of bone material are inevitable, the structural integrity of the bones will be maintained longer. Financial advisers tell everybody to start saving as young as possible, and this concept is even more important for prevention of osteoporosis.
Osteoporosis is commonly associated with women, but the prevalence among men is now the same as it was in women about 25 yr ago. This trend has been attributed to lifestyle changes, which include inadequate intake of calcium and less-than-optimal levels of physical activity. The personal lifestyle changes due to osteoporosis and the public health care costs of osteoporosis are so great that they are considered further in chapter 23.
Bone Failure in Relation to Bone Development, Age, or Activity
The modern automotive industry attempts to protect passengers by enclosing them in a cage that is lined by material that cushions the forces involved in a collision. In similar fashion, the soft tissues of the human body, including fat and muscle, perform the role of a crumple zone. However, despite this protection, sometimes the energy involved in a mishap is enough to cause a fracture anyway. Bone failure may be related to bone development, age, or activity.
Certain types of fractures are associated with particular stages of development. The flexible bones of a child tend to splinter in a manner similar to the broken branch of a growing tree; this type of fracture is called a greenstick fracture. Late in life the fracture is more likely to be of the brittle type because of the increased stiffness of old bones. The bones of adults who are elderly are also more likely to fail because of the decreased bone mass and density in osteoporosis.
Specific fractures are related to certain ages. For example, fractures of the neck of the femur (commonly called hip fractures) are common in women in the elderly population. Incidence of forearm fracture increases near the peak of the pubescent growth spurt. This is associated with increased porosity of compact bone as the remodelling space increases to provide calcium to the rapid-growth regions. A second increase in wrist fractures is seen in women who are elderly; this is related to the combination of osteoporosis and increased incidence of falls. In developing children, the cartilaginous growth plate may also fail; this most often occurs at the zone of maturation, or hypertrophy (see figure 5.2).
The development of bone has implications for potential injury to the musculoskeletal system. For example, injury to the cartilaginous growth plates can be produced by single excessive forces causing trauma or by repetitive compressive forces, or tensile forces produced by muscle-tendon pull on an area of developing bone.
In sports medicine, specific injuries have been attributed to certain activities. One of the best examples is Little League elbow, described in more detail in chapter 24. Other examples of the relationships between activity and types of growth-plate problems are wrist injuries in gymnasts and tibial tuberosity avulsions in sports involving sprinting and jumping. Pain over the tibial tuberosity (just below the knee cap) in a prepubescent child is indicative of Osgood-Schlatter's condition.
Effects of Various Factors on Range of Motion
Range of motion at joints is affected by joint structure and the mechanical properties of the tissues associated with the joints. A general perception is that joint range of motion decreases during life. Although this is the general trend, the rate of loss is not constant. Ranges of joint motion are very large in a newborn baby. As an example, the range of ankle dorsiflexion in a newborn is limited only by the contact of the top of the foot against the shin. Try this movement yourself to indicate how much flexibility you have lost since birth.
Between the ages of 6 and 15 yr, there is a general trend for joint range of motion to decrease in boys, whereas for girls the effects are variable and joint dependent. Girls are generally more flexible than boys during childhood and adolescence. Changes in joint ranges of motion between adolescence and young adulthood are variable and, as discussed in chapter 6, appear to be related to physical activity. Similarly, the general trend for joint flexibility to decrease during ageing may not be completely explained by biological ageing processes.
Ageing people may suffer arthritis (“inflammation of joints”), which markedly restricts range of motion. Rheumatoid arthritis involves inflammation of the synovial membrane, while osteoarthritis involves degeneration of the articular cartilage (see figure 3.6). In the total adult population, about three times as many women have osteoarthritis as do men. Thus there is some genetic determination of osteoarthritis, to which are added the effects of environmental factors. It would appear that the risk factors for rheumatoid arthritis are mainly related to a family history of the condition.
Age-Related Changes in the Muscular System
Mesoderm, which forms toward the end of the third week of development, eventually forms the muscle and connective tissues of the body. Some of the mesoderm is segmented such that it is in lumps on either side of the midline, and these lumps form the muscles of the trunk. Other mesoderm forms the muscles of the limbs; this tissue migrates into the limbs when they first appear. Limb muscles migrate toward their final position during further development.
Certain precursor cells differentiate into myoblasts (muscle-forming cells) that fuse longitudinally to form the long muscles and muscle fibres we can observe. The number of muscle fibres appears to be genetically determined so that large people often have more muscle fibres than persons who are smaller in stature. As muscles grow in length, the number of sarcomeres increases. Although direct evidence is sparse, many believe that the stimulus for growth of muscle is the bone growth that pulls on the attached ends of the muscle. Developmental growth occurs in both muscle length and cross-sectional area, and these factors may also be affected by activity. The effects of physical training on muscle is summarised in chapters 6, 10, and 13.
In an infant, muscle tissue accounts for about 25% of total body weight, but in a young adult the proportion of muscle is more than 40%. In terms of growth, muscle increases from about 850 g (1.9 lb) at birth to about 30 kg (66 lb) in a young adult male weighing 70 kg (154 lb).
Strength, which may be defined as the capacity to produce force against an external resistance, is related to the cross-sectional areas of the muscles producing the force. Muscle volume peaks at about 30 yr of age and then gradually decreases. The atrophy of skeletal muscle associated with disuse is much faster than any ageing-related atrophy. Muscle elasticity also decreases during ageing so that muscles are stiffer and less extensible. This is one contributing factor to the loss of joint range of motion described earlier.
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Training can increase both muscle strength and fiber contraction speed
Adaptations are common following injury and surgical treatment.
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Biomechanical Adaptations to Injury
Adaptations are common following injury and surgical treatment. One interesting example found in the orthopaedics literature is a phenomenon known as quadriceps avoidance gait. This particular adaptation occurs in ACL-deficient (ACLD) patients; that is, patients who have suffered a complete rupture of the ACL. ACLD patients usually have difficulty with movements involving either lateral thrusts (e.g., cutting from side to side) or twisting (i.e., rotation of the femur relative to the tibia in the transverse plane).
Gait analysis has been done on ACLD patients during various activities of daily living, including level walking, jogging, walking up and down stairs, and even running, cutting, and pivoting. Kinematic and force-plate data were used to estimate the net muscle moments exerted at the knee in all three planes of movement: flexion-extension, abduction-
adduction, and internal-external rotation. Interestingly, the greatest functional changes between ACLD patients and healthy subjects occurred while walking at preferred speeds on level ground. During the midstance portion of the gait cycle, the ACLD patients exhibited a net extensor moment that was significantly lower than normal (figure 10.9, solid line). One interpretation of these findings is that the ACLD patients walk by reducing the demand on their quadriceps during stance (i.e., the quadriceps muscles are activated less in the ACL-deficient patients, giving rise to a quadriceps-avoidance gait pattern).
It is important to realise that a lower net extensor moment at the knee does not necessarily mean that the moment exerted by quadriceps is lower. Because the net moment at the knee is a combination of quadriceps and hamstrings muscle action, a lower extensor moment could result from an increase in the flexor moment applied by hamstrings. However, muscle electromyography results tend to support the interpretation that quadriceps moment is lower because the activation level of the quadriceps is lower in the ACLD patients.
Gait analysis results have also shown that the ACLD patients reduce their net extensor moment by 25% during jogging compared with a 100% reduction in walking. However, the net extensor moment in stair climbing was the same in ACLD patients and normal subjects. When taken together, these results suggest that the observed quadriceps avoidance in ACLD patients is not a result of quadriceps weakness but rather is due to a change in neuromuscular control.
Dependence of Motor Performance on Changes in Muscle Properties
Strength and quickness are critical determinants of performance in explosive movements such as jumping, throwing, and sprinting. Together, these two factors determine the amount of power developed instantaneously during a task (i.e., muscle power is given by the product of muscle force and muscle contraction speed; see “In Focus: At What Speed Must a Muscle Shorten to Develop Maximum Power?”). As discussed earlier in this chapter, resistance strength training can alter both muscle force and fibre contraction speed. This section describes the effects that changes in the contractile properties of muscle have on motor performance, specifically, how biomechanical performance depends on muscle strength and muscle fibre contraction speed.
Vertical jumping is one of the most heavily studied motor tasks. Many scientists have studied jumping with the aim of learning more about the biomechanics and control of this task and, specifically, how performance may be influenced by training. In a recent jump-training program that incorporated stretching, plyometric exercises, and weight lifting, female high school volleyball players demonstrated a mean increase of 4 cm (1.5 in.) in jump height after training. This represented an almost 10% increase in jump height as a result of the 6 wk period of training. Even larger increases in vertical jumping performance have been documented in the literature. One extreme example is the 1984 United States Olympic gold medal volleyball team, which showed a 10 cm (4 in.) increase in jump height after 2 yr of jump training.
Using Computer Modelling to Study Vertical Jumping Performance
It is gratifying to learn that jumping performance can be increased significantly by strength and neuromuscular training. However, it is difficult, if not impossible, to explain why these increases occur. Noninvasive measurements of biomechanical performance cannot pinpoint the factor or factors responsible for the increase in jump height. The reason is that several properties of the neuromuscular and musculoskeletal systems change simultaneously during the training regimen. Alternatively, computer models may be used to study the relationships between training effects and performance; that is, a model of the neuromusculoskeletal system may be used to predict how changes to specific parameters affect the performance of a motor
task.
Models of the body similar to the one shown in figure 8.7 have been used to study how changes in muscle strength, muscle contraction speed, and motor unit recruitment affect jump height. The values of each of these parameters in the model were increased by amounts consistent with results obtained from strength-training programs. For example, the peak isometric strength and maximum shortening velocity of each muscle in the model were increased by 20% and the activation level of each muscle was increased by 10%. The three training effects were first applied to all muscles simultaneously and then to the ankle plantarflexors, knee extensors, and hip extensors separately. In this way, the modelling results were used to determine whether it is better to train all the leg muscles simultaneously or to isolate specific muscle groups such as the quadriceps (knee extensors) and gluteus maximus (hip
extensors).
Insights Into the Effects of Training Provided by Computer Models
Increasing the peak isometric force of all the leg muscles in the model by 20% produced the largest increase in jump height. In this case, jump height increased by 7 cm, or 5% of that obtained before the simulated training effect. Increasing the maximum shortening velocity of each muscle by 20% or the activation level of each muscle by 10% increased jump height by only 4 cm, or 3% of that obtained for the nominal (untrained) model. When all three training effects were introduced simultaneously, jumping performance increased by nearly 17 cm, or 12% of that calculated for the untrained model. Thus, training programs that increase strength, fibre contraction speed, and motor recruitment (i.e., activation level) of all the leg muscles simultaneously are most beneficial to overall jumping performance.
The modelling results also suggest that training the knee extensors is better than training either the ankle plantarflexors or the hip extensors. When peak isometric force and maximum shortening velocity of the quadriceps were increased by 20% and quadriceps activation was simultaneously increased by 10%, jump height increased by almost 10 cm, or 7% of the value calculated for the untrained model. The same changes made to either the ankle plantarflexors or the hip extensors produced increases in jump height of only 3 cm, or 2% of the value obtained for the untrained model.
The latter result is a little puzzling because the model calculations had previously shown that the quadriceps and gluteus maximus are the major energy producers—the prime movers—of the body in vertical jumping (see “In Focus: Which Muscles Are Most Important to Vertical Jumping Performance?” in chapter 8). It should be noted here that each time a change was introduced to the model, a new optimal pattern of muscle activations was found by re-solving the optimisation problem for a maximum-height jump. Thus, one interpretation of the result is that what matters most in terms of performance is the quadriceps:gluteus maximus muscle strength ratio and not the absolute strength of these muscles. In other words, jumping performance is most sensitive to a change in the knee-extensor:hip-extensor muscle strength ratio. Increasing quadriceps strength by 20% increases the knee-extensor:hip-extensor muscle strength ratio in the model, whereas increasing gluteus maximus muscle strength decreases this ratio. The same line of reasoning may be used to explain why increasing ankle plantarflexor muscle strength by 20% leads to an increase in jump height of just 3 cm in the model.
Training for strength is also better than training for quickness or speed. Figure 10.10 shows the effects of increasing muscle strength and muscle contraction speed on vertical jump height as predicted by the four-segment, eight-muscle, sagittal-plane model of the body described in chapter 8 (see figure 8.7). An increase in muscle strength was simulated in the model by simultaneously increasing body weight because muscle strength increases in proportion to muscle mass. Thus, changes in body strength:weight ratio are represented in figure 10.10 rather than changes in muscle strength alone. Also, jumping performance is normalised in these results by dividing by the value of jump height calculated for the untrained model. Similarly, body strength:weight ratio and muscle contraction speed are each normalised by dividing by the value of body strength:weight ratio and muscle contraction speed in the untrained model, respectively.
The simulation results show that the slope of the line predicted for changes in body strength:weight ratio is twice as large as that obtained when changes in muscle fibre contraction speed are made (compare solid and dashed lines in figure 10.10). Thus, muscle strength has a greater effect on vertical jump height than muscle fibre contraction speed, even when the accompanying increase in body mass is taken into account.
Another important lesson learned from the modelling studies is that musculoskeletal changes must be accompanied by appropriate changes in neuromuscular control; otherwise, the expected improvement in motor performance will not be seen. In vertical jumping, if the pattern of muscle activations remains unchanged subsequent to strength training, jump height actually decreases relative to the untrained state. Figure 10.11 shows a simulated jump in which the strength of the knee-extensor muscles has been increased by 20% but the control exerted over the joints is the same as that calculated for the untrained model before the training effect was introduced. When the pattern of muscle activations was not optimised to match the changes introduced to the neuromusculoskeletal model, the body left the ground prematurely (i.e., the centre of mass was at a lower height at liftoff than is optimal for a maximal jump).
One consequence of not optimising the pattern of muscle activations (i.e., the controls) is that a larger fraction of the total work produced by the muscles goes into rotating the body segments rather than accelerating the centre of mass upward. In a maximum-height jump, approximately 90% of the total work done by the leg muscles is used to propel the centre of mass upward. This number is closer to 80% when muscle coordination (i.e., the sequence and timing of muscle activations) is not optimal. So, even though jumping performance depends heavily on muscle strength, and to a lesser extent on muscle fibre contraction speed, optimal performance is also intimately related to neuromuscular control. For this reason, and as discussed at the beginning of this chapter, jump-training programs now focus on maneuvers that blend muscle strength with neural control.
Summary
Training can change both muscle strength and fibre contraction speed. Strength may be increased by increasing motor unit recruitment (net neural drive to the muscle) or by increasing muscle fibre size. Contraction speed may be altered by changing the shape of a muscle's force-velocity curve or by changing the value of its intrinsic maximum shortening velocity. Early in a training program (2-8 wk), increases in muscle strength are brought by neural adaptations rather than by increases in muscle size. Training can also change the way muscle action is coordinated during activity (i.e., neuromuscular control). Neuromuscular training programs are usually designed to improve stability (balance and coordination) and proprioception (joint position sense) in addition to muscle strength. Training is vital for improving biomechanical performance and for preventing injuries during sport.
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The three components of motivation affect exercise adherence
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
Effects of Psychological Factors on Exercise
The three components of motivation—direction, intensity, and persistence—mentioned in chapter 19 in relation to sport apply to exercise motivation as well. Direction clearly relates to whether an individual chooses the direction of the gym or the couch, the stairs or the elevator, or the pool or the bath. Once the direction of exercise has been chosen, intensity and persistence become important. For example, as a New Year's resolution two individuals may have both decided to join a gym. They have both chosen the direction of exercise. During their first day at the gym, one rides the bicycle for 30 min, tries the stepper and the rowing machine, completes multiple sets on the different weights available, and finishes off with an aerobics class. The other individual begins with just a few minutes on the bicycle and then completes just one set of each of the different weights exercises using light weights. The first person in this example exhibits high intensity and the second person exhibits low intensity. If we stop at this point in the example, you may conclude that the first person is more motivated than the second. Your opinion may change when the third component of motivation—persistence—is considered. Two days later the first individual is home in bed, too sore to move (thinking that exercise is painful and should be avoided whenever possible). The second individual, however, is once again at the gym adding a couple of minutes to his time on the bicycle and sticking to his light-weights workout.
Exercise Participation Motivation
Exercise participation motivation refers primarily to the direction component of motivation. Exercise participation motivation is the initiation of exercise. A variety of factors influence whether individuals initiate an exercise program.
Knowledge, attitudes, and beliefs about exercise influence motivation toward exercise participation. Individuals who understand the importance and value of regular exercise are more likely to initiate an exercise program than those who do not. Similarly, if people have positive attitudes about the value and importance of regular exercise they will have greater motivation to participate in exercise than will people with negative attitudes.
Valuing the importance of exercise, however, is not the only determinant of exercise participation. Beliefs about ourselves influence motivation as well. Even if individuals understand that exercise is important, they will be unlikely to begin an exercise program if they believe that they cannot succeed at it. If they believe that the exercise program is too difficult or that it requires more fitness, strength, coordination, or time than they have, it is doubtful that they will join the program. This confidence in one's ability to succeed at an exercise program is called exercise self-efficacy. It is logical that self-efficacy would influence behaviour. How likely would people be to do something they were convinced they could not do, particularly if they had to pay to do it? How likely would they be to invest any energy in pursuing that activity? Most people in this situation would not attempt the activity. People who have such feelings are described as having low self-efficacy. As self-efficacy, or one's belief in one's ability to succeed at a particular task, increases, so does the likelihood of undertaking that task.
How can we enhance exercise participation motivation? Educating people about the importance and value of exercise can be a valuable first step because individuals who understand the merit of exercise are more likely to adopt an exercise program. Unfortunately, imparting knowledge is not enough. A lot of us do things that we know are not good for us and, similarly, do not do things that we know are good for us. Enhancing the exercise self-efficacy of individuals will increase their motivation. Demonstrating how individuals can control their own activity is useful. Some people have low self-efficacy about exercise because they believe that they are too unfit to begin exercising. They may equate exercise with young, thin, Lycra-clad gym enthusiasts whom they see in the media. Programs that emphasise choice of activities, illustrate exercisers similar in age and fitness level to the potential exercisers, and reveal that exercising can be enjoyable may increase exercise participation motivation. Exercise programs that begin with activities that the individuals already know they are capable of doing, such as walking and climbing stairs, may also increase self-efficacy and thereby increase motivation. Imagery may also help individuals enhance their self-efficacy beliefs (see “Imagery, Exercise, and Self-Efficacy”).
Exercise Adherence Motivation
Although many people get motivated to begin an exercise program, many of the people who begin fail to continue. Approximately 50% of individuals who begin a regular physical-activity program drop out within the first 6 mo. These people had exercise participation motivation, but they lack exercise adherence motivation, the persistence component of motivation.
Biological, psychological, sensory, and situational factors all interact to influence exercise adherence. Biologically, body composition, aerobic fitness, and the presence of disease influence adherence. Unfortunately, it is usually the people who could gain the most from exercise who are the least likely to adhere. Overweight or obese, unfit, or chronically ill people are less likely to adhere to an exercise program than are thinner, fitter, and healthier people.
As is the case with exercise participation motivation, attitudes and beliefs influence exercise adherence motivation. Attitudes and beliefs about the importance of exercise play a role in adherence, but so too do individuals' expectations about the effects that exercise is having on them personally. For example, if individuals believe that major changes in fitness and body composition should occur after 6 wk of regular exercise and they do not perceive major improvement in their own bodies after 6 wk, they may believe that exercise does not do what it should and therefore quit. Even though it is unrealistic to believe that 6 wk of exercise can make up for 6 yr of inactivity, it is individuals' beliefs, not reality, that influence behaviour. Therefore, when introducing newcomers to exercise it is important that they have realistic expectations about the time and effort required and the anticipated effects of the proposed program.
In addition to attitudes and beliefs, other psychological factors influence exercise adherence motivation. Extroverts (people who are social and outgoing) tend to adhere to exercise programs better than do introverts. Exercise programs that are executed in the presence of other people are probably more comfortable for extroverts than for introverts. Extroverts tend to enjoy the interaction with class members and exercise partners, possibly encouraging their adherence. Introverts, on the other hand, may adhere better to individual, home-based exercise programs. The bulk of the research on exercise adherence has involved programs that take place on site at fitness facilities with other people. This setting may have led to the conclusion that extroverts are better adherers than introverts. If the research had been done on independent, home-based exercise programs, it might have been found that introverts were better adherers. Clearly efforts need to be made to match the social environment of the exercise program to the personality of the exerciser.
Individuals with high levels of self-motivation are more likely to adhere to exercise programs than are individuals with low levels of self-motivation. It is logical that highly self-motivated people have better adherence rates. The challenge is to help those individuals with low levels of self-motivation. One of the most effective methods of helping these individuals is to encourage their involvement in the goal-setting process.
Goal Setting
Setting goals can help enhance motivation for a number of reasons. Goal setting addresses all three components of motivation. Goals give direction by providing a target. Intensity and effort also can be enhanced because goals provide reasons for participating in the activity. We are all more likely to put in effort when we feel there is a reason for doing so. If individuals are given two jobs to do at work, one that has a particular target and objective and one that seems vague and purposeless, into which job are they more likely to put their effort? Having goals helps focus attention and effort. In addition, goals can augment persistence by fostering new strategies. If individuals have a goal to which they are committed and initial tactics appear unsuccessful, they will search for alternative strategies to achieve their aim. If the goal had not been set in the first place, instead of persisting with different plans of action, they would likely give up.
Goals are also beneficial because they reflect improvement. Too often people make short-term comparisons regarding their strength, fitness, flexibility, or weight. Because the positive effects of exercise take time to emerge, improvements being made are often not noticed when people use a short timeframe for comparison. If a goal is achieved, evidence of improvement exists.
Goal setting involves a number of steps:
- setting the goal,
- setting a target date by which to achieve the goal,
- determining strategies to achieve the goal, and
- evaluating the goal on a regular basis.
If a target date is not set (e.g., “One of these days I'll ride the exercise bike continuously for an hour”), the goal is really just a dream. For this idea to be a goal, the individual needs to set a specific date by which to achieve the behaviour. Goal setting usually involves both long-term and short-term goals. The long-term goal provides direction; the short-term goals provide the increase in intensity and effort. People often err by setting only long-term goals. They begin to work toward achieving the goal, but success seems so far away that they give up before they get there. If someone had decided to ride a bike for 1 h and was currently having trouble lasting 10 min, 1 h would seem virtually impossible. Achieving short-term goals along the way to the long-term goal boosts confidence and motivation because it is obvious that the effort is worthwhile because improvement is being made. Target dates are set for each short-term goal in a progressive order until the long-term goal is achieved. This pattern of goal setting can be considered as a staircase, where each short-term goal is a step on the way to the long-term goal (see figure 20.1).
For goals to be effective, however, they need to be properly set. Goals can be considered to be good if they meet certain criteria (table 20.1). Goals need to be challenging but realistic. If goals are not challenging, they probably are not requiring any real change in behaviour and therefore will have little effect. However, if goals are so challenging that they are unrealistic, people are setting themselves up for failure. Continued failure leads to lowered confidence and less motivation.
Goals also need to be specific and measurable. Saying “I want to be fitter” or “My goal is to be stronger” does not provide any way of knowing when success has been achieved. What is “fitter”? How strong is “stronger”? There needs to be some way of knowing whether the goal has been achieved when the target date arrives. The easiest way of making goals specific is to make goals numerical. Numbers can easily be used to quantify time spent exercising, distance travelled, repetitions accomplished, weight lifted, or exercise sessions attended.
In addition to being specific and measurable, goals need to be positive. If an exerciser makes it her goal not to recline and rest during the abdominals section of her aerobics class, she will be thinking about reclining and resting. If instead she makes it her goal to complete first 1 min and then 2 min of the abdominal exercises, she will be thinking about doing the exercises, thus increasing her chances of doing them. If we tell you not to think about pink elephants, what is the first thing you think about? Similarly, if you set a goal of not letting your back arch off the bench when you do bench presses, you will be thinking about your back arching. When you think about your back arching, your brain may be sending messages to the muscles that make your back arch (just like in the psychoneuromuscular theory of imagery discussed in chapter 19). By making your goal “not arching,” you may actually be increasing the likelihood of arching your back. A goal of keeping your back pushed flat against the bench would be much more effective. Goals should stipulate the desired behaviour. Positively worded goals help you think about, plan for, and prepare to do what it is you want to do.
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Lifespan changes in skeletal system can lead to injury
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages.
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Age-Related Changes in the Skeletal and Articular Systems
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages. During the early fetal growth period there is rapid multiplication of cells (hyperplasia), whereas during the pubertal period growth is caused mainly by development and enlargement of existing cells (hypertrophy). Development can also occur via the replacement of one type of tissue by another.
Stages in the Development of Bone
Adult human bones appear in a variety of shapes and sizes. Their basic shape is genetically determined, but their final shape is influenced greatly by the environment in which they develop. Environmental influences include mechanical factors, such as muscle forces acting on the developing bone, and metabolic factors, which include the supply of nutrients.
Typical long bones, such as the humerus in the arm and the femur in the thigh, develop via a process of endochondral ossification in which a cartilaginous model precedes the bone formation. The cartilage is eventually replaced by bone. The cartilage model initially is small, and its replacement by bone begins at an early stage of development. The primary ossification centres appear near the middle of the shaft of the future long bone at about 8 wk after fertilisation, when the embryo is about 35 mm (1.4 in.) long (see figure 5.1).
The cartilage model of the future bone grows before it is replaced by bone. This replacement occurs in a series of stages. Initially, the cartilage model grows by two processes: an increase in the number of cartilage cells and then an increase in the size of each cell. Next, the gel-like matrix surrounding the cartilage cells is calcified. This hardening of the tissue surrounding the cells effectively cuts off their supply of nutrients and prevents the removal of metabolic waste products. These imprisoned cartilage cells eventually die so that the calcified cartilage has a honeycomb appearance. Invading blood vessels bring nutrients and bone-forming cells (osteoblasts) to the calcified cartilage to lay down new bone. This new bone has a disorganised appearance. If ossification were the final stage, a long bone would not develop its hollow structure. What happens instead is that some of the newly laid bone is removed from internal sites by special bone-eroding cells. These osteoclasts work with osteoblasts during remodelling of the bone, causing the cortex to drift away from the central axis of the shaft during growth. Changes in shaft diameter and compact bone thickness can occur at any age by a remodelling process. Bone can respond to changes in mechanical stimuli by changing its structure.
One type of remodelling that occurs is the replacement of the originally laid bone by compact bone that has an organised structure (figure 3.1). Most primary centres of ossification (in the developing shaft) appear before birth, whereas most secondary centres of ossification (in the developing ends of the bone) appear after birth. The times of appearance of these secondary centres of ossification vary widely. The centre in the femur near the knee is normally present at birth, but the centre in the clavicle (collarbone) closest to the sternum (breastbone) does not appear until about 18 yr of age.
Growth in Length and Width of Bone
When both the shaft and ends of a long bone are developing they are separated by a growth plate, or epiphyseal plate. In this region, all the stages of bone development can be seen under a microscope (see figure 5.2). Each zone in figure 5.2 represents a stage in the growth of bone and then replacement of cartilage by bone; that is, the zones illustrated do not migrate downward, but rather retain their position and change their nature. This effectively increases the distance between the growth plates at each end of the bone. The process illustrated continues for almost 20 yr. Growth in length of the long bones ceases when the process stops and epiphyseal plates fuse and disappear. Actually, the growth stops because the cartilage cells no longer respond to hormonal influence.
http://www.humankinetics.com/AcuCustom/Sitename/DAM/093/fig_5_Main.2.png
Although growth in height ceases at a certain age, changes in thickness of the compact bone of the shaft and the density of spongy bone can occur at any time of life (see chapter 6). Growth in the thickness and diameter of long bones occurs by appositional growth. In this process, bone is normally added on the outside of the shaft and removed from the inside of the shaft.
Skeletal Composition Changes Across the Life Span
The relative proportions of the inorganic salt crystals and the organic collagen change throughout the life span. In a child, the flexibility of the bone is related to the large proportion of collagen. It is thought that the rapid growth of children's bones and the consequent amount of remodelling results in less-than-optimal mineralisation. In a newborn baby, the skeleton accounts for about 13% of the total body weight, but about two-thirds of the skeleton is cartilaginous.
In bone of a young adult the inorganic material is close to the optimal two-thirds mineralisation, so the bone is strong and tough. Though the proportion of skeletal material in the mature adult is also about 13% of total body weight, just over 10% of the skeleton is cartilaginous. Of the bone, four-fifths is compact and one-fifth is spongy.
Eventually, aged bone becomes brittle. This change relates partly to the increased proportion of the inorganic salts, but of more significance is the reduced total mass of bone material in the aged musculoskeletal system. Bone mass decreases mainly because of a decrease in the number of trabeculae in spongy bone (see “In Focus: Why Trunk Height Decreases With Ageing”) combined with resorption and consequent thinning of the trabeculae. In compact bone, the size of the Haversian canals (see figure 3.1) increases and the cross-sectional area of the bone surrounding each Haversian canal decreases. This increased porosity of bone, in turn, causes decreased bone density (mass per unit volume of tissue). In an 80-yr-old male the density of bone in the spine is often 55% of the value found in a 20-yr-old; in females who are elderly, it is often only 40% of its peak value. With ageing, bone also becomes stiffer, partly because of the collagen cross-linking. A consequence of these mechanical changes is that aged bone can absorb less energy before it fractures, so it is more liable to fail when subjected to large forces.
Osteoporosis
For most tissues in the body, their mass at any time is determined by the balance between the simultaneous formation and destruction of the tissue. Osteopenia (“bone poverty”) is reduced bone density, particularly in women during the postmenopausal years, when the dramatic decrease in the female hormone estrogen allows the rate of synthesis to be lower than the rate of resorption. The initial effects are mainly to the spongy bone, but later the compact bone in the shafts of long bones also decreases in thickness.
According to the World Health Organization, osteoporosis (“bone porosity”) is present when the bone density is more than 2.5 standard deviations below the average bone mass of a young adult woman. In general, the porosity of bone in the human femur doubles from 40 to 80 yr of age and people with osteoporosis show an even greater change. Thus, osteoporotic bone is even less able to absorb energy before it fails than is normally aged bone. Moreover, even though a chemical analysis of osteoporotic bone material may indicate slightly increased levels of calcium per unit mass of bone, the overall mass of bone decreases so markedly that there is less calcium stored in the skeleton overall.
During maturation, the density and calcium content of bones is increased due to genetic predisposition aided by the interaction between dietary calcium and exercise. It is now thought that peak bone mass occurs at about 16 to 20 yr of age in women, so some health authorities claim osteoporosis is a pediatric disease and have promoted the idea of “bone banks” to help prevent osteoporosis. The idea is that if a person's lifestyle during maturation and early adulthood promoted maximum deposition of bone material, then later in life, when withdrawals of bone material are inevitable, the structural integrity of the bones will be maintained longer. Financial advisers tell everybody to start saving as young as possible, and this concept is even more important for prevention of osteoporosis.
Osteoporosis is commonly associated with women, but the prevalence among men is now the same as it was in women about 25 yr ago. This trend has been attributed to lifestyle changes, which include inadequate intake of calcium and less-than-optimal levels of physical activity. The personal lifestyle changes due to osteoporosis and the public health care costs of osteoporosis are so great that they are considered further in chapter 23.
Bone Failure in Relation to Bone Development, Age, or Activity
The modern automotive industry attempts to protect passengers by enclosing them in a cage that is lined by material that cushions the forces involved in a collision. In similar fashion, the soft tissues of the human body, including fat and muscle, perform the role of a crumple zone. However, despite this protection, sometimes the energy involved in a mishap is enough to cause a fracture anyway. Bone failure may be related to bone development, age, or activity.
Certain types of fractures are associated with particular stages of development. The flexible bones of a child tend to splinter in a manner similar to the broken branch of a growing tree; this type of fracture is called a greenstick fracture. Late in life the fracture is more likely to be of the brittle type because of the increased stiffness of old bones. The bones of adults who are elderly are also more likely to fail because of the decreased bone mass and density in osteoporosis.
Specific fractures are related to certain ages. For example, fractures of the neck of the femur (commonly called hip fractures) are common in women in the elderly population. Incidence of forearm fracture increases near the peak of the pubescent growth spurt. This is associated with increased porosity of compact bone as the remodelling space increases to provide calcium to the rapid-growth regions. A second increase in wrist fractures is seen in women who are elderly; this is related to the combination of osteoporosis and increased incidence of falls. In developing children, the cartilaginous growth plate may also fail; this most often occurs at the zone of maturation, or hypertrophy (see figure 5.2).
The development of bone has implications for potential injury to the musculoskeletal system. For example, injury to the cartilaginous growth plates can be produced by single excessive forces causing trauma or by repetitive compressive forces, or tensile forces produced by muscle-tendon pull on an area of developing bone.
In sports medicine, specific injuries have been attributed to certain activities. One of the best examples is Little League elbow, described in more detail in chapter 24. Other examples of the relationships between activity and types of growth-plate problems are wrist injuries in gymnasts and tibial tuberosity avulsions in sports involving sprinting and jumping. Pain over the tibial tuberosity (just below the knee cap) in a prepubescent child is indicative of Osgood-Schlatter's condition.
Effects of Various Factors on Range of Motion
Range of motion at joints is affected by joint structure and the mechanical properties of the tissues associated with the joints. A general perception is that joint range of motion decreases during life. Although this is the general trend, the rate of loss is not constant. Ranges of joint motion are very large in a newborn baby. As an example, the range of ankle dorsiflexion in a newborn is limited only by the contact of the top of the foot against the shin. Try this movement yourself to indicate how much flexibility you have lost since birth.
Between the ages of 6 and 15 yr, there is a general trend for joint range of motion to decrease in boys, whereas for girls the effects are variable and joint dependent. Girls are generally more flexible than boys during childhood and adolescence. Changes in joint ranges of motion between adolescence and young adulthood are variable and, as discussed in chapter 6, appear to be related to physical activity. Similarly, the general trend for joint flexibility to decrease during ageing may not be completely explained by biological ageing processes.
Ageing people may suffer arthritis (“inflammation of joints”), which markedly restricts range of motion. Rheumatoid arthritis involves inflammation of the synovial membrane, while osteoarthritis involves degeneration of the articular cartilage (see figure 3.6). In the total adult population, about three times as many women have osteoarthritis as do men. Thus there is some genetic determination of osteoarthritis, to which are added the effects of environmental factors. It would appear that the risk factors for rheumatoid arthritis are mainly related to a family history of the condition.
Age-Related Changes in the Muscular System
Mesoderm, which forms toward the end of the third week of development, eventually forms the muscle and connective tissues of the body. Some of the mesoderm is segmented such that it is in lumps on either side of the midline, and these lumps form the muscles of the trunk. Other mesoderm forms the muscles of the limbs; this tissue migrates into the limbs when they first appear. Limb muscles migrate toward their final position during further development.
Certain precursor cells differentiate into myoblasts (muscle-forming cells) that fuse longitudinally to form the long muscles and muscle fibres we can observe. The number of muscle fibres appears to be genetically determined so that large people often have more muscle fibres than persons who are smaller in stature. As muscles grow in length, the number of sarcomeres increases. Although direct evidence is sparse, many believe that the stimulus for growth of muscle is the bone growth that pulls on the attached ends of the muscle. Developmental growth occurs in both muscle length and cross-sectional area, and these factors may also be affected by activity. The effects of physical training on muscle is summarised in chapters 6, 10, and 13.
In an infant, muscle tissue accounts for about 25% of total body weight, but in a young adult the proportion of muscle is more than 40%. In terms of growth, muscle increases from about 850 g (1.9 lb) at birth to about 30 kg (66 lb) in a young adult male weighing 70 kg (154 lb).
Strength, which may be defined as the capacity to produce force against an external resistance, is related to the cross-sectional areas of the muscles producing the force. Muscle volume peaks at about 30 yr of age and then gradually decreases. The atrophy of skeletal muscle associated with disuse is much faster than any ageing-related atrophy. Muscle elasticity also decreases during ageing so that muscles are stiffer and less extensible. This is one contributing factor to the loss of joint range of motion described earlier.
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Training can increase both muscle strength and fiber contraction speed
Adaptations are common following injury and surgical treatment.
Biophysical Foundations of Human Movement, Third Edition.
Biomechanical Adaptations to Injury
Adaptations are common following injury and surgical treatment. One interesting example found in the orthopaedics literature is a phenomenon known as quadriceps avoidance gait. This particular adaptation occurs in ACL-deficient (ACLD) patients; that is, patients who have suffered a complete rupture of the ACL. ACLD patients usually have difficulty with movements involving either lateral thrusts (e.g., cutting from side to side) or twisting (i.e., rotation of the femur relative to the tibia in the transverse plane).
Gait analysis has been done on ACLD patients during various activities of daily living, including level walking, jogging, walking up and down stairs, and even running, cutting, and pivoting. Kinematic and force-plate data were used to estimate the net muscle moments exerted at the knee in all three planes of movement: flexion-extension, abduction-
adduction, and internal-external rotation. Interestingly, the greatest functional changes between ACLD patients and healthy subjects occurred while walking at preferred speeds on level ground. During the midstance portion of the gait cycle, the ACLD patients exhibited a net extensor moment that was significantly lower than normal (figure 10.9, solid line). One interpretation of these findings is that the ACLD patients walk by reducing the demand on their quadriceps during stance (i.e., the quadriceps muscles are activated less in the ACL-deficient patients, giving rise to a quadriceps-avoidance gait pattern).
It is important to realise that a lower net extensor moment at the knee does not necessarily mean that the moment exerted by quadriceps is lower. Because the net moment at the knee is a combination of quadriceps and hamstrings muscle action, a lower extensor moment could result from an increase in the flexor moment applied by hamstrings. However, muscle electromyography results tend to support the interpretation that quadriceps moment is lower because the activation level of the quadriceps is lower in the ACLD patients.
Gait analysis results have also shown that the ACLD patients reduce their net extensor moment by 25% during jogging compared with a 100% reduction in walking. However, the net extensor moment in stair climbing was the same in ACLD patients and normal subjects. When taken together, these results suggest that the observed quadriceps avoidance in ACLD patients is not a result of quadriceps weakness but rather is due to a change in neuromuscular control.
Dependence of Motor Performance on Changes in Muscle Properties
Strength and quickness are critical determinants of performance in explosive movements such as jumping, throwing, and sprinting. Together, these two factors determine the amount of power developed instantaneously during a task (i.e., muscle power is given by the product of muscle force and muscle contraction speed; see “In Focus: At What Speed Must a Muscle Shorten to Develop Maximum Power?”). As discussed earlier in this chapter, resistance strength training can alter both muscle force and fibre contraction speed. This section describes the effects that changes in the contractile properties of muscle have on motor performance, specifically, how biomechanical performance depends on muscle strength and muscle fibre contraction speed.
Vertical jumping is one of the most heavily studied motor tasks. Many scientists have studied jumping with the aim of learning more about the biomechanics and control of this task and, specifically, how performance may be influenced by training. In a recent jump-training program that incorporated stretching, plyometric exercises, and weight lifting, female high school volleyball players demonstrated a mean increase of 4 cm (1.5 in.) in jump height after training. This represented an almost 10% increase in jump height as a result of the 6 wk period of training. Even larger increases in vertical jumping performance have been documented in the literature. One extreme example is the 1984 United States Olympic gold medal volleyball team, which showed a 10 cm (4 in.) increase in jump height after 2 yr of jump training.
Using Computer Modelling to Study Vertical Jumping Performance
It is gratifying to learn that jumping performance can be increased significantly by strength and neuromuscular training. However, it is difficult, if not impossible, to explain why these increases occur. Noninvasive measurements of biomechanical performance cannot pinpoint the factor or factors responsible for the increase in jump height. The reason is that several properties of the neuromuscular and musculoskeletal systems change simultaneously during the training regimen. Alternatively, computer models may be used to study the relationships between training effects and performance; that is, a model of the neuromusculoskeletal system may be used to predict how changes to specific parameters affect the performance of a motor
task.
Models of the body similar to the one shown in figure 8.7 have been used to study how changes in muscle strength, muscle contraction speed, and motor unit recruitment affect jump height. The values of each of these parameters in the model were increased by amounts consistent with results obtained from strength-training programs. For example, the peak isometric strength and maximum shortening velocity of each muscle in the model were increased by 20% and the activation level of each muscle was increased by 10%. The three training effects were first applied to all muscles simultaneously and then to the ankle plantarflexors, knee extensors, and hip extensors separately. In this way, the modelling results were used to determine whether it is better to train all the leg muscles simultaneously or to isolate specific muscle groups such as the quadriceps (knee extensors) and gluteus maximus (hip
extensors).
Insights Into the Effects of Training Provided by Computer Models
Increasing the peak isometric force of all the leg muscles in the model by 20% produced the largest increase in jump height. In this case, jump height increased by 7 cm, or 5% of that obtained before the simulated training effect. Increasing the maximum shortening velocity of each muscle by 20% or the activation level of each muscle by 10% increased jump height by only 4 cm, or 3% of that obtained for the nominal (untrained) model. When all three training effects were introduced simultaneously, jumping performance increased by nearly 17 cm, or 12% of that calculated for the untrained model. Thus, training programs that increase strength, fibre contraction speed, and motor recruitment (i.e., activation level) of all the leg muscles simultaneously are most beneficial to overall jumping performance.
The modelling results also suggest that training the knee extensors is better than training either the ankle plantarflexors or the hip extensors. When peak isometric force and maximum shortening velocity of the quadriceps were increased by 20% and quadriceps activation was simultaneously increased by 10%, jump height increased by almost 10 cm, or 7% of the value calculated for the untrained model. The same changes made to either the ankle plantarflexors or the hip extensors produced increases in jump height of only 3 cm, or 2% of the value obtained for the untrained model.
The latter result is a little puzzling because the model calculations had previously shown that the quadriceps and gluteus maximus are the major energy producers—the prime movers—of the body in vertical jumping (see “In Focus: Which Muscles Are Most Important to Vertical Jumping Performance?” in chapter 8). It should be noted here that each time a change was introduced to the model, a new optimal pattern of muscle activations was found by re-solving the optimisation problem for a maximum-height jump. Thus, one interpretation of the result is that what matters most in terms of performance is the quadriceps:gluteus maximus muscle strength ratio and not the absolute strength of these muscles. In other words, jumping performance is most sensitive to a change in the knee-extensor:hip-extensor muscle strength ratio. Increasing quadriceps strength by 20% increases the knee-extensor:hip-extensor muscle strength ratio in the model, whereas increasing gluteus maximus muscle strength decreases this ratio. The same line of reasoning may be used to explain why increasing ankle plantarflexor muscle strength by 20% leads to an increase in jump height of just 3 cm in the model.
Training for strength is also better than training for quickness or speed. Figure 10.10 shows the effects of increasing muscle strength and muscle contraction speed on vertical jump height as predicted by the four-segment, eight-muscle, sagittal-plane model of the body described in chapter 8 (see figure 8.7). An increase in muscle strength was simulated in the model by simultaneously increasing body weight because muscle strength increases in proportion to muscle mass. Thus, changes in body strength:weight ratio are represented in figure 10.10 rather than changes in muscle strength alone. Also, jumping performance is normalised in these results by dividing by the value of jump height calculated for the untrained model. Similarly, body strength:weight ratio and muscle contraction speed are each normalised by dividing by the value of body strength:weight ratio and muscle contraction speed in the untrained model, respectively.
The simulation results show that the slope of the line predicted for changes in body strength:weight ratio is twice as large as that obtained when changes in muscle fibre contraction speed are made (compare solid and dashed lines in figure 10.10). Thus, muscle strength has a greater effect on vertical jump height than muscle fibre contraction speed, even when the accompanying increase in body mass is taken into account.
Another important lesson learned from the modelling studies is that musculoskeletal changes must be accompanied by appropriate changes in neuromuscular control; otherwise, the expected improvement in motor performance will not be seen. In vertical jumping, if the pattern of muscle activations remains unchanged subsequent to strength training, jump height actually decreases relative to the untrained state. Figure 10.11 shows a simulated jump in which the strength of the knee-extensor muscles has been increased by 20% but the control exerted over the joints is the same as that calculated for the untrained model before the training effect was introduced. When the pattern of muscle activations was not optimised to match the changes introduced to the neuromusculoskeletal model, the body left the ground prematurely (i.e., the centre of mass was at a lower height at liftoff than is optimal for a maximal jump).
One consequence of not optimising the pattern of muscle activations (i.e., the controls) is that a larger fraction of the total work produced by the muscles goes into rotating the body segments rather than accelerating the centre of mass upward. In a maximum-height jump, approximately 90% of the total work done by the leg muscles is used to propel the centre of mass upward. This number is closer to 80% when muscle coordination (i.e., the sequence and timing of muscle activations) is not optimal. So, even though jumping performance depends heavily on muscle strength, and to a lesser extent on muscle fibre contraction speed, optimal performance is also intimately related to neuromuscular control. For this reason, and as discussed at the beginning of this chapter, jump-training programs now focus on maneuvers that blend muscle strength with neural control.
Summary
Training can change both muscle strength and fibre contraction speed. Strength may be increased by increasing motor unit recruitment (net neural drive to the muscle) or by increasing muscle fibre size. Contraction speed may be altered by changing the shape of a muscle's force-velocity curve or by changing the value of its intrinsic maximum shortening velocity. Early in a training program (2-8 wk), increases in muscle strength are brought by neural adaptations rather than by increases in muscle size. Training can also change the way muscle action is coordinated during activity (i.e., neuromuscular control). Neuromuscular training programs are usually designed to improve stability (balance and coordination) and proprioception (joint position sense) in addition to muscle strength. Training is vital for improving biomechanical performance and for preventing injuries during sport.
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The three components of motivation affect exercise adherence
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
Effects of Psychological Factors on Exercise
The three components of motivation—direction, intensity, and persistence—mentioned in chapter 19 in relation to sport apply to exercise motivation as well. Direction clearly relates to whether an individual chooses the direction of the gym or the couch, the stairs or the elevator, or the pool or the bath. Once the direction of exercise has been chosen, intensity and persistence become important. For example, as a New Year's resolution two individuals may have both decided to join a gym. They have both chosen the direction of exercise. During their first day at the gym, one rides the bicycle for 30 min, tries the stepper and the rowing machine, completes multiple sets on the different weights available, and finishes off with an aerobics class. The other individual begins with just a few minutes on the bicycle and then completes just one set of each of the different weights exercises using light weights. The first person in this example exhibits high intensity and the second person exhibits low intensity. If we stop at this point in the example, you may conclude that the first person is more motivated than the second. Your opinion may change when the third component of motivation—persistence—is considered. Two days later the first individual is home in bed, too sore to move (thinking that exercise is painful and should be avoided whenever possible). The second individual, however, is once again at the gym adding a couple of minutes to his time on the bicycle and sticking to his light-weights workout.
Exercise Participation Motivation
Exercise participation motivation refers primarily to the direction component of motivation. Exercise participation motivation is the initiation of exercise. A variety of factors influence whether individuals initiate an exercise program.
Knowledge, attitudes, and beliefs about exercise influence motivation toward exercise participation. Individuals who understand the importance and value of regular exercise are more likely to initiate an exercise program than those who do not. Similarly, if people have positive attitudes about the value and importance of regular exercise they will have greater motivation to participate in exercise than will people with negative attitudes.
Valuing the importance of exercise, however, is not the only determinant of exercise participation. Beliefs about ourselves influence motivation as well. Even if individuals understand that exercise is important, they will be unlikely to begin an exercise program if they believe that they cannot succeed at it. If they believe that the exercise program is too difficult or that it requires more fitness, strength, coordination, or time than they have, it is doubtful that they will join the program. This confidence in one's ability to succeed at an exercise program is called exercise self-efficacy. It is logical that self-efficacy would influence behaviour. How likely would people be to do something they were convinced they could not do, particularly if they had to pay to do it? How likely would they be to invest any energy in pursuing that activity? Most people in this situation would not attempt the activity. People who have such feelings are described as having low self-efficacy. As self-efficacy, or one's belief in one's ability to succeed at a particular task, increases, so does the likelihood of undertaking that task.
How can we enhance exercise participation motivation? Educating people about the importance and value of exercise can be a valuable first step because individuals who understand the merit of exercise are more likely to adopt an exercise program. Unfortunately, imparting knowledge is not enough. A lot of us do things that we know are not good for us and, similarly, do not do things that we know are good for us. Enhancing the exercise self-efficacy of individuals will increase their motivation. Demonstrating how individuals can control their own activity is useful. Some people have low self-efficacy about exercise because they believe that they are too unfit to begin exercising. They may equate exercise with young, thin, Lycra-clad gym enthusiasts whom they see in the media. Programs that emphasise choice of activities, illustrate exercisers similar in age and fitness level to the potential exercisers, and reveal that exercising can be enjoyable may increase exercise participation motivation. Exercise programs that begin with activities that the individuals already know they are capable of doing, such as walking and climbing stairs, may also increase self-efficacy and thereby increase motivation. Imagery may also help individuals enhance their self-efficacy beliefs (see “Imagery, Exercise, and Self-Efficacy”).
Exercise Adherence Motivation
Although many people get motivated to begin an exercise program, many of the people who begin fail to continue. Approximately 50% of individuals who begin a regular physical-activity program drop out within the first 6 mo. These people had exercise participation motivation, but they lack exercise adherence motivation, the persistence component of motivation.
Biological, psychological, sensory, and situational factors all interact to influence exercise adherence. Biologically, body composition, aerobic fitness, and the presence of disease influence adherence. Unfortunately, it is usually the people who could gain the most from exercise who are the least likely to adhere. Overweight or obese, unfit, or chronically ill people are less likely to adhere to an exercise program than are thinner, fitter, and healthier people.
As is the case with exercise participation motivation, attitudes and beliefs influence exercise adherence motivation. Attitudes and beliefs about the importance of exercise play a role in adherence, but so too do individuals' expectations about the effects that exercise is having on them personally. For example, if individuals believe that major changes in fitness and body composition should occur after 6 wk of regular exercise and they do not perceive major improvement in their own bodies after 6 wk, they may believe that exercise does not do what it should and therefore quit. Even though it is unrealistic to believe that 6 wk of exercise can make up for 6 yr of inactivity, it is individuals' beliefs, not reality, that influence behaviour. Therefore, when introducing newcomers to exercise it is important that they have realistic expectations about the time and effort required and the anticipated effects of the proposed program.
In addition to attitudes and beliefs, other psychological factors influence exercise adherence motivation. Extroverts (people who are social and outgoing) tend to adhere to exercise programs better than do introverts. Exercise programs that are executed in the presence of other people are probably more comfortable for extroverts than for introverts. Extroverts tend to enjoy the interaction with class members and exercise partners, possibly encouraging their adherence. Introverts, on the other hand, may adhere better to individual, home-based exercise programs. The bulk of the research on exercise adherence has involved programs that take place on site at fitness facilities with other people. This setting may have led to the conclusion that extroverts are better adherers than introverts. If the research had been done on independent, home-based exercise programs, it might have been found that introverts were better adherers. Clearly efforts need to be made to match the social environment of the exercise program to the personality of the exerciser.
Individuals with high levels of self-motivation are more likely to adhere to exercise programs than are individuals with low levels of self-motivation. It is logical that highly self-motivated people have better adherence rates. The challenge is to help those individuals with low levels of self-motivation. One of the most effective methods of helping these individuals is to encourage their involvement in the goal-setting process.
Goal Setting
Setting goals can help enhance motivation for a number of reasons. Goal setting addresses all three components of motivation. Goals give direction by providing a target. Intensity and effort also can be enhanced because goals provide reasons for participating in the activity. We are all more likely to put in effort when we feel there is a reason for doing so. If individuals are given two jobs to do at work, one that has a particular target and objective and one that seems vague and purposeless, into which job are they more likely to put their effort? Having goals helps focus attention and effort. In addition, goals can augment persistence by fostering new strategies. If individuals have a goal to which they are committed and initial tactics appear unsuccessful, they will search for alternative strategies to achieve their aim. If the goal had not been set in the first place, instead of persisting with different plans of action, they would likely give up.
Goals are also beneficial because they reflect improvement. Too often people make short-term comparisons regarding their strength, fitness, flexibility, or weight. Because the positive effects of exercise take time to emerge, improvements being made are often not noticed when people use a short timeframe for comparison. If a goal is achieved, evidence of improvement exists.
Goal setting involves a number of steps:
- setting the goal,
- setting a target date by which to achieve the goal,
- determining strategies to achieve the goal, and
- evaluating the goal on a regular basis.
If a target date is not set (e.g., “One of these days I'll ride the exercise bike continuously for an hour”), the goal is really just a dream. For this idea to be a goal, the individual needs to set a specific date by which to achieve the behaviour. Goal setting usually involves both long-term and short-term goals. The long-term goal provides direction; the short-term goals provide the increase in intensity and effort. People often err by setting only long-term goals. They begin to work toward achieving the goal, but success seems so far away that they give up before they get there. If someone had decided to ride a bike for 1 h and was currently having trouble lasting 10 min, 1 h would seem virtually impossible. Achieving short-term goals along the way to the long-term goal boosts confidence and motivation because it is obvious that the effort is worthwhile because improvement is being made. Target dates are set for each short-term goal in a progressive order until the long-term goal is achieved. This pattern of goal setting can be considered as a staircase, where each short-term goal is a step on the way to the long-term goal (see figure 20.1).
For goals to be effective, however, they need to be properly set. Goals can be considered to be good if they meet certain criteria (table 20.1). Goals need to be challenging but realistic. If goals are not challenging, they probably are not requiring any real change in behaviour and therefore will have little effect. However, if goals are so challenging that they are unrealistic, people are setting themselves up for failure. Continued failure leads to lowered confidence and less motivation.
Goals also need to be specific and measurable. Saying “I want to be fitter” or “My goal is to be stronger” does not provide any way of knowing when success has been achieved. What is “fitter”? How strong is “stronger”? There needs to be some way of knowing whether the goal has been achieved when the target date arrives. The easiest way of making goals specific is to make goals numerical. Numbers can easily be used to quantify time spent exercising, distance travelled, repetitions accomplished, weight lifted, or exercise sessions attended.
In addition to being specific and measurable, goals need to be positive. If an exerciser makes it her goal not to recline and rest during the abdominals section of her aerobics class, she will be thinking about reclining and resting. If instead she makes it her goal to complete first 1 min and then 2 min of the abdominal exercises, she will be thinking about doing the exercises, thus increasing her chances of doing them. If we tell you not to think about pink elephants, what is the first thing you think about? Similarly, if you set a goal of not letting your back arch off the bench when you do bench presses, you will be thinking about your back arching. When you think about your back arching, your brain may be sending messages to the muscles that make your back arch (just like in the psychoneuromuscular theory of imagery discussed in chapter 19). By making your goal “not arching,” you may actually be increasing the likelihood of arching your back. A goal of keeping your back pushed flat against the bench would be much more effective. Goals should stipulate the desired behaviour. Positively worded goals help you think about, plan for, and prepare to do what it is you want to do.
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Lifespan changes in skeletal system can lead to injury
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages.
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Age-Related Changes in the Skeletal and Articular Systems
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages. During the early fetal growth period there is rapid multiplication of cells (hyperplasia), whereas during the pubertal period growth is caused mainly by development and enlargement of existing cells (hypertrophy). Development can also occur via the replacement of one type of tissue by another.
Stages in the Development of Bone
Adult human bones appear in a variety of shapes and sizes. Their basic shape is genetically determined, but their final shape is influenced greatly by the environment in which they develop. Environmental influences include mechanical factors, such as muscle forces acting on the developing bone, and metabolic factors, which include the supply of nutrients.
Typical long bones, such as the humerus in the arm and the femur in the thigh, develop via a process of endochondral ossification in which a cartilaginous model precedes the bone formation. The cartilage is eventually replaced by bone. The cartilage model initially is small, and its replacement by bone begins at an early stage of development. The primary ossification centres appear near the middle of the shaft of the future long bone at about 8 wk after fertilisation, when the embryo is about 35 mm (1.4 in.) long (see figure 5.1).
The cartilage model of the future bone grows before it is replaced by bone. This replacement occurs in a series of stages. Initially, the cartilage model grows by two processes: an increase in the number of cartilage cells and then an increase in the size of each cell. Next, the gel-like matrix surrounding the cartilage cells is calcified. This hardening of the tissue surrounding the cells effectively cuts off their supply of nutrients and prevents the removal of metabolic waste products. These imprisoned cartilage cells eventually die so that the calcified cartilage has a honeycomb appearance. Invading blood vessels bring nutrients and bone-forming cells (osteoblasts) to the calcified cartilage to lay down new bone. This new bone has a disorganised appearance. If ossification were the final stage, a long bone would not develop its hollow structure. What happens instead is that some of the newly laid bone is removed from internal sites by special bone-eroding cells. These osteoclasts work with osteoblasts during remodelling of the bone, causing the cortex to drift away from the central axis of the shaft during growth. Changes in shaft diameter and compact bone thickness can occur at any age by a remodelling process. Bone can respond to changes in mechanical stimuli by changing its structure.
One type of remodelling that occurs is the replacement of the originally laid bone by compact bone that has an organised structure (figure 3.1). Most primary centres of ossification (in the developing shaft) appear before birth, whereas most secondary centres of ossification (in the developing ends of the bone) appear after birth. The times of appearance of these secondary centres of ossification vary widely. The centre in the femur near the knee is normally present at birth, but the centre in the clavicle (collarbone) closest to the sternum (breastbone) does not appear until about 18 yr of age.
Growth in Length and Width of Bone
When both the shaft and ends of a long bone are developing they are separated by a growth plate, or epiphyseal plate. In this region, all the stages of bone development can be seen under a microscope (see figure 5.2). Each zone in figure 5.2 represents a stage in the growth of bone and then replacement of cartilage by bone; that is, the zones illustrated do not migrate downward, but rather retain their position and change their nature. This effectively increases the distance between the growth plates at each end of the bone. The process illustrated continues for almost 20 yr. Growth in length of the long bones ceases when the process stops and epiphyseal plates fuse and disappear. Actually, the growth stops because the cartilage cells no longer respond to hormonal influence.
http://www.humankinetics.com/AcuCustom/Sitename/DAM/093/fig_5_Main.2.png
Although growth in height ceases at a certain age, changes in thickness of the compact bone of the shaft and the density of spongy bone can occur at any time of life (see chapter 6). Growth in the thickness and diameter of long bones occurs by appositional growth. In this process, bone is normally added on the outside of the shaft and removed from the inside of the shaft.
Skeletal Composition Changes Across the Life Span
The relative proportions of the inorganic salt crystals and the organic collagen change throughout the life span. In a child, the flexibility of the bone is related to the large proportion of collagen. It is thought that the rapid growth of children's bones and the consequent amount of remodelling results in less-than-optimal mineralisation. In a newborn baby, the skeleton accounts for about 13% of the total body weight, but about two-thirds of the skeleton is cartilaginous.
In bone of a young adult the inorganic material is close to the optimal two-thirds mineralisation, so the bone is strong and tough. Though the proportion of skeletal material in the mature adult is also about 13% of total body weight, just over 10% of the skeleton is cartilaginous. Of the bone, four-fifths is compact and one-fifth is spongy.
Eventually, aged bone becomes brittle. This change relates partly to the increased proportion of the inorganic salts, but of more significance is the reduced total mass of bone material in the aged musculoskeletal system. Bone mass decreases mainly because of a decrease in the number of trabeculae in spongy bone (see “In Focus: Why Trunk Height Decreases With Ageing”) combined with resorption and consequent thinning of the trabeculae. In compact bone, the size of the Haversian canals (see figure 3.1) increases and the cross-sectional area of the bone surrounding each Haversian canal decreases. This increased porosity of bone, in turn, causes decreased bone density (mass per unit volume of tissue). In an 80-yr-old male the density of bone in the spine is often 55% of the value found in a 20-yr-old; in females who are elderly, it is often only 40% of its peak value. With ageing, bone also becomes stiffer, partly because of the collagen cross-linking. A consequence of these mechanical changes is that aged bone can absorb less energy before it fractures, so it is more liable to fail when subjected to large forces.
Osteoporosis
For most tissues in the body, their mass at any time is determined by the balance between the simultaneous formation and destruction of the tissue. Osteopenia (“bone poverty”) is reduced bone density, particularly in women during the postmenopausal years, when the dramatic decrease in the female hormone estrogen allows the rate of synthesis to be lower than the rate of resorption. The initial effects are mainly to the spongy bone, but later the compact bone in the shafts of long bones also decreases in thickness.
According to the World Health Organization, osteoporosis (“bone porosity”) is present when the bone density is more than 2.5 standard deviations below the average bone mass of a young adult woman. In general, the porosity of bone in the human femur doubles from 40 to 80 yr of age and people with osteoporosis show an even greater change. Thus, osteoporotic bone is even less able to absorb energy before it fails than is normally aged bone. Moreover, even though a chemical analysis of osteoporotic bone material may indicate slightly increased levels of calcium per unit mass of bone, the overall mass of bone decreases so markedly that there is less calcium stored in the skeleton overall.
During maturation, the density and calcium content of bones is increased due to genetic predisposition aided by the interaction between dietary calcium and exercise. It is now thought that peak bone mass occurs at about 16 to 20 yr of age in women, so some health authorities claim osteoporosis is a pediatric disease and have promoted the idea of “bone banks” to help prevent osteoporosis. The idea is that if a person's lifestyle during maturation and early adulthood promoted maximum deposition of bone material, then later in life, when withdrawals of bone material are inevitable, the structural integrity of the bones will be maintained longer. Financial advisers tell everybody to start saving as young as possible, and this concept is even more important for prevention of osteoporosis.
Osteoporosis is commonly associated with women, but the prevalence among men is now the same as it was in women about 25 yr ago. This trend has been attributed to lifestyle changes, which include inadequate intake of calcium and less-than-optimal levels of physical activity. The personal lifestyle changes due to osteoporosis and the public health care costs of osteoporosis are so great that they are considered further in chapter 23.
Bone Failure in Relation to Bone Development, Age, or Activity
The modern automotive industry attempts to protect passengers by enclosing them in a cage that is lined by material that cushions the forces involved in a collision. In similar fashion, the soft tissues of the human body, including fat and muscle, perform the role of a crumple zone. However, despite this protection, sometimes the energy involved in a mishap is enough to cause a fracture anyway. Bone failure may be related to bone development, age, or activity.
Certain types of fractures are associated with particular stages of development. The flexible bones of a child tend to splinter in a manner similar to the broken branch of a growing tree; this type of fracture is called a greenstick fracture. Late in life the fracture is more likely to be of the brittle type because of the increased stiffness of old bones. The bones of adults who are elderly are also more likely to fail because of the decreased bone mass and density in osteoporosis.
Specific fractures are related to certain ages. For example, fractures of the neck of the femur (commonly called hip fractures) are common in women in the elderly population. Incidence of forearm fracture increases near the peak of the pubescent growth spurt. This is associated with increased porosity of compact bone as the remodelling space increases to provide calcium to the rapid-growth regions. A second increase in wrist fractures is seen in women who are elderly; this is related to the combination of osteoporosis and increased incidence of falls. In developing children, the cartilaginous growth plate may also fail; this most often occurs at the zone of maturation, or hypertrophy (see figure 5.2).
The development of bone has implications for potential injury to the musculoskeletal system. For example, injury to the cartilaginous growth plates can be produced by single excessive forces causing trauma or by repetitive compressive forces, or tensile forces produced by muscle-tendon pull on an area of developing bone.
In sports medicine, specific injuries have been attributed to certain activities. One of the best examples is Little League elbow, described in more detail in chapter 24. Other examples of the relationships between activity and types of growth-plate problems are wrist injuries in gymnasts and tibial tuberosity avulsions in sports involving sprinting and jumping. Pain over the tibial tuberosity (just below the knee cap) in a prepubescent child is indicative of Osgood-Schlatter's condition.
Effects of Various Factors on Range of Motion
Range of motion at joints is affected by joint structure and the mechanical properties of the tissues associated with the joints. A general perception is that joint range of motion decreases during life. Although this is the general trend, the rate of loss is not constant. Ranges of joint motion are very large in a newborn baby. As an example, the range of ankle dorsiflexion in a newborn is limited only by the contact of the top of the foot against the shin. Try this movement yourself to indicate how much flexibility you have lost since birth.
Between the ages of 6 and 15 yr, there is a general trend for joint range of motion to decrease in boys, whereas for girls the effects are variable and joint dependent. Girls are generally more flexible than boys during childhood and adolescence. Changes in joint ranges of motion between adolescence and young adulthood are variable and, as discussed in chapter 6, appear to be related to physical activity. Similarly, the general trend for joint flexibility to decrease during ageing may not be completely explained by biological ageing processes.
Ageing people may suffer arthritis (“inflammation of joints”), which markedly restricts range of motion. Rheumatoid arthritis involves inflammation of the synovial membrane, while osteoarthritis involves degeneration of the articular cartilage (see figure 3.6). In the total adult population, about three times as many women have osteoarthritis as do men. Thus there is some genetic determination of osteoarthritis, to which are added the effects of environmental factors. It would appear that the risk factors for rheumatoid arthritis are mainly related to a family history of the condition.
Age-Related Changes in the Muscular System
Mesoderm, which forms toward the end of the third week of development, eventually forms the muscle and connective tissues of the body. Some of the mesoderm is segmented such that it is in lumps on either side of the midline, and these lumps form the muscles of the trunk. Other mesoderm forms the muscles of the limbs; this tissue migrates into the limbs when they first appear. Limb muscles migrate toward their final position during further development.
Certain precursor cells differentiate into myoblasts (muscle-forming cells) that fuse longitudinally to form the long muscles and muscle fibres we can observe. The number of muscle fibres appears to be genetically determined so that large people often have more muscle fibres than persons who are smaller in stature. As muscles grow in length, the number of sarcomeres increases. Although direct evidence is sparse, many believe that the stimulus for growth of muscle is the bone growth that pulls on the attached ends of the muscle. Developmental growth occurs in both muscle length and cross-sectional area, and these factors may also be affected by activity. The effects of physical training on muscle is summarised in chapters 6, 10, and 13.
In an infant, muscle tissue accounts for about 25% of total body weight, but in a young adult the proportion of muscle is more than 40%. In terms of growth, muscle increases from about 850 g (1.9 lb) at birth to about 30 kg (66 lb) in a young adult male weighing 70 kg (154 lb).
Strength, which may be defined as the capacity to produce force against an external resistance, is related to the cross-sectional areas of the muscles producing the force. Muscle volume peaks at about 30 yr of age and then gradually decreases. The atrophy of skeletal muscle associated with disuse is much faster than any ageing-related atrophy. Muscle elasticity also decreases during ageing so that muscles are stiffer and less extensible. This is one contributing factor to the loss of joint range of motion described earlier.
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Training can increase both muscle strength and fiber contraction speed
Adaptations are common following injury and surgical treatment.
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Biomechanical Adaptations to Injury
Adaptations are common following injury and surgical treatment. One interesting example found in the orthopaedics literature is a phenomenon known as quadriceps avoidance gait. This particular adaptation occurs in ACL-deficient (ACLD) patients; that is, patients who have suffered a complete rupture of the ACL. ACLD patients usually have difficulty with movements involving either lateral thrusts (e.g., cutting from side to side) or twisting (i.e., rotation of the femur relative to the tibia in the transverse plane).
Gait analysis has been done on ACLD patients during various activities of daily living, including level walking, jogging, walking up and down stairs, and even running, cutting, and pivoting. Kinematic and force-plate data were used to estimate the net muscle moments exerted at the knee in all three planes of movement: flexion-extension, abduction-
adduction, and internal-external rotation. Interestingly, the greatest functional changes between ACLD patients and healthy subjects occurred while walking at preferred speeds on level ground. During the midstance portion of the gait cycle, the ACLD patients exhibited a net extensor moment that was significantly lower than normal (figure 10.9, solid line). One interpretation of these findings is that the ACLD patients walk by reducing the demand on their quadriceps during stance (i.e., the quadriceps muscles are activated less in the ACL-deficient patients, giving rise to a quadriceps-avoidance gait pattern).
It is important to realise that a lower net extensor moment at the knee does not necessarily mean that the moment exerted by quadriceps is lower. Because the net moment at the knee is a combination of quadriceps and hamstrings muscle action, a lower extensor moment could result from an increase in the flexor moment applied by hamstrings. However, muscle electromyography results tend to support the interpretation that quadriceps moment is lower because the activation level of the quadriceps is lower in the ACLD patients.
Gait analysis results have also shown that the ACLD patients reduce their net extensor moment by 25% during jogging compared with a 100% reduction in walking. However, the net extensor moment in stair climbing was the same in ACLD patients and normal subjects. When taken together, these results suggest that the observed quadriceps avoidance in ACLD patients is not a result of quadriceps weakness but rather is due to a change in neuromuscular control.
Dependence of Motor Performance on Changes in Muscle Properties
Strength and quickness are critical determinants of performance in explosive movements such as jumping, throwing, and sprinting. Together, these two factors determine the amount of power developed instantaneously during a task (i.e., muscle power is given by the product of muscle force and muscle contraction speed; see “In Focus: At What Speed Must a Muscle Shorten to Develop Maximum Power?”). As discussed earlier in this chapter, resistance strength training can alter both muscle force and fibre contraction speed. This section describes the effects that changes in the contractile properties of muscle have on motor performance, specifically, how biomechanical performance depends on muscle strength and muscle fibre contraction speed.
Vertical jumping is one of the most heavily studied motor tasks. Many scientists have studied jumping with the aim of learning more about the biomechanics and control of this task and, specifically, how performance may be influenced by training. In a recent jump-training program that incorporated stretching, plyometric exercises, and weight lifting, female high school volleyball players demonstrated a mean increase of 4 cm (1.5 in.) in jump height after training. This represented an almost 10% increase in jump height as a result of the 6 wk period of training. Even larger increases in vertical jumping performance have been documented in the literature. One extreme example is the 1984 United States Olympic gold medal volleyball team, which showed a 10 cm (4 in.) increase in jump height after 2 yr of jump training.
Using Computer Modelling to Study Vertical Jumping Performance
It is gratifying to learn that jumping performance can be increased significantly by strength and neuromuscular training. However, it is difficult, if not impossible, to explain why these increases occur. Noninvasive measurements of biomechanical performance cannot pinpoint the factor or factors responsible for the increase in jump height. The reason is that several properties of the neuromuscular and musculoskeletal systems change simultaneously during the training regimen. Alternatively, computer models may be used to study the relationships between training effects and performance; that is, a model of the neuromusculoskeletal system may be used to predict how changes to specific parameters affect the performance of a motor
task.
Models of the body similar to the one shown in figure 8.7 have been used to study how changes in muscle strength, muscle contraction speed, and motor unit recruitment affect jump height. The values of each of these parameters in the model were increased by amounts consistent with results obtained from strength-training programs. For example, the peak isometric strength and maximum shortening velocity of each muscle in the model were increased by 20% and the activation level of each muscle was increased by 10%. The three training effects were first applied to all muscles simultaneously and then to the ankle plantarflexors, knee extensors, and hip extensors separately. In this way, the modelling results were used to determine whether it is better to train all the leg muscles simultaneously or to isolate specific muscle groups such as the quadriceps (knee extensors) and gluteus maximus (hip
extensors).
Insights Into the Effects of Training Provided by Computer Models
Increasing the peak isometric force of all the leg muscles in the model by 20% produced the largest increase in jump height. In this case, jump height increased by 7 cm, or 5% of that obtained before the simulated training effect. Increasing the maximum shortening velocity of each muscle by 20% or the activation level of each muscle by 10% increased jump height by only 4 cm, or 3% of that obtained for the nominal (untrained) model. When all three training effects were introduced simultaneously, jumping performance increased by nearly 17 cm, or 12% of that calculated for the untrained model. Thus, training programs that increase strength, fibre contraction speed, and motor recruitment (i.e., activation level) of all the leg muscles simultaneously are most beneficial to overall jumping performance.
The modelling results also suggest that training the knee extensors is better than training either the ankle plantarflexors or the hip extensors. When peak isometric force and maximum shortening velocity of the quadriceps were increased by 20% and quadriceps activation was simultaneously increased by 10%, jump height increased by almost 10 cm, or 7% of the value calculated for the untrained model. The same changes made to either the ankle plantarflexors or the hip extensors produced increases in jump height of only 3 cm, or 2% of the value obtained for the untrained model.
The latter result is a little puzzling because the model calculations had previously shown that the quadriceps and gluteus maximus are the major energy producers—the prime movers—of the body in vertical jumping (see “In Focus: Which Muscles Are Most Important to Vertical Jumping Performance?” in chapter 8). It should be noted here that each time a change was introduced to the model, a new optimal pattern of muscle activations was found by re-solving the optimisation problem for a maximum-height jump. Thus, one interpretation of the result is that what matters most in terms of performance is the quadriceps:gluteus maximus muscle strength ratio and not the absolute strength of these muscles. In other words, jumping performance is most sensitive to a change in the knee-extensor:hip-extensor muscle strength ratio. Increasing quadriceps strength by 20% increases the knee-extensor:hip-extensor muscle strength ratio in the model, whereas increasing gluteus maximus muscle strength decreases this ratio. The same line of reasoning may be used to explain why increasing ankle plantarflexor muscle strength by 20% leads to an increase in jump height of just 3 cm in the model.
Training for strength is also better than training for quickness or speed. Figure 10.10 shows the effects of increasing muscle strength and muscle contraction speed on vertical jump height as predicted by the four-segment, eight-muscle, sagittal-plane model of the body described in chapter 8 (see figure 8.7). An increase in muscle strength was simulated in the model by simultaneously increasing body weight because muscle strength increases in proportion to muscle mass. Thus, changes in body strength:weight ratio are represented in figure 10.10 rather than changes in muscle strength alone. Also, jumping performance is normalised in these results by dividing by the value of jump height calculated for the untrained model. Similarly, body strength:weight ratio and muscle contraction speed are each normalised by dividing by the value of body strength:weight ratio and muscle contraction speed in the untrained model, respectively.
The simulation results show that the slope of the line predicted for changes in body strength:weight ratio is twice as large as that obtained when changes in muscle fibre contraction speed are made (compare solid and dashed lines in figure 10.10). Thus, muscle strength has a greater effect on vertical jump height than muscle fibre contraction speed, even when the accompanying increase in body mass is taken into account.
Another important lesson learned from the modelling studies is that musculoskeletal changes must be accompanied by appropriate changes in neuromuscular control; otherwise, the expected improvement in motor performance will not be seen. In vertical jumping, if the pattern of muscle activations remains unchanged subsequent to strength training, jump height actually decreases relative to the untrained state. Figure 10.11 shows a simulated jump in which the strength of the knee-extensor muscles has been increased by 20% but the control exerted over the joints is the same as that calculated for the untrained model before the training effect was introduced. When the pattern of muscle activations was not optimised to match the changes introduced to the neuromusculoskeletal model, the body left the ground prematurely (i.e., the centre of mass was at a lower height at liftoff than is optimal for a maximal jump).
One consequence of not optimising the pattern of muscle activations (i.e., the controls) is that a larger fraction of the total work produced by the muscles goes into rotating the body segments rather than accelerating the centre of mass upward. In a maximum-height jump, approximately 90% of the total work done by the leg muscles is used to propel the centre of mass upward. This number is closer to 80% when muscle coordination (i.e., the sequence and timing of muscle activations) is not optimal. So, even though jumping performance depends heavily on muscle strength, and to a lesser extent on muscle fibre contraction speed, optimal performance is also intimately related to neuromuscular control. For this reason, and as discussed at the beginning of this chapter, jump-training programs now focus on maneuvers that blend muscle strength with neural control.
Summary
Training can change both muscle strength and fibre contraction speed. Strength may be increased by increasing motor unit recruitment (net neural drive to the muscle) or by increasing muscle fibre size. Contraction speed may be altered by changing the shape of a muscle's force-velocity curve or by changing the value of its intrinsic maximum shortening velocity. Early in a training program (2-8 wk), increases in muscle strength are brought by neural adaptations rather than by increases in muscle size. Training can also change the way muscle action is coordinated during activity (i.e., neuromuscular control). Neuromuscular training programs are usually designed to improve stability (balance and coordination) and proprioception (joint position sense) in addition to muscle strength. Training is vital for improving biomechanical performance and for preventing injuries during sport.
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The three components of motivation affect exercise adherence
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
Effects of Psychological Factors on Exercise
The three components of motivation—direction, intensity, and persistence—mentioned in chapter 19 in relation to sport apply to exercise motivation as well. Direction clearly relates to whether an individual chooses the direction of the gym or the couch, the stairs or the elevator, or the pool or the bath. Once the direction of exercise has been chosen, intensity and persistence become important. For example, as a New Year's resolution two individuals may have both decided to join a gym. They have both chosen the direction of exercise. During their first day at the gym, one rides the bicycle for 30 min, tries the stepper and the rowing machine, completes multiple sets on the different weights available, and finishes off with an aerobics class. The other individual begins with just a few minutes on the bicycle and then completes just one set of each of the different weights exercises using light weights. The first person in this example exhibits high intensity and the second person exhibits low intensity. If we stop at this point in the example, you may conclude that the first person is more motivated than the second. Your opinion may change when the third component of motivation—persistence—is considered. Two days later the first individual is home in bed, too sore to move (thinking that exercise is painful and should be avoided whenever possible). The second individual, however, is once again at the gym adding a couple of minutes to his time on the bicycle and sticking to his light-weights workout.
Exercise Participation Motivation
Exercise participation motivation refers primarily to the direction component of motivation. Exercise participation motivation is the initiation of exercise. A variety of factors influence whether individuals initiate an exercise program.
Knowledge, attitudes, and beliefs about exercise influence motivation toward exercise participation. Individuals who understand the importance and value of regular exercise are more likely to initiate an exercise program than those who do not. Similarly, if people have positive attitudes about the value and importance of regular exercise they will have greater motivation to participate in exercise than will people with negative attitudes.
Valuing the importance of exercise, however, is not the only determinant of exercise participation. Beliefs about ourselves influence motivation as well. Even if individuals understand that exercise is important, they will be unlikely to begin an exercise program if they believe that they cannot succeed at it. If they believe that the exercise program is too difficult or that it requires more fitness, strength, coordination, or time than they have, it is doubtful that they will join the program. This confidence in one's ability to succeed at an exercise program is called exercise self-efficacy. It is logical that self-efficacy would influence behaviour. How likely would people be to do something they were convinced they could not do, particularly if they had to pay to do it? How likely would they be to invest any energy in pursuing that activity? Most people in this situation would not attempt the activity. People who have such feelings are described as having low self-efficacy. As self-efficacy, or one's belief in one's ability to succeed at a particular task, increases, so does the likelihood of undertaking that task.
How can we enhance exercise participation motivation? Educating people about the importance and value of exercise can be a valuable first step because individuals who understand the merit of exercise are more likely to adopt an exercise program. Unfortunately, imparting knowledge is not enough. A lot of us do things that we know are not good for us and, similarly, do not do things that we know are good for us. Enhancing the exercise self-efficacy of individuals will increase their motivation. Demonstrating how individuals can control their own activity is useful. Some people have low self-efficacy about exercise because they believe that they are too unfit to begin exercising. They may equate exercise with young, thin, Lycra-clad gym enthusiasts whom they see in the media. Programs that emphasise choice of activities, illustrate exercisers similar in age and fitness level to the potential exercisers, and reveal that exercising can be enjoyable may increase exercise participation motivation. Exercise programs that begin with activities that the individuals already know they are capable of doing, such as walking and climbing stairs, may also increase self-efficacy and thereby increase motivation. Imagery may also help individuals enhance their self-efficacy beliefs (see “Imagery, Exercise, and Self-Efficacy”).
Exercise Adherence Motivation
Although many people get motivated to begin an exercise program, many of the people who begin fail to continue. Approximately 50% of individuals who begin a regular physical-activity program drop out within the first 6 mo. These people had exercise participation motivation, but they lack exercise adherence motivation, the persistence component of motivation.
Biological, psychological, sensory, and situational factors all interact to influence exercise adherence. Biologically, body composition, aerobic fitness, and the presence of disease influence adherence. Unfortunately, it is usually the people who could gain the most from exercise who are the least likely to adhere. Overweight or obese, unfit, or chronically ill people are less likely to adhere to an exercise program than are thinner, fitter, and healthier people.
As is the case with exercise participation motivation, attitudes and beliefs influence exercise adherence motivation. Attitudes and beliefs about the importance of exercise play a role in adherence, but so too do individuals' expectations about the effects that exercise is having on them personally. For example, if individuals believe that major changes in fitness and body composition should occur after 6 wk of regular exercise and they do not perceive major improvement in their own bodies after 6 wk, they may believe that exercise does not do what it should and therefore quit. Even though it is unrealistic to believe that 6 wk of exercise can make up for 6 yr of inactivity, it is individuals' beliefs, not reality, that influence behaviour. Therefore, when introducing newcomers to exercise it is important that they have realistic expectations about the time and effort required and the anticipated effects of the proposed program.
In addition to attitudes and beliefs, other psychological factors influence exercise adherence motivation. Extroverts (people who are social and outgoing) tend to adhere to exercise programs better than do introverts. Exercise programs that are executed in the presence of other people are probably more comfortable for extroverts than for introverts. Extroverts tend to enjoy the interaction with class members and exercise partners, possibly encouraging their adherence. Introverts, on the other hand, may adhere better to individual, home-based exercise programs. The bulk of the research on exercise adherence has involved programs that take place on site at fitness facilities with other people. This setting may have led to the conclusion that extroverts are better adherers than introverts. If the research had been done on independent, home-based exercise programs, it might have been found that introverts were better adherers. Clearly efforts need to be made to match the social environment of the exercise program to the personality of the exerciser.
Individuals with high levels of self-motivation are more likely to adhere to exercise programs than are individuals with low levels of self-motivation. It is logical that highly self-motivated people have better adherence rates. The challenge is to help those individuals with low levels of self-motivation. One of the most effective methods of helping these individuals is to encourage their involvement in the goal-setting process.
Goal Setting
Setting goals can help enhance motivation for a number of reasons. Goal setting addresses all three components of motivation. Goals give direction by providing a target. Intensity and effort also can be enhanced because goals provide reasons for participating in the activity. We are all more likely to put in effort when we feel there is a reason for doing so. If individuals are given two jobs to do at work, one that has a particular target and objective and one that seems vague and purposeless, into which job are they more likely to put their effort? Having goals helps focus attention and effort. In addition, goals can augment persistence by fostering new strategies. If individuals have a goal to which they are committed and initial tactics appear unsuccessful, they will search for alternative strategies to achieve their aim. If the goal had not been set in the first place, instead of persisting with different plans of action, they would likely give up.
Goals are also beneficial because they reflect improvement. Too often people make short-term comparisons regarding their strength, fitness, flexibility, or weight. Because the positive effects of exercise take time to emerge, improvements being made are often not noticed when people use a short timeframe for comparison. If a goal is achieved, evidence of improvement exists.
Goal setting involves a number of steps:
- setting the goal,
- setting a target date by which to achieve the goal,
- determining strategies to achieve the goal, and
- evaluating the goal on a regular basis.
If a target date is not set (e.g., “One of these days I'll ride the exercise bike continuously for an hour”), the goal is really just a dream. For this idea to be a goal, the individual needs to set a specific date by which to achieve the behaviour. Goal setting usually involves both long-term and short-term goals. The long-term goal provides direction; the short-term goals provide the increase in intensity and effort. People often err by setting only long-term goals. They begin to work toward achieving the goal, but success seems so far away that they give up before they get there. If someone had decided to ride a bike for 1 h and was currently having trouble lasting 10 min, 1 h would seem virtually impossible. Achieving short-term goals along the way to the long-term goal boosts confidence and motivation because it is obvious that the effort is worthwhile because improvement is being made. Target dates are set for each short-term goal in a progressive order until the long-term goal is achieved. This pattern of goal setting can be considered as a staircase, where each short-term goal is a step on the way to the long-term goal (see figure 20.1).
For goals to be effective, however, they need to be properly set. Goals can be considered to be good if they meet certain criteria (table 20.1). Goals need to be challenging but realistic. If goals are not challenging, they probably are not requiring any real change in behaviour and therefore will have little effect. However, if goals are so challenging that they are unrealistic, people are setting themselves up for failure. Continued failure leads to lowered confidence and less motivation.
Goals also need to be specific and measurable. Saying “I want to be fitter” or “My goal is to be stronger” does not provide any way of knowing when success has been achieved. What is “fitter”? How strong is “stronger”? There needs to be some way of knowing whether the goal has been achieved when the target date arrives. The easiest way of making goals specific is to make goals numerical. Numbers can easily be used to quantify time spent exercising, distance travelled, repetitions accomplished, weight lifted, or exercise sessions attended.
In addition to being specific and measurable, goals need to be positive. If an exerciser makes it her goal not to recline and rest during the abdominals section of her aerobics class, she will be thinking about reclining and resting. If instead she makes it her goal to complete first 1 min and then 2 min of the abdominal exercises, she will be thinking about doing the exercises, thus increasing her chances of doing them. If we tell you not to think about pink elephants, what is the first thing you think about? Similarly, if you set a goal of not letting your back arch off the bench when you do bench presses, you will be thinking about your back arching. When you think about your back arching, your brain may be sending messages to the muscles that make your back arch (just like in the psychoneuromuscular theory of imagery discussed in chapter 19). By making your goal “not arching,” you may actually be increasing the likelihood of arching your back. A goal of keeping your back pushed flat against the bench would be much more effective. Goals should stipulate the desired behaviour. Positively worded goals help you think about, plan for, and prepare to do what it is you want to do.
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Lifespan changes in skeletal system can lead to injury
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages.
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Age-Related Changes in the Skeletal and Articular Systems
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages. During the early fetal growth period there is rapid multiplication of cells (hyperplasia), whereas during the pubertal period growth is caused mainly by development and enlargement of existing cells (hypertrophy). Development can also occur via the replacement of one type of tissue by another.
Stages in the Development of Bone
Adult human bones appear in a variety of shapes and sizes. Their basic shape is genetically determined, but their final shape is influenced greatly by the environment in which they develop. Environmental influences include mechanical factors, such as muscle forces acting on the developing bone, and metabolic factors, which include the supply of nutrients.
Typical long bones, such as the humerus in the arm and the femur in the thigh, develop via a process of endochondral ossification in which a cartilaginous model precedes the bone formation. The cartilage is eventually replaced by bone. The cartilage model initially is small, and its replacement by bone begins at an early stage of development. The primary ossification centres appear near the middle of the shaft of the future long bone at about 8 wk after fertilisation, when the embryo is about 35 mm (1.4 in.) long (see figure 5.1).
The cartilage model of the future bone grows before it is replaced by bone. This replacement occurs in a series of stages. Initially, the cartilage model grows by two processes: an increase in the number of cartilage cells and then an increase in the size of each cell. Next, the gel-like matrix surrounding the cartilage cells is calcified. This hardening of the tissue surrounding the cells effectively cuts off their supply of nutrients and prevents the removal of metabolic waste products. These imprisoned cartilage cells eventually die so that the calcified cartilage has a honeycomb appearance. Invading blood vessels bring nutrients and bone-forming cells (osteoblasts) to the calcified cartilage to lay down new bone. This new bone has a disorganised appearance. If ossification were the final stage, a long bone would not develop its hollow structure. What happens instead is that some of the newly laid bone is removed from internal sites by special bone-eroding cells. These osteoclasts work with osteoblasts during remodelling of the bone, causing the cortex to drift away from the central axis of the shaft during growth. Changes in shaft diameter and compact bone thickness can occur at any age by a remodelling process. Bone can respond to changes in mechanical stimuli by changing its structure.
One type of remodelling that occurs is the replacement of the originally laid bone by compact bone that has an organised structure (figure 3.1). Most primary centres of ossification (in the developing shaft) appear before birth, whereas most secondary centres of ossification (in the developing ends of the bone) appear after birth. The times of appearance of these secondary centres of ossification vary widely. The centre in the femur near the knee is normally present at birth, but the centre in the clavicle (collarbone) closest to the sternum (breastbone) does not appear until about 18 yr of age.
Growth in Length and Width of Bone
When both the shaft and ends of a long bone are developing they are separated by a growth plate, or epiphyseal plate. In this region, all the stages of bone development can be seen under a microscope (see figure 5.2). Each zone in figure 5.2 represents a stage in the growth of bone and then replacement of cartilage by bone; that is, the zones illustrated do not migrate downward, but rather retain their position and change their nature. This effectively increases the distance between the growth plates at each end of the bone. The process illustrated continues for almost 20 yr. Growth in length of the long bones ceases when the process stops and epiphyseal plates fuse and disappear. Actually, the growth stops because the cartilage cells no longer respond to hormonal influence.
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Although growth in height ceases at a certain age, changes in thickness of the compact bone of the shaft and the density of spongy bone can occur at any time of life (see chapter 6). Growth in the thickness and diameter of long bones occurs by appositional growth. In this process, bone is normally added on the outside of the shaft and removed from the inside of the shaft.
Skeletal Composition Changes Across the Life Span
The relative proportions of the inorganic salt crystals and the organic collagen change throughout the life span. In a child, the flexibility of the bone is related to the large proportion of collagen. It is thought that the rapid growth of children's bones and the consequent amount of remodelling results in less-than-optimal mineralisation. In a newborn baby, the skeleton accounts for about 13% of the total body weight, but about two-thirds of the skeleton is cartilaginous.
In bone of a young adult the inorganic material is close to the optimal two-thirds mineralisation, so the bone is strong and tough. Though the proportion of skeletal material in the mature adult is also about 13% of total body weight, just over 10% of the skeleton is cartilaginous. Of the bone, four-fifths is compact and one-fifth is spongy.
Eventually, aged bone becomes brittle. This change relates partly to the increased proportion of the inorganic salts, but of more significance is the reduced total mass of bone material in the aged musculoskeletal system. Bone mass decreases mainly because of a decrease in the number of trabeculae in spongy bone (see “In Focus: Why Trunk Height Decreases With Ageing”) combined with resorption and consequent thinning of the trabeculae. In compact bone, the size of the Haversian canals (see figure 3.1) increases and the cross-sectional area of the bone surrounding each Haversian canal decreases. This increased porosity of bone, in turn, causes decreased bone density (mass per unit volume of tissue). In an 80-yr-old male the density of bone in the spine is often 55% of the value found in a 20-yr-old; in females who are elderly, it is often only 40% of its peak value. With ageing, bone also becomes stiffer, partly because of the collagen cross-linking. A consequence of these mechanical changes is that aged bone can absorb less energy before it fractures, so it is more liable to fail when subjected to large forces.
Osteoporosis
For most tissues in the body, their mass at any time is determined by the balance between the simultaneous formation and destruction of the tissue. Osteopenia (“bone poverty”) is reduced bone density, particularly in women during the postmenopausal years, when the dramatic decrease in the female hormone estrogen allows the rate of synthesis to be lower than the rate of resorption. The initial effects are mainly to the spongy bone, but later the compact bone in the shafts of long bones also decreases in thickness.
According to the World Health Organization, osteoporosis (“bone porosity”) is present when the bone density is more than 2.5 standard deviations below the average bone mass of a young adult woman. In general, the porosity of bone in the human femur doubles from 40 to 80 yr of age and people with osteoporosis show an even greater change. Thus, osteoporotic bone is even less able to absorb energy before it fails than is normally aged bone. Moreover, even though a chemical analysis of osteoporotic bone material may indicate slightly increased levels of calcium per unit mass of bone, the overall mass of bone decreases so markedly that there is less calcium stored in the skeleton overall.
During maturation, the density and calcium content of bones is increased due to genetic predisposition aided by the interaction between dietary calcium and exercise. It is now thought that peak bone mass occurs at about 16 to 20 yr of age in women, so some health authorities claim osteoporosis is a pediatric disease and have promoted the idea of “bone banks” to help prevent osteoporosis. The idea is that if a person's lifestyle during maturation and early adulthood promoted maximum deposition of bone material, then later in life, when withdrawals of bone material are inevitable, the structural integrity of the bones will be maintained longer. Financial advisers tell everybody to start saving as young as possible, and this concept is even more important for prevention of osteoporosis.
Osteoporosis is commonly associated with women, but the prevalence among men is now the same as it was in women about 25 yr ago. This trend has been attributed to lifestyle changes, which include inadequate intake of calcium and less-than-optimal levels of physical activity. The personal lifestyle changes due to osteoporosis and the public health care costs of osteoporosis are so great that they are considered further in chapter 23.
Bone Failure in Relation to Bone Development, Age, or Activity
The modern automotive industry attempts to protect passengers by enclosing them in a cage that is lined by material that cushions the forces involved in a collision. In similar fashion, the soft tissues of the human body, including fat and muscle, perform the role of a crumple zone. However, despite this protection, sometimes the energy involved in a mishap is enough to cause a fracture anyway. Bone failure may be related to bone development, age, or activity.
Certain types of fractures are associated with particular stages of development. The flexible bones of a child tend to splinter in a manner similar to the broken branch of a growing tree; this type of fracture is called a greenstick fracture. Late in life the fracture is more likely to be of the brittle type because of the increased stiffness of old bones. The bones of adults who are elderly are also more likely to fail because of the decreased bone mass and density in osteoporosis.
Specific fractures are related to certain ages. For example, fractures of the neck of the femur (commonly called hip fractures) are common in women in the elderly population. Incidence of forearm fracture increases near the peak of the pubescent growth spurt. This is associated with increased porosity of compact bone as the remodelling space increases to provide calcium to the rapid-growth regions. A second increase in wrist fractures is seen in women who are elderly; this is related to the combination of osteoporosis and increased incidence of falls. In developing children, the cartilaginous growth plate may also fail; this most often occurs at the zone of maturation, or hypertrophy (see figure 5.2).
The development of bone has implications for potential injury to the musculoskeletal system. For example, injury to the cartilaginous growth plates can be produced by single excessive forces causing trauma or by repetitive compressive forces, or tensile forces produced by muscle-tendon pull on an area of developing bone.
In sports medicine, specific injuries have been attributed to certain activities. One of the best examples is Little League elbow, described in more detail in chapter 24. Other examples of the relationships between activity and types of growth-plate problems are wrist injuries in gymnasts and tibial tuberosity avulsions in sports involving sprinting and jumping. Pain over the tibial tuberosity (just below the knee cap) in a prepubescent child is indicative of Osgood-Schlatter's condition.
Effects of Various Factors on Range of Motion
Range of motion at joints is affected by joint structure and the mechanical properties of the tissues associated with the joints. A general perception is that joint range of motion decreases during life. Although this is the general trend, the rate of loss is not constant. Ranges of joint motion are very large in a newborn baby. As an example, the range of ankle dorsiflexion in a newborn is limited only by the contact of the top of the foot against the shin. Try this movement yourself to indicate how much flexibility you have lost since birth.
Between the ages of 6 and 15 yr, there is a general trend for joint range of motion to decrease in boys, whereas for girls the effects are variable and joint dependent. Girls are generally more flexible than boys during childhood and adolescence. Changes in joint ranges of motion between adolescence and young adulthood are variable and, as discussed in chapter 6, appear to be related to physical activity. Similarly, the general trend for joint flexibility to decrease during ageing may not be completely explained by biological ageing processes.
Ageing people may suffer arthritis (“inflammation of joints”), which markedly restricts range of motion. Rheumatoid arthritis involves inflammation of the synovial membrane, while osteoarthritis involves degeneration of the articular cartilage (see figure 3.6). In the total adult population, about three times as many women have osteoarthritis as do men. Thus there is some genetic determination of osteoarthritis, to which are added the effects of environmental factors. It would appear that the risk factors for rheumatoid arthritis are mainly related to a family history of the condition.
Age-Related Changes in the Muscular System
Mesoderm, which forms toward the end of the third week of development, eventually forms the muscle and connective tissues of the body. Some of the mesoderm is segmented such that it is in lumps on either side of the midline, and these lumps form the muscles of the trunk. Other mesoderm forms the muscles of the limbs; this tissue migrates into the limbs when they first appear. Limb muscles migrate toward their final position during further development.
Certain precursor cells differentiate into myoblasts (muscle-forming cells) that fuse longitudinally to form the long muscles and muscle fibres we can observe. The number of muscle fibres appears to be genetically determined so that large people often have more muscle fibres than persons who are smaller in stature. As muscles grow in length, the number of sarcomeres increases. Although direct evidence is sparse, many believe that the stimulus for growth of muscle is the bone growth that pulls on the attached ends of the muscle. Developmental growth occurs in both muscle length and cross-sectional area, and these factors may also be affected by activity. The effects of physical training on muscle is summarised in chapters 6, 10, and 13.
In an infant, muscle tissue accounts for about 25% of total body weight, but in a young adult the proportion of muscle is more than 40%. In terms of growth, muscle increases from about 850 g (1.9 lb) at birth to about 30 kg (66 lb) in a young adult male weighing 70 kg (154 lb).
Strength, which may be defined as the capacity to produce force against an external resistance, is related to the cross-sectional areas of the muscles producing the force. Muscle volume peaks at about 30 yr of age and then gradually decreases. The atrophy of skeletal muscle associated with disuse is much faster than any ageing-related atrophy. Muscle elasticity also decreases during ageing so that muscles are stiffer and less extensible. This is one contributing factor to the loss of joint range of motion described earlier.
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Training can increase both muscle strength and fiber contraction speed
Adaptations are common following injury and surgical treatment.
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Biomechanical Adaptations to Injury
Adaptations are common following injury and surgical treatment. One interesting example found in the orthopaedics literature is a phenomenon known as quadriceps avoidance gait. This particular adaptation occurs in ACL-deficient (ACLD) patients; that is, patients who have suffered a complete rupture of the ACL. ACLD patients usually have difficulty with movements involving either lateral thrusts (e.g., cutting from side to side) or twisting (i.e., rotation of the femur relative to the tibia in the transverse plane).
Gait analysis has been done on ACLD patients during various activities of daily living, including level walking, jogging, walking up and down stairs, and even running, cutting, and pivoting. Kinematic and force-plate data were used to estimate the net muscle moments exerted at the knee in all three planes of movement: flexion-extension, abduction-
adduction, and internal-external rotation. Interestingly, the greatest functional changes between ACLD patients and healthy subjects occurred while walking at preferred speeds on level ground. During the midstance portion of the gait cycle, the ACLD patients exhibited a net extensor moment that was significantly lower than normal (figure 10.9, solid line). One interpretation of these findings is that the ACLD patients walk by reducing the demand on their quadriceps during stance (i.e., the quadriceps muscles are activated less in the ACL-deficient patients, giving rise to a quadriceps-avoidance gait pattern).
It is important to realise that a lower net extensor moment at the knee does not necessarily mean that the moment exerted by quadriceps is lower. Because the net moment at the knee is a combination of quadriceps and hamstrings muscle action, a lower extensor moment could result from an increase in the flexor moment applied by hamstrings. However, muscle electromyography results tend to support the interpretation that quadriceps moment is lower because the activation level of the quadriceps is lower in the ACLD patients.
Gait analysis results have also shown that the ACLD patients reduce their net extensor moment by 25% during jogging compared with a 100% reduction in walking. However, the net extensor moment in stair climbing was the same in ACLD patients and normal subjects. When taken together, these results suggest that the observed quadriceps avoidance in ACLD patients is not a result of quadriceps weakness but rather is due to a change in neuromuscular control.
Dependence of Motor Performance on Changes in Muscle Properties
Strength and quickness are critical determinants of performance in explosive movements such as jumping, throwing, and sprinting. Together, these two factors determine the amount of power developed instantaneously during a task (i.e., muscle power is given by the product of muscle force and muscle contraction speed; see “In Focus: At What Speed Must a Muscle Shorten to Develop Maximum Power?”). As discussed earlier in this chapter, resistance strength training can alter both muscle force and fibre contraction speed. This section describes the effects that changes in the contractile properties of muscle have on motor performance, specifically, how biomechanical performance depends on muscle strength and muscle fibre contraction speed.
Vertical jumping is one of the most heavily studied motor tasks. Many scientists have studied jumping with the aim of learning more about the biomechanics and control of this task and, specifically, how performance may be influenced by training. In a recent jump-training program that incorporated stretching, plyometric exercises, and weight lifting, female high school volleyball players demonstrated a mean increase of 4 cm (1.5 in.) in jump height after training. This represented an almost 10% increase in jump height as a result of the 6 wk period of training. Even larger increases in vertical jumping performance have been documented in the literature. One extreme example is the 1984 United States Olympic gold medal volleyball team, which showed a 10 cm (4 in.) increase in jump height after 2 yr of jump training.
Using Computer Modelling to Study Vertical Jumping Performance
It is gratifying to learn that jumping performance can be increased significantly by strength and neuromuscular training. However, it is difficult, if not impossible, to explain why these increases occur. Noninvasive measurements of biomechanical performance cannot pinpoint the factor or factors responsible for the increase in jump height. The reason is that several properties of the neuromuscular and musculoskeletal systems change simultaneously during the training regimen. Alternatively, computer models may be used to study the relationships between training effects and performance; that is, a model of the neuromusculoskeletal system may be used to predict how changes to specific parameters affect the performance of a motor
task.
Models of the body similar to the one shown in figure 8.7 have been used to study how changes in muscle strength, muscle contraction speed, and motor unit recruitment affect jump height. The values of each of these parameters in the model were increased by amounts consistent with results obtained from strength-training programs. For example, the peak isometric strength and maximum shortening velocity of each muscle in the model were increased by 20% and the activation level of each muscle was increased by 10%. The three training effects were first applied to all muscles simultaneously and then to the ankle plantarflexors, knee extensors, and hip extensors separately. In this way, the modelling results were used to determine whether it is better to train all the leg muscles simultaneously or to isolate specific muscle groups such as the quadriceps (knee extensors) and gluteus maximus (hip
extensors).
Insights Into the Effects of Training Provided by Computer Models
Increasing the peak isometric force of all the leg muscles in the model by 20% produced the largest increase in jump height. In this case, jump height increased by 7 cm, or 5% of that obtained before the simulated training effect. Increasing the maximum shortening velocity of each muscle by 20% or the activation level of each muscle by 10% increased jump height by only 4 cm, or 3% of that obtained for the nominal (untrained) model. When all three training effects were introduced simultaneously, jumping performance increased by nearly 17 cm, or 12% of that calculated for the untrained model. Thus, training programs that increase strength, fibre contraction speed, and motor recruitment (i.e., activation level) of all the leg muscles simultaneously are most beneficial to overall jumping performance.
The modelling results also suggest that training the knee extensors is better than training either the ankle plantarflexors or the hip extensors. When peak isometric force and maximum shortening velocity of the quadriceps were increased by 20% and quadriceps activation was simultaneously increased by 10%, jump height increased by almost 10 cm, or 7% of the value calculated for the untrained model. The same changes made to either the ankle plantarflexors or the hip extensors produced increases in jump height of only 3 cm, or 2% of the value obtained for the untrained model.
The latter result is a little puzzling because the model calculations had previously shown that the quadriceps and gluteus maximus are the major energy producers—the prime movers—of the body in vertical jumping (see “In Focus: Which Muscles Are Most Important to Vertical Jumping Performance?” in chapter 8). It should be noted here that each time a change was introduced to the model, a new optimal pattern of muscle activations was found by re-solving the optimisation problem for a maximum-height jump. Thus, one interpretation of the result is that what matters most in terms of performance is the quadriceps:gluteus maximus muscle strength ratio and not the absolute strength of these muscles. In other words, jumping performance is most sensitive to a change in the knee-extensor:hip-extensor muscle strength ratio. Increasing quadriceps strength by 20% increases the knee-extensor:hip-extensor muscle strength ratio in the model, whereas increasing gluteus maximus muscle strength decreases this ratio. The same line of reasoning may be used to explain why increasing ankle plantarflexor muscle strength by 20% leads to an increase in jump height of just 3 cm in the model.
Training for strength is also better than training for quickness or speed. Figure 10.10 shows the effects of increasing muscle strength and muscle contraction speed on vertical jump height as predicted by the four-segment, eight-muscle, sagittal-plane model of the body described in chapter 8 (see figure 8.7). An increase in muscle strength was simulated in the model by simultaneously increasing body weight because muscle strength increases in proportion to muscle mass. Thus, changes in body strength:weight ratio are represented in figure 10.10 rather than changes in muscle strength alone. Also, jumping performance is normalised in these results by dividing by the value of jump height calculated for the untrained model. Similarly, body strength:weight ratio and muscle contraction speed are each normalised by dividing by the value of body strength:weight ratio and muscle contraction speed in the untrained model, respectively.
The simulation results show that the slope of the line predicted for changes in body strength:weight ratio is twice as large as that obtained when changes in muscle fibre contraction speed are made (compare solid and dashed lines in figure 10.10). Thus, muscle strength has a greater effect on vertical jump height than muscle fibre contraction speed, even when the accompanying increase in body mass is taken into account.
Another important lesson learned from the modelling studies is that musculoskeletal changes must be accompanied by appropriate changes in neuromuscular control; otherwise, the expected improvement in motor performance will not be seen. In vertical jumping, if the pattern of muscle activations remains unchanged subsequent to strength training, jump height actually decreases relative to the untrained state. Figure 10.11 shows a simulated jump in which the strength of the knee-extensor muscles has been increased by 20% but the control exerted over the joints is the same as that calculated for the untrained model before the training effect was introduced. When the pattern of muscle activations was not optimised to match the changes introduced to the neuromusculoskeletal model, the body left the ground prematurely (i.e., the centre of mass was at a lower height at liftoff than is optimal for a maximal jump).
One consequence of not optimising the pattern of muscle activations (i.e., the controls) is that a larger fraction of the total work produced by the muscles goes into rotating the body segments rather than accelerating the centre of mass upward. In a maximum-height jump, approximately 90% of the total work done by the leg muscles is used to propel the centre of mass upward. This number is closer to 80% when muscle coordination (i.e., the sequence and timing of muscle activations) is not optimal. So, even though jumping performance depends heavily on muscle strength, and to a lesser extent on muscle fibre contraction speed, optimal performance is also intimately related to neuromuscular control. For this reason, and as discussed at the beginning of this chapter, jump-training programs now focus on maneuvers that blend muscle strength with neural control.
Summary
Training can change both muscle strength and fibre contraction speed. Strength may be increased by increasing motor unit recruitment (net neural drive to the muscle) or by increasing muscle fibre size. Contraction speed may be altered by changing the shape of a muscle's force-velocity curve or by changing the value of its intrinsic maximum shortening velocity. Early in a training program (2-8 wk), increases in muscle strength are brought by neural adaptations rather than by increases in muscle size. Training can also change the way muscle action is coordinated during activity (i.e., neuromuscular control). Neuromuscular training programs are usually designed to improve stability (balance and coordination) and proprioception (joint position sense) in addition to muscle strength. Training is vital for improving biomechanical performance and for preventing injuries during sport.
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The three components of motivation affect exercise adherence
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
Effects of Psychological Factors on Exercise
The three components of motivation—direction, intensity, and persistence—mentioned in chapter 19 in relation to sport apply to exercise motivation as well. Direction clearly relates to whether an individual chooses the direction of the gym or the couch, the stairs or the elevator, or the pool or the bath. Once the direction of exercise has been chosen, intensity and persistence become important. For example, as a New Year's resolution two individuals may have both decided to join a gym. They have both chosen the direction of exercise. During their first day at the gym, one rides the bicycle for 30 min, tries the stepper and the rowing machine, completes multiple sets on the different weights available, and finishes off with an aerobics class. The other individual begins with just a few minutes on the bicycle and then completes just one set of each of the different weights exercises using light weights. The first person in this example exhibits high intensity and the second person exhibits low intensity. If we stop at this point in the example, you may conclude that the first person is more motivated than the second. Your opinion may change when the third component of motivation—persistence—is considered. Two days later the first individual is home in bed, too sore to move (thinking that exercise is painful and should be avoided whenever possible). The second individual, however, is once again at the gym adding a couple of minutes to his time on the bicycle and sticking to his light-weights workout.
Exercise Participation Motivation
Exercise participation motivation refers primarily to the direction component of motivation. Exercise participation motivation is the initiation of exercise. A variety of factors influence whether individuals initiate an exercise program.
Knowledge, attitudes, and beliefs about exercise influence motivation toward exercise participation. Individuals who understand the importance and value of regular exercise are more likely to initiate an exercise program than those who do not. Similarly, if people have positive attitudes about the value and importance of regular exercise they will have greater motivation to participate in exercise than will people with negative attitudes.
Valuing the importance of exercise, however, is not the only determinant of exercise participation. Beliefs about ourselves influence motivation as well. Even if individuals understand that exercise is important, they will be unlikely to begin an exercise program if they believe that they cannot succeed at it. If they believe that the exercise program is too difficult or that it requires more fitness, strength, coordination, or time than they have, it is doubtful that they will join the program. This confidence in one's ability to succeed at an exercise program is called exercise self-efficacy. It is logical that self-efficacy would influence behaviour. How likely would people be to do something they were convinced they could not do, particularly if they had to pay to do it? How likely would they be to invest any energy in pursuing that activity? Most people in this situation would not attempt the activity. People who have such feelings are described as having low self-efficacy. As self-efficacy, or one's belief in one's ability to succeed at a particular task, increases, so does the likelihood of undertaking that task.
How can we enhance exercise participation motivation? Educating people about the importance and value of exercise can be a valuable first step because individuals who understand the merit of exercise are more likely to adopt an exercise program. Unfortunately, imparting knowledge is not enough. A lot of us do things that we know are not good for us and, similarly, do not do things that we know are good for us. Enhancing the exercise self-efficacy of individuals will increase their motivation. Demonstrating how individuals can control their own activity is useful. Some people have low self-efficacy about exercise because they believe that they are too unfit to begin exercising. They may equate exercise with young, thin, Lycra-clad gym enthusiasts whom they see in the media. Programs that emphasise choice of activities, illustrate exercisers similar in age and fitness level to the potential exercisers, and reveal that exercising can be enjoyable may increase exercise participation motivation. Exercise programs that begin with activities that the individuals already know they are capable of doing, such as walking and climbing stairs, may also increase self-efficacy and thereby increase motivation. Imagery may also help individuals enhance their self-efficacy beliefs (see “Imagery, Exercise, and Self-Efficacy”).
Exercise Adherence Motivation
Although many people get motivated to begin an exercise program, many of the people who begin fail to continue. Approximately 50% of individuals who begin a regular physical-activity program drop out within the first 6 mo. These people had exercise participation motivation, but they lack exercise adherence motivation, the persistence component of motivation.
Biological, psychological, sensory, and situational factors all interact to influence exercise adherence. Biologically, body composition, aerobic fitness, and the presence of disease influence adherence. Unfortunately, it is usually the people who could gain the most from exercise who are the least likely to adhere. Overweight or obese, unfit, or chronically ill people are less likely to adhere to an exercise program than are thinner, fitter, and healthier people.
As is the case with exercise participation motivation, attitudes and beliefs influence exercise adherence motivation. Attitudes and beliefs about the importance of exercise play a role in adherence, but so too do individuals' expectations about the effects that exercise is having on them personally. For example, if individuals believe that major changes in fitness and body composition should occur after 6 wk of regular exercise and they do not perceive major improvement in their own bodies after 6 wk, they may believe that exercise does not do what it should and therefore quit. Even though it is unrealistic to believe that 6 wk of exercise can make up for 6 yr of inactivity, it is individuals' beliefs, not reality, that influence behaviour. Therefore, when introducing newcomers to exercise it is important that they have realistic expectations about the time and effort required and the anticipated effects of the proposed program.
In addition to attitudes and beliefs, other psychological factors influence exercise adherence motivation. Extroverts (people who are social and outgoing) tend to adhere to exercise programs better than do introverts. Exercise programs that are executed in the presence of other people are probably more comfortable for extroverts than for introverts. Extroverts tend to enjoy the interaction with class members and exercise partners, possibly encouraging their adherence. Introverts, on the other hand, may adhere better to individual, home-based exercise programs. The bulk of the research on exercise adherence has involved programs that take place on site at fitness facilities with other people. This setting may have led to the conclusion that extroverts are better adherers than introverts. If the research had been done on independent, home-based exercise programs, it might have been found that introverts were better adherers. Clearly efforts need to be made to match the social environment of the exercise program to the personality of the exerciser.
Individuals with high levels of self-motivation are more likely to adhere to exercise programs than are individuals with low levels of self-motivation. It is logical that highly self-motivated people have better adherence rates. The challenge is to help those individuals with low levels of self-motivation. One of the most effective methods of helping these individuals is to encourage their involvement in the goal-setting process.
Goal Setting
Setting goals can help enhance motivation for a number of reasons. Goal setting addresses all three components of motivation. Goals give direction by providing a target. Intensity and effort also can be enhanced because goals provide reasons for participating in the activity. We are all more likely to put in effort when we feel there is a reason for doing so. If individuals are given two jobs to do at work, one that has a particular target and objective and one that seems vague and purposeless, into which job are they more likely to put their effort? Having goals helps focus attention and effort. In addition, goals can augment persistence by fostering new strategies. If individuals have a goal to which they are committed and initial tactics appear unsuccessful, they will search for alternative strategies to achieve their aim. If the goal had not been set in the first place, instead of persisting with different plans of action, they would likely give up.
Goals are also beneficial because they reflect improvement. Too often people make short-term comparisons regarding their strength, fitness, flexibility, or weight. Because the positive effects of exercise take time to emerge, improvements being made are often not noticed when people use a short timeframe for comparison. If a goal is achieved, evidence of improvement exists.
Goal setting involves a number of steps:
- setting the goal,
- setting a target date by which to achieve the goal,
- determining strategies to achieve the goal, and
- evaluating the goal on a regular basis.
If a target date is not set (e.g., “One of these days I'll ride the exercise bike continuously for an hour”), the goal is really just a dream. For this idea to be a goal, the individual needs to set a specific date by which to achieve the behaviour. Goal setting usually involves both long-term and short-term goals. The long-term goal provides direction; the short-term goals provide the increase in intensity and effort. People often err by setting only long-term goals. They begin to work toward achieving the goal, but success seems so far away that they give up before they get there. If someone had decided to ride a bike for 1 h and was currently having trouble lasting 10 min, 1 h would seem virtually impossible. Achieving short-term goals along the way to the long-term goal boosts confidence and motivation because it is obvious that the effort is worthwhile because improvement is being made. Target dates are set for each short-term goal in a progressive order until the long-term goal is achieved. This pattern of goal setting can be considered as a staircase, where each short-term goal is a step on the way to the long-term goal (see figure 20.1).
For goals to be effective, however, they need to be properly set. Goals can be considered to be good if they meet certain criteria (table 20.1). Goals need to be challenging but realistic. If goals are not challenging, they probably are not requiring any real change in behaviour and therefore will have little effect. However, if goals are so challenging that they are unrealistic, people are setting themselves up for failure. Continued failure leads to lowered confidence and less motivation.
Goals also need to be specific and measurable. Saying “I want to be fitter” or “My goal is to be stronger” does not provide any way of knowing when success has been achieved. What is “fitter”? How strong is “stronger”? There needs to be some way of knowing whether the goal has been achieved when the target date arrives. The easiest way of making goals specific is to make goals numerical. Numbers can easily be used to quantify time spent exercising, distance travelled, repetitions accomplished, weight lifted, or exercise sessions attended.
In addition to being specific and measurable, goals need to be positive. If an exerciser makes it her goal not to recline and rest during the abdominals section of her aerobics class, she will be thinking about reclining and resting. If instead she makes it her goal to complete first 1 min and then 2 min of the abdominal exercises, she will be thinking about doing the exercises, thus increasing her chances of doing them. If we tell you not to think about pink elephants, what is the first thing you think about? Similarly, if you set a goal of not letting your back arch off the bench when you do bench presses, you will be thinking about your back arching. When you think about your back arching, your brain may be sending messages to the muscles that make your back arch (just like in the psychoneuromuscular theory of imagery discussed in chapter 19). By making your goal “not arching,” you may actually be increasing the likelihood of arching your back. A goal of keeping your back pushed flat against the bench would be much more effective. Goals should stipulate the desired behaviour. Positively worded goals help you think about, plan for, and prepare to do what it is you want to do.
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Lifespan changes in skeletal system can lead to injury
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages.
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Age-Related Changes in the Skeletal and Articular Systems
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages. During the early fetal growth period there is rapid multiplication of cells (hyperplasia), whereas during the pubertal period growth is caused mainly by development and enlargement of existing cells (hypertrophy). Development can also occur via the replacement of one type of tissue by another.
Stages in the Development of Bone
Adult human bones appear in a variety of shapes and sizes. Their basic shape is genetically determined, but their final shape is influenced greatly by the environment in which they develop. Environmental influences include mechanical factors, such as muscle forces acting on the developing bone, and metabolic factors, which include the supply of nutrients.
Typical long bones, such as the humerus in the arm and the femur in the thigh, develop via a process of endochondral ossification in which a cartilaginous model precedes the bone formation. The cartilage is eventually replaced by bone. The cartilage model initially is small, and its replacement by bone begins at an early stage of development. The primary ossification centres appear near the middle of the shaft of the future long bone at about 8 wk after fertilisation, when the embryo is about 35 mm (1.4 in.) long (see figure 5.1).
The cartilage model of the future bone grows before it is replaced by bone. This replacement occurs in a series of stages. Initially, the cartilage model grows by two processes: an increase in the number of cartilage cells and then an increase in the size of each cell. Next, the gel-like matrix surrounding the cartilage cells is calcified. This hardening of the tissue surrounding the cells effectively cuts off their supply of nutrients and prevents the removal of metabolic waste products. These imprisoned cartilage cells eventually die so that the calcified cartilage has a honeycomb appearance. Invading blood vessels bring nutrients and bone-forming cells (osteoblasts) to the calcified cartilage to lay down new bone. This new bone has a disorganised appearance. If ossification were the final stage, a long bone would not develop its hollow structure. What happens instead is that some of the newly laid bone is removed from internal sites by special bone-eroding cells. These osteoclasts work with osteoblasts during remodelling of the bone, causing the cortex to drift away from the central axis of the shaft during growth. Changes in shaft diameter and compact bone thickness can occur at any age by a remodelling process. Bone can respond to changes in mechanical stimuli by changing its structure.
One type of remodelling that occurs is the replacement of the originally laid bone by compact bone that has an organised structure (figure 3.1). Most primary centres of ossification (in the developing shaft) appear before birth, whereas most secondary centres of ossification (in the developing ends of the bone) appear after birth. The times of appearance of these secondary centres of ossification vary widely. The centre in the femur near the knee is normally present at birth, but the centre in the clavicle (collarbone) closest to the sternum (breastbone) does not appear until about 18 yr of age.
Growth in Length and Width of Bone
When both the shaft and ends of a long bone are developing they are separated by a growth plate, or epiphyseal plate. In this region, all the stages of bone development can be seen under a microscope (see figure 5.2). Each zone in figure 5.2 represents a stage in the growth of bone and then replacement of cartilage by bone; that is, the zones illustrated do not migrate downward, but rather retain their position and change their nature. This effectively increases the distance between the growth plates at each end of the bone. The process illustrated continues for almost 20 yr. Growth in length of the long bones ceases when the process stops and epiphyseal plates fuse and disappear. Actually, the growth stops because the cartilage cells no longer respond to hormonal influence.
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Although growth in height ceases at a certain age, changes in thickness of the compact bone of the shaft and the density of spongy bone can occur at any time of life (see chapter 6). Growth in the thickness and diameter of long bones occurs by appositional growth. In this process, bone is normally added on the outside of the shaft and removed from the inside of the shaft.
Skeletal Composition Changes Across the Life Span
The relative proportions of the inorganic salt crystals and the organic collagen change throughout the life span. In a child, the flexibility of the bone is related to the large proportion of collagen. It is thought that the rapid growth of children's bones and the consequent amount of remodelling results in less-than-optimal mineralisation. In a newborn baby, the skeleton accounts for about 13% of the total body weight, but about two-thirds of the skeleton is cartilaginous.
In bone of a young adult the inorganic material is close to the optimal two-thirds mineralisation, so the bone is strong and tough. Though the proportion of skeletal material in the mature adult is also about 13% of total body weight, just over 10% of the skeleton is cartilaginous. Of the bone, four-fifths is compact and one-fifth is spongy.
Eventually, aged bone becomes brittle. This change relates partly to the increased proportion of the inorganic salts, but of more significance is the reduced total mass of bone material in the aged musculoskeletal system. Bone mass decreases mainly because of a decrease in the number of trabeculae in spongy bone (see “In Focus: Why Trunk Height Decreases With Ageing”) combined with resorption and consequent thinning of the trabeculae. In compact bone, the size of the Haversian canals (see figure 3.1) increases and the cross-sectional area of the bone surrounding each Haversian canal decreases. This increased porosity of bone, in turn, causes decreased bone density (mass per unit volume of tissue). In an 80-yr-old male the density of bone in the spine is often 55% of the value found in a 20-yr-old; in females who are elderly, it is often only 40% of its peak value. With ageing, bone also becomes stiffer, partly because of the collagen cross-linking. A consequence of these mechanical changes is that aged bone can absorb less energy before it fractures, so it is more liable to fail when subjected to large forces.
Osteoporosis
For most tissues in the body, their mass at any time is determined by the balance between the simultaneous formation and destruction of the tissue. Osteopenia (“bone poverty”) is reduced bone density, particularly in women during the postmenopausal years, when the dramatic decrease in the female hormone estrogen allows the rate of synthesis to be lower than the rate of resorption. The initial effects are mainly to the spongy bone, but later the compact bone in the shafts of long bones also decreases in thickness.
According to the World Health Organization, osteoporosis (“bone porosity”) is present when the bone density is more than 2.5 standard deviations below the average bone mass of a young adult woman. In general, the porosity of bone in the human femur doubles from 40 to 80 yr of age and people with osteoporosis show an even greater change. Thus, osteoporotic bone is even less able to absorb energy before it fails than is normally aged bone. Moreover, even though a chemical analysis of osteoporotic bone material may indicate slightly increased levels of calcium per unit mass of bone, the overall mass of bone decreases so markedly that there is less calcium stored in the skeleton overall.
During maturation, the density and calcium content of bones is increased due to genetic predisposition aided by the interaction between dietary calcium and exercise. It is now thought that peak bone mass occurs at about 16 to 20 yr of age in women, so some health authorities claim osteoporosis is a pediatric disease and have promoted the idea of “bone banks” to help prevent osteoporosis. The idea is that if a person's lifestyle during maturation and early adulthood promoted maximum deposition of bone material, then later in life, when withdrawals of bone material are inevitable, the structural integrity of the bones will be maintained longer. Financial advisers tell everybody to start saving as young as possible, and this concept is even more important for prevention of osteoporosis.
Osteoporosis is commonly associated with women, but the prevalence among men is now the same as it was in women about 25 yr ago. This trend has been attributed to lifestyle changes, which include inadequate intake of calcium and less-than-optimal levels of physical activity. The personal lifestyle changes due to osteoporosis and the public health care costs of osteoporosis are so great that they are considered further in chapter 23.
Bone Failure in Relation to Bone Development, Age, or Activity
The modern automotive industry attempts to protect passengers by enclosing them in a cage that is lined by material that cushions the forces involved in a collision. In similar fashion, the soft tissues of the human body, including fat and muscle, perform the role of a crumple zone. However, despite this protection, sometimes the energy involved in a mishap is enough to cause a fracture anyway. Bone failure may be related to bone development, age, or activity.
Certain types of fractures are associated with particular stages of development. The flexible bones of a child tend to splinter in a manner similar to the broken branch of a growing tree; this type of fracture is called a greenstick fracture. Late in life the fracture is more likely to be of the brittle type because of the increased stiffness of old bones. The bones of adults who are elderly are also more likely to fail because of the decreased bone mass and density in osteoporosis.
Specific fractures are related to certain ages. For example, fractures of the neck of the femur (commonly called hip fractures) are common in women in the elderly population. Incidence of forearm fracture increases near the peak of the pubescent growth spurt. This is associated with increased porosity of compact bone as the remodelling space increases to provide calcium to the rapid-growth regions. A second increase in wrist fractures is seen in women who are elderly; this is related to the combination of osteoporosis and increased incidence of falls. In developing children, the cartilaginous growth plate may also fail; this most often occurs at the zone of maturation, or hypertrophy (see figure 5.2).
The development of bone has implications for potential injury to the musculoskeletal system. For example, injury to the cartilaginous growth plates can be produced by single excessive forces causing trauma or by repetitive compressive forces, or tensile forces produced by muscle-tendon pull on an area of developing bone.
In sports medicine, specific injuries have been attributed to certain activities. One of the best examples is Little League elbow, described in more detail in chapter 24. Other examples of the relationships between activity and types of growth-plate problems are wrist injuries in gymnasts and tibial tuberosity avulsions in sports involving sprinting and jumping. Pain over the tibial tuberosity (just below the knee cap) in a prepubescent child is indicative of Osgood-Schlatter's condition.
Effects of Various Factors on Range of Motion
Range of motion at joints is affected by joint structure and the mechanical properties of the tissues associated with the joints. A general perception is that joint range of motion decreases during life. Although this is the general trend, the rate of loss is not constant. Ranges of joint motion are very large in a newborn baby. As an example, the range of ankle dorsiflexion in a newborn is limited only by the contact of the top of the foot against the shin. Try this movement yourself to indicate how much flexibility you have lost since birth.
Between the ages of 6 and 15 yr, there is a general trend for joint range of motion to decrease in boys, whereas for girls the effects are variable and joint dependent. Girls are generally more flexible than boys during childhood and adolescence. Changes in joint ranges of motion between adolescence and young adulthood are variable and, as discussed in chapter 6, appear to be related to physical activity. Similarly, the general trend for joint flexibility to decrease during ageing may not be completely explained by biological ageing processes.
Ageing people may suffer arthritis (“inflammation of joints”), which markedly restricts range of motion. Rheumatoid arthritis involves inflammation of the synovial membrane, while osteoarthritis involves degeneration of the articular cartilage (see figure 3.6). In the total adult population, about three times as many women have osteoarthritis as do men. Thus there is some genetic determination of osteoarthritis, to which are added the effects of environmental factors. It would appear that the risk factors for rheumatoid arthritis are mainly related to a family history of the condition.
Age-Related Changes in the Muscular System
Mesoderm, which forms toward the end of the third week of development, eventually forms the muscle and connective tissues of the body. Some of the mesoderm is segmented such that it is in lumps on either side of the midline, and these lumps form the muscles of the trunk. Other mesoderm forms the muscles of the limbs; this tissue migrates into the limbs when they first appear. Limb muscles migrate toward their final position during further development.
Certain precursor cells differentiate into myoblasts (muscle-forming cells) that fuse longitudinally to form the long muscles and muscle fibres we can observe. The number of muscle fibres appears to be genetically determined so that large people often have more muscle fibres than persons who are smaller in stature. As muscles grow in length, the number of sarcomeres increases. Although direct evidence is sparse, many believe that the stimulus for growth of muscle is the bone growth that pulls on the attached ends of the muscle. Developmental growth occurs in both muscle length and cross-sectional area, and these factors may also be affected by activity. The effects of physical training on muscle is summarised in chapters 6, 10, and 13.
In an infant, muscle tissue accounts for about 25% of total body weight, but in a young adult the proportion of muscle is more than 40%. In terms of growth, muscle increases from about 850 g (1.9 lb) at birth to about 30 kg (66 lb) in a young adult male weighing 70 kg (154 lb).
Strength, which may be defined as the capacity to produce force against an external resistance, is related to the cross-sectional areas of the muscles producing the force. Muscle volume peaks at about 30 yr of age and then gradually decreases. The atrophy of skeletal muscle associated with disuse is much faster than any ageing-related atrophy. Muscle elasticity also decreases during ageing so that muscles are stiffer and less extensible. This is one contributing factor to the loss of joint range of motion described earlier.
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Training can increase both muscle strength and fiber contraction speed
Adaptations are common following injury and surgical treatment.
Biophysical Foundations of Human Movement, Third Edition.
Biomechanical Adaptations to Injury
Adaptations are common following injury and surgical treatment. One interesting example found in the orthopaedics literature is a phenomenon known as quadriceps avoidance gait. This particular adaptation occurs in ACL-deficient (ACLD) patients; that is, patients who have suffered a complete rupture of the ACL. ACLD patients usually have difficulty with movements involving either lateral thrusts (e.g., cutting from side to side) or twisting (i.e., rotation of the femur relative to the tibia in the transverse plane).
Gait analysis has been done on ACLD patients during various activities of daily living, including level walking, jogging, walking up and down stairs, and even running, cutting, and pivoting. Kinematic and force-plate data were used to estimate the net muscle moments exerted at the knee in all three planes of movement: flexion-extension, abduction-
adduction, and internal-external rotation. Interestingly, the greatest functional changes between ACLD patients and healthy subjects occurred while walking at preferred speeds on level ground. During the midstance portion of the gait cycle, the ACLD patients exhibited a net extensor moment that was significantly lower than normal (figure 10.9, solid line). One interpretation of these findings is that the ACLD patients walk by reducing the demand on their quadriceps during stance (i.e., the quadriceps muscles are activated less in the ACL-deficient patients, giving rise to a quadriceps-avoidance gait pattern).
It is important to realise that a lower net extensor moment at the knee does not necessarily mean that the moment exerted by quadriceps is lower. Because the net moment at the knee is a combination of quadriceps and hamstrings muscle action, a lower extensor moment could result from an increase in the flexor moment applied by hamstrings. However, muscle electromyography results tend to support the interpretation that quadriceps moment is lower because the activation level of the quadriceps is lower in the ACLD patients.
Gait analysis results have also shown that the ACLD patients reduce their net extensor moment by 25% during jogging compared with a 100% reduction in walking. However, the net extensor moment in stair climbing was the same in ACLD patients and normal subjects. When taken together, these results suggest that the observed quadriceps avoidance in ACLD patients is not a result of quadriceps weakness but rather is due to a change in neuromuscular control.
Dependence of Motor Performance on Changes in Muscle Properties
Strength and quickness are critical determinants of performance in explosive movements such as jumping, throwing, and sprinting. Together, these two factors determine the amount of power developed instantaneously during a task (i.e., muscle power is given by the product of muscle force and muscle contraction speed; see “In Focus: At What Speed Must a Muscle Shorten to Develop Maximum Power?”). As discussed earlier in this chapter, resistance strength training can alter both muscle force and fibre contraction speed. This section describes the effects that changes in the contractile properties of muscle have on motor performance, specifically, how biomechanical performance depends on muscle strength and muscle fibre contraction speed.
Vertical jumping is one of the most heavily studied motor tasks. Many scientists have studied jumping with the aim of learning more about the biomechanics and control of this task and, specifically, how performance may be influenced by training. In a recent jump-training program that incorporated stretching, plyometric exercises, and weight lifting, female high school volleyball players demonstrated a mean increase of 4 cm (1.5 in.) in jump height after training. This represented an almost 10% increase in jump height as a result of the 6 wk period of training. Even larger increases in vertical jumping performance have been documented in the literature. One extreme example is the 1984 United States Olympic gold medal volleyball team, which showed a 10 cm (4 in.) increase in jump height after 2 yr of jump training.
Using Computer Modelling to Study Vertical Jumping Performance
It is gratifying to learn that jumping performance can be increased significantly by strength and neuromuscular training. However, it is difficult, if not impossible, to explain why these increases occur. Noninvasive measurements of biomechanical performance cannot pinpoint the factor or factors responsible for the increase in jump height. The reason is that several properties of the neuromuscular and musculoskeletal systems change simultaneously during the training regimen. Alternatively, computer models may be used to study the relationships between training effects and performance; that is, a model of the neuromusculoskeletal system may be used to predict how changes to specific parameters affect the performance of a motor
task.
Models of the body similar to the one shown in figure 8.7 have been used to study how changes in muscle strength, muscle contraction speed, and motor unit recruitment affect jump height. The values of each of these parameters in the model were increased by amounts consistent with results obtained from strength-training programs. For example, the peak isometric strength and maximum shortening velocity of each muscle in the model were increased by 20% and the activation level of each muscle was increased by 10%. The three training effects were first applied to all muscles simultaneously and then to the ankle plantarflexors, knee extensors, and hip extensors separately. In this way, the modelling results were used to determine whether it is better to train all the leg muscles simultaneously or to isolate specific muscle groups such as the quadriceps (knee extensors) and gluteus maximus (hip
extensors).
Insights Into the Effects of Training Provided by Computer Models
Increasing the peak isometric force of all the leg muscles in the model by 20% produced the largest increase in jump height. In this case, jump height increased by 7 cm, or 5% of that obtained before the simulated training effect. Increasing the maximum shortening velocity of each muscle by 20% or the activation level of each muscle by 10% increased jump height by only 4 cm, or 3% of that obtained for the nominal (untrained) model. When all three training effects were introduced simultaneously, jumping performance increased by nearly 17 cm, or 12% of that calculated for the untrained model. Thus, training programs that increase strength, fibre contraction speed, and motor recruitment (i.e., activation level) of all the leg muscles simultaneously are most beneficial to overall jumping performance.
The modelling results also suggest that training the knee extensors is better than training either the ankle plantarflexors or the hip extensors. When peak isometric force and maximum shortening velocity of the quadriceps were increased by 20% and quadriceps activation was simultaneously increased by 10%, jump height increased by almost 10 cm, or 7% of the value calculated for the untrained model. The same changes made to either the ankle plantarflexors or the hip extensors produced increases in jump height of only 3 cm, or 2% of the value obtained for the untrained model.
The latter result is a little puzzling because the model calculations had previously shown that the quadriceps and gluteus maximus are the major energy producers—the prime movers—of the body in vertical jumping (see “In Focus: Which Muscles Are Most Important to Vertical Jumping Performance?” in chapter 8). It should be noted here that each time a change was introduced to the model, a new optimal pattern of muscle activations was found by re-solving the optimisation problem for a maximum-height jump. Thus, one interpretation of the result is that what matters most in terms of performance is the quadriceps:gluteus maximus muscle strength ratio and not the absolute strength of these muscles. In other words, jumping performance is most sensitive to a change in the knee-extensor:hip-extensor muscle strength ratio. Increasing quadriceps strength by 20% increases the knee-extensor:hip-extensor muscle strength ratio in the model, whereas increasing gluteus maximus muscle strength decreases this ratio. The same line of reasoning may be used to explain why increasing ankle plantarflexor muscle strength by 20% leads to an increase in jump height of just 3 cm in the model.
Training for strength is also better than training for quickness or speed. Figure 10.10 shows the effects of increasing muscle strength and muscle contraction speed on vertical jump height as predicted by the four-segment, eight-muscle, sagittal-plane model of the body described in chapter 8 (see figure 8.7). An increase in muscle strength was simulated in the model by simultaneously increasing body weight because muscle strength increases in proportion to muscle mass. Thus, changes in body strength:weight ratio are represented in figure 10.10 rather than changes in muscle strength alone. Also, jumping performance is normalised in these results by dividing by the value of jump height calculated for the untrained model. Similarly, body strength:weight ratio and muscle contraction speed are each normalised by dividing by the value of body strength:weight ratio and muscle contraction speed in the untrained model, respectively.
The simulation results show that the slope of the line predicted for changes in body strength:weight ratio is twice as large as that obtained when changes in muscle fibre contraction speed are made (compare solid and dashed lines in figure 10.10). Thus, muscle strength has a greater effect on vertical jump height than muscle fibre contraction speed, even when the accompanying increase in body mass is taken into account.
Another important lesson learned from the modelling studies is that musculoskeletal changes must be accompanied by appropriate changes in neuromuscular control; otherwise, the expected improvement in motor performance will not be seen. In vertical jumping, if the pattern of muscle activations remains unchanged subsequent to strength training, jump height actually decreases relative to the untrained state. Figure 10.11 shows a simulated jump in which the strength of the knee-extensor muscles has been increased by 20% but the control exerted over the joints is the same as that calculated for the untrained model before the training effect was introduced. When the pattern of muscle activations was not optimised to match the changes introduced to the neuromusculoskeletal model, the body left the ground prematurely (i.e., the centre of mass was at a lower height at liftoff than is optimal for a maximal jump).
One consequence of not optimising the pattern of muscle activations (i.e., the controls) is that a larger fraction of the total work produced by the muscles goes into rotating the body segments rather than accelerating the centre of mass upward. In a maximum-height jump, approximately 90% of the total work done by the leg muscles is used to propel the centre of mass upward. This number is closer to 80% when muscle coordination (i.e., the sequence and timing of muscle activations) is not optimal. So, even though jumping performance depends heavily on muscle strength, and to a lesser extent on muscle fibre contraction speed, optimal performance is also intimately related to neuromuscular control. For this reason, and as discussed at the beginning of this chapter, jump-training programs now focus on maneuvers that blend muscle strength with neural control.
Summary
Training can change both muscle strength and fibre contraction speed. Strength may be increased by increasing motor unit recruitment (net neural drive to the muscle) or by increasing muscle fibre size. Contraction speed may be altered by changing the shape of a muscle's force-velocity curve or by changing the value of its intrinsic maximum shortening velocity. Early in a training program (2-8 wk), increases in muscle strength are brought by neural adaptations rather than by increases in muscle size. Training can also change the way muscle action is coordinated during activity (i.e., neuromuscular control). Neuromuscular training programs are usually designed to improve stability (balance and coordination) and proprioception (joint position sense) in addition to muscle strength. Training is vital for improving biomechanical performance and for preventing injuries during sport.
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The three components of motivation affect exercise adherence
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
Effects of Psychological Factors on Exercise
The three components of motivation—direction, intensity, and persistence—mentioned in chapter 19 in relation to sport apply to exercise motivation as well. Direction clearly relates to whether an individual chooses the direction of the gym or the couch, the stairs or the elevator, or the pool or the bath. Once the direction of exercise has been chosen, intensity and persistence become important. For example, as a New Year's resolution two individuals may have both decided to join a gym. They have both chosen the direction of exercise. During their first day at the gym, one rides the bicycle for 30 min, tries the stepper and the rowing machine, completes multiple sets on the different weights available, and finishes off with an aerobics class. The other individual begins with just a few minutes on the bicycle and then completes just one set of each of the different weights exercises using light weights. The first person in this example exhibits high intensity and the second person exhibits low intensity. If we stop at this point in the example, you may conclude that the first person is more motivated than the second. Your opinion may change when the third component of motivation—persistence—is considered. Two days later the first individual is home in bed, too sore to move (thinking that exercise is painful and should be avoided whenever possible). The second individual, however, is once again at the gym adding a couple of minutes to his time on the bicycle and sticking to his light-weights workout.
Exercise Participation Motivation
Exercise participation motivation refers primarily to the direction component of motivation. Exercise participation motivation is the initiation of exercise. A variety of factors influence whether individuals initiate an exercise program.
Knowledge, attitudes, and beliefs about exercise influence motivation toward exercise participation. Individuals who understand the importance and value of regular exercise are more likely to initiate an exercise program than those who do not. Similarly, if people have positive attitudes about the value and importance of regular exercise they will have greater motivation to participate in exercise than will people with negative attitudes.
Valuing the importance of exercise, however, is not the only determinant of exercise participation. Beliefs about ourselves influence motivation as well. Even if individuals understand that exercise is important, they will be unlikely to begin an exercise program if they believe that they cannot succeed at it. If they believe that the exercise program is too difficult or that it requires more fitness, strength, coordination, or time than they have, it is doubtful that they will join the program. This confidence in one's ability to succeed at an exercise program is called exercise self-efficacy. It is logical that self-efficacy would influence behaviour. How likely would people be to do something they were convinced they could not do, particularly if they had to pay to do it? How likely would they be to invest any energy in pursuing that activity? Most people in this situation would not attempt the activity. People who have such feelings are described as having low self-efficacy. As self-efficacy, or one's belief in one's ability to succeed at a particular task, increases, so does the likelihood of undertaking that task.
How can we enhance exercise participation motivation? Educating people about the importance and value of exercise can be a valuable first step because individuals who understand the merit of exercise are more likely to adopt an exercise program. Unfortunately, imparting knowledge is not enough. A lot of us do things that we know are not good for us and, similarly, do not do things that we know are good for us. Enhancing the exercise self-efficacy of individuals will increase their motivation. Demonstrating how individuals can control their own activity is useful. Some people have low self-efficacy about exercise because they believe that they are too unfit to begin exercising. They may equate exercise with young, thin, Lycra-clad gym enthusiasts whom they see in the media. Programs that emphasise choice of activities, illustrate exercisers similar in age and fitness level to the potential exercisers, and reveal that exercising can be enjoyable may increase exercise participation motivation. Exercise programs that begin with activities that the individuals already know they are capable of doing, such as walking and climbing stairs, may also increase self-efficacy and thereby increase motivation. Imagery may also help individuals enhance their self-efficacy beliefs (see “Imagery, Exercise, and Self-Efficacy”).
Exercise Adherence Motivation
Although many people get motivated to begin an exercise program, many of the people who begin fail to continue. Approximately 50% of individuals who begin a regular physical-activity program drop out within the first 6 mo. These people had exercise participation motivation, but they lack exercise adherence motivation, the persistence component of motivation.
Biological, psychological, sensory, and situational factors all interact to influence exercise adherence. Biologically, body composition, aerobic fitness, and the presence of disease influence adherence. Unfortunately, it is usually the people who could gain the most from exercise who are the least likely to adhere. Overweight or obese, unfit, or chronically ill people are less likely to adhere to an exercise program than are thinner, fitter, and healthier people.
As is the case with exercise participation motivation, attitudes and beliefs influence exercise adherence motivation. Attitudes and beliefs about the importance of exercise play a role in adherence, but so too do individuals' expectations about the effects that exercise is having on them personally. For example, if individuals believe that major changes in fitness and body composition should occur after 6 wk of regular exercise and they do not perceive major improvement in their own bodies after 6 wk, they may believe that exercise does not do what it should and therefore quit. Even though it is unrealistic to believe that 6 wk of exercise can make up for 6 yr of inactivity, it is individuals' beliefs, not reality, that influence behaviour. Therefore, when introducing newcomers to exercise it is important that they have realistic expectations about the time and effort required and the anticipated effects of the proposed program.
In addition to attitudes and beliefs, other psychological factors influence exercise adherence motivation. Extroverts (people who are social and outgoing) tend to adhere to exercise programs better than do introverts. Exercise programs that are executed in the presence of other people are probably more comfortable for extroverts than for introverts. Extroverts tend to enjoy the interaction with class members and exercise partners, possibly encouraging their adherence. Introverts, on the other hand, may adhere better to individual, home-based exercise programs. The bulk of the research on exercise adherence has involved programs that take place on site at fitness facilities with other people. This setting may have led to the conclusion that extroverts are better adherers than introverts. If the research had been done on independent, home-based exercise programs, it might have been found that introverts were better adherers. Clearly efforts need to be made to match the social environment of the exercise program to the personality of the exerciser.
Individuals with high levels of self-motivation are more likely to adhere to exercise programs than are individuals with low levels of self-motivation. It is logical that highly self-motivated people have better adherence rates. The challenge is to help those individuals with low levels of self-motivation. One of the most effective methods of helping these individuals is to encourage their involvement in the goal-setting process.
Goal Setting
Setting goals can help enhance motivation for a number of reasons. Goal setting addresses all three components of motivation. Goals give direction by providing a target. Intensity and effort also can be enhanced because goals provide reasons for participating in the activity. We are all more likely to put in effort when we feel there is a reason for doing so. If individuals are given two jobs to do at work, one that has a particular target and objective and one that seems vague and purposeless, into which job are they more likely to put their effort? Having goals helps focus attention and effort. In addition, goals can augment persistence by fostering new strategies. If individuals have a goal to which they are committed and initial tactics appear unsuccessful, they will search for alternative strategies to achieve their aim. If the goal had not been set in the first place, instead of persisting with different plans of action, they would likely give up.
Goals are also beneficial because they reflect improvement. Too often people make short-term comparisons regarding their strength, fitness, flexibility, or weight. Because the positive effects of exercise take time to emerge, improvements being made are often not noticed when people use a short timeframe for comparison. If a goal is achieved, evidence of improvement exists.
Goal setting involves a number of steps:
- setting the goal,
- setting a target date by which to achieve the goal,
- determining strategies to achieve the goal, and
- evaluating the goal on a regular basis.
If a target date is not set (e.g., “One of these days I'll ride the exercise bike continuously for an hour”), the goal is really just a dream. For this idea to be a goal, the individual needs to set a specific date by which to achieve the behaviour. Goal setting usually involves both long-term and short-term goals. The long-term goal provides direction; the short-term goals provide the increase in intensity and effort. People often err by setting only long-term goals. They begin to work toward achieving the goal, but success seems so far away that they give up before they get there. If someone had decided to ride a bike for 1 h and was currently having trouble lasting 10 min, 1 h would seem virtually impossible. Achieving short-term goals along the way to the long-term goal boosts confidence and motivation because it is obvious that the effort is worthwhile because improvement is being made. Target dates are set for each short-term goal in a progressive order until the long-term goal is achieved. This pattern of goal setting can be considered as a staircase, where each short-term goal is a step on the way to the long-term goal (see figure 20.1).
For goals to be effective, however, they need to be properly set. Goals can be considered to be good if they meet certain criteria (table 20.1). Goals need to be challenging but realistic. If goals are not challenging, they probably are not requiring any real change in behaviour and therefore will have little effect. However, if goals are so challenging that they are unrealistic, people are setting themselves up for failure. Continued failure leads to lowered confidence and less motivation.
Goals also need to be specific and measurable. Saying “I want to be fitter” or “My goal is to be stronger” does not provide any way of knowing when success has been achieved. What is “fitter”? How strong is “stronger”? There needs to be some way of knowing whether the goal has been achieved when the target date arrives. The easiest way of making goals specific is to make goals numerical. Numbers can easily be used to quantify time spent exercising, distance travelled, repetitions accomplished, weight lifted, or exercise sessions attended.
In addition to being specific and measurable, goals need to be positive. If an exerciser makes it her goal not to recline and rest during the abdominals section of her aerobics class, she will be thinking about reclining and resting. If instead she makes it her goal to complete first 1 min and then 2 min of the abdominal exercises, she will be thinking about doing the exercises, thus increasing her chances of doing them. If we tell you not to think about pink elephants, what is the first thing you think about? Similarly, if you set a goal of not letting your back arch off the bench when you do bench presses, you will be thinking about your back arching. When you think about your back arching, your brain may be sending messages to the muscles that make your back arch (just like in the psychoneuromuscular theory of imagery discussed in chapter 19). By making your goal “not arching,” you may actually be increasing the likelihood of arching your back. A goal of keeping your back pushed flat against the bench would be much more effective. Goals should stipulate the desired behaviour. Positively worded goals help you think about, plan for, and prepare to do what it is you want to do.
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Lifespan changes in skeletal system can lead to injury
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages.
Biophysical Foundations of Human Movement, Third Edition.
Age-Related Changes in the Skeletal and Articular Systems
For the musculoskeletal system, the major periods of growth are the fetal and pubertal stages. During the early fetal growth period there is rapid multiplication of cells (hyperplasia), whereas during the pubertal period growth is caused mainly by development and enlargement of existing cells (hypertrophy). Development can also occur via the replacement of one type of tissue by another.
Stages in the Development of Bone
Adult human bones appear in a variety of shapes and sizes. Their basic shape is genetically determined, but their final shape is influenced greatly by the environment in which they develop. Environmental influences include mechanical factors, such as muscle forces acting on the developing bone, and metabolic factors, which include the supply of nutrients.
Typical long bones, such as the humerus in the arm and the femur in the thigh, develop via a process of endochondral ossification in which a cartilaginous model precedes the bone formation. The cartilage is eventually replaced by bone. The cartilage model initially is small, and its replacement by bone begins at an early stage of development. The primary ossification centres appear near the middle of the shaft of the future long bone at about 8 wk after fertilisation, when the embryo is about 35 mm (1.4 in.) long (see figure 5.1).
The cartilage model of the future bone grows before it is replaced by bone. This replacement occurs in a series of stages. Initially, the cartilage model grows by two processes: an increase in the number of cartilage cells and then an increase in the size of each cell. Next, the gel-like matrix surrounding the cartilage cells is calcified. This hardening of the tissue surrounding the cells effectively cuts off their supply of nutrients and prevents the removal of metabolic waste products. These imprisoned cartilage cells eventually die so that the calcified cartilage has a honeycomb appearance. Invading blood vessels bring nutrients and bone-forming cells (osteoblasts) to the calcified cartilage to lay down new bone. This new bone has a disorganised appearance. If ossification were the final stage, a long bone would not develop its hollow structure. What happens instead is that some of the newly laid bone is removed from internal sites by special bone-eroding cells. These osteoclasts work with osteoblasts during remodelling of the bone, causing the cortex to drift away from the central axis of the shaft during growth. Changes in shaft diameter and compact bone thickness can occur at any age by a remodelling process. Bone can respond to changes in mechanical stimuli by changing its structure.
One type of remodelling that occurs is the replacement of the originally laid bone by compact bone that has an organised structure (figure 3.1). Most primary centres of ossification (in the developing shaft) appear before birth, whereas most secondary centres of ossification (in the developing ends of the bone) appear after birth. The times of appearance of these secondary centres of ossification vary widely. The centre in the femur near the knee is normally present at birth, but the centre in the clavicle (collarbone) closest to the sternum (breastbone) does not appear until about 18 yr of age.
Growth in Length and Width of Bone
When both the shaft and ends of a long bone are developing they are separated by a growth plate, or epiphyseal plate. In this region, all the stages of bone development can be seen under a microscope (see figure 5.2). Each zone in figure 5.2 represents a stage in the growth of bone and then replacement of cartilage by bone; that is, the zones illustrated do not migrate downward, but rather retain their position and change their nature. This effectively increases the distance between the growth plates at each end of the bone. The process illustrated continues for almost 20 yr. Growth in length of the long bones ceases when the process stops and epiphyseal plates fuse and disappear. Actually, the growth stops because the cartilage cells no longer respond to hormonal influence.
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Although growth in height ceases at a certain age, changes in thickness of the compact bone of the shaft and the density of spongy bone can occur at any time of life (see chapter 6). Growth in the thickness and diameter of long bones occurs by appositional growth. In this process, bone is normally added on the outside of the shaft and removed from the inside of the shaft.
Skeletal Composition Changes Across the Life Span
The relative proportions of the inorganic salt crystals and the organic collagen change throughout the life span. In a child, the flexibility of the bone is related to the large proportion of collagen. It is thought that the rapid growth of children's bones and the consequent amount of remodelling results in less-than-optimal mineralisation. In a newborn baby, the skeleton accounts for about 13% of the total body weight, but about two-thirds of the skeleton is cartilaginous.
In bone of a young adult the inorganic material is close to the optimal two-thirds mineralisation, so the bone is strong and tough. Though the proportion of skeletal material in the mature adult is also about 13% of total body weight, just over 10% of the skeleton is cartilaginous. Of the bone, four-fifths is compact and one-fifth is spongy.
Eventually, aged bone becomes brittle. This change relates partly to the increased proportion of the inorganic salts, but of more significance is the reduced total mass of bone material in the aged musculoskeletal system. Bone mass decreases mainly because of a decrease in the number of trabeculae in spongy bone (see “In Focus: Why Trunk Height Decreases With Ageing”) combined with resorption and consequent thinning of the trabeculae. In compact bone, the size of the Haversian canals (see figure 3.1) increases and the cross-sectional area of the bone surrounding each Haversian canal decreases. This increased porosity of bone, in turn, causes decreased bone density (mass per unit volume of tissue). In an 80-yr-old male the density of bone in the spine is often 55% of the value found in a 20-yr-old; in females who are elderly, it is often only 40% of its peak value. With ageing, bone also becomes stiffer, partly because of the collagen cross-linking. A consequence of these mechanical changes is that aged bone can absorb less energy before it fractures, so it is more liable to fail when subjected to large forces.
Osteoporosis
For most tissues in the body, their mass at any time is determined by the balance between the simultaneous formation and destruction of the tissue. Osteopenia (“bone poverty”) is reduced bone density, particularly in women during the postmenopausal years, when the dramatic decrease in the female hormone estrogen allows the rate of synthesis to be lower than the rate of resorption. The initial effects are mainly to the spongy bone, but later the compact bone in the shafts of long bones also decreases in thickness.
According to the World Health Organization, osteoporosis (“bone porosity”) is present when the bone density is more than 2.5 standard deviations below the average bone mass of a young adult woman. In general, the porosity of bone in the human femur doubles from 40 to 80 yr of age and people with osteoporosis show an even greater change. Thus, osteoporotic bone is even less able to absorb energy before it fails than is normally aged bone. Moreover, even though a chemical analysis of osteoporotic bone material may indicate slightly increased levels of calcium per unit mass of bone, the overall mass of bone decreases so markedly that there is less calcium stored in the skeleton overall.
During maturation, the density and calcium content of bones is increased due to genetic predisposition aided by the interaction between dietary calcium and exercise. It is now thought that peak bone mass occurs at about 16 to 20 yr of age in women, so some health authorities claim osteoporosis is a pediatric disease and have promoted the idea of “bone banks” to help prevent osteoporosis. The idea is that if a person's lifestyle during maturation and early adulthood promoted maximum deposition of bone material, then later in life, when withdrawals of bone material are inevitable, the structural integrity of the bones will be maintained longer. Financial advisers tell everybody to start saving as young as possible, and this concept is even more important for prevention of osteoporosis.
Osteoporosis is commonly associated with women, but the prevalence among men is now the same as it was in women about 25 yr ago. This trend has been attributed to lifestyle changes, which include inadequate intake of calcium and less-than-optimal levels of physical activity. The personal lifestyle changes due to osteoporosis and the public health care costs of osteoporosis are so great that they are considered further in chapter 23.
Bone Failure in Relation to Bone Development, Age, or Activity
The modern automotive industry attempts to protect passengers by enclosing them in a cage that is lined by material that cushions the forces involved in a collision. In similar fashion, the soft tissues of the human body, including fat and muscle, perform the role of a crumple zone. However, despite this protection, sometimes the energy involved in a mishap is enough to cause a fracture anyway. Bone failure may be related to bone development, age, or activity.
Certain types of fractures are associated with particular stages of development. The flexible bones of a child tend to splinter in a manner similar to the broken branch of a growing tree; this type of fracture is called a greenstick fracture. Late in life the fracture is more likely to be of the brittle type because of the increased stiffness of old bones. The bones of adults who are elderly are also more likely to fail because of the decreased bone mass and density in osteoporosis.
Specific fractures are related to certain ages. For example, fractures of the neck of the femur (commonly called hip fractures) are common in women in the elderly population. Incidence of forearm fracture increases near the peak of the pubescent growth spurt. This is associated with increased porosity of compact bone as the remodelling space increases to provide calcium to the rapid-growth regions. A second increase in wrist fractures is seen in women who are elderly; this is related to the combination of osteoporosis and increased incidence of falls. In developing children, the cartilaginous growth plate may also fail; this most often occurs at the zone of maturation, or hypertrophy (see figure 5.2).
The development of bone has implications for potential injury to the musculoskeletal system. For example, injury to the cartilaginous growth plates can be produced by single excessive forces causing trauma or by repetitive compressive forces, or tensile forces produced by muscle-tendon pull on an area of developing bone.
In sports medicine, specific injuries have been attributed to certain activities. One of the best examples is Little League elbow, described in more detail in chapter 24. Other examples of the relationships between activity and types of growth-plate problems are wrist injuries in gymnasts and tibial tuberosity avulsions in sports involving sprinting and jumping. Pain over the tibial tuberosity (just below the knee cap) in a prepubescent child is indicative of Osgood-Schlatter's condition.
Effects of Various Factors on Range of Motion
Range of motion at joints is affected by joint structure and the mechanical properties of the tissues associated with the joints. A general perception is that joint range of motion decreases during life. Although this is the general trend, the rate of loss is not constant. Ranges of joint motion are very large in a newborn baby. As an example, the range of ankle dorsiflexion in a newborn is limited only by the contact of the top of the foot against the shin. Try this movement yourself to indicate how much flexibility you have lost since birth.
Between the ages of 6 and 15 yr, there is a general trend for joint range of motion to decrease in boys, whereas for girls the effects are variable and joint dependent. Girls are generally more flexible than boys during childhood and adolescence. Changes in joint ranges of motion between adolescence and young adulthood are variable and, as discussed in chapter 6, appear to be related to physical activity. Similarly, the general trend for joint flexibility to decrease during ageing may not be completely explained by biological ageing processes.
Ageing people may suffer arthritis (“inflammation of joints”), which markedly restricts range of motion. Rheumatoid arthritis involves inflammation of the synovial membrane, while osteoarthritis involves degeneration of the articular cartilage (see figure 3.6). In the total adult population, about three times as many women have osteoarthritis as do men. Thus there is some genetic determination of osteoarthritis, to which are added the effects of environmental factors. It would appear that the risk factors for rheumatoid arthritis are mainly related to a family history of the condition.
Age-Related Changes in the Muscular System
Mesoderm, which forms toward the end of the third week of development, eventually forms the muscle and connective tissues of the body. Some of the mesoderm is segmented such that it is in lumps on either side of the midline, and these lumps form the muscles of the trunk. Other mesoderm forms the muscles of the limbs; this tissue migrates into the limbs when they first appear. Limb muscles migrate toward their final position during further development.
Certain precursor cells differentiate into myoblasts (muscle-forming cells) that fuse longitudinally to form the long muscles and muscle fibres we can observe. The number of muscle fibres appears to be genetically determined so that large people often have more muscle fibres than persons who are smaller in stature. As muscles grow in length, the number of sarcomeres increases. Although direct evidence is sparse, many believe that the stimulus for growth of muscle is the bone growth that pulls on the attached ends of the muscle. Developmental growth occurs in both muscle length and cross-sectional area, and these factors may also be affected by activity. The effects of physical training on muscle is summarised in chapters 6, 10, and 13.
In an infant, muscle tissue accounts for about 25% of total body weight, but in a young adult the proportion of muscle is more than 40%. In terms of growth, muscle increases from about 850 g (1.9 lb) at birth to about 30 kg (66 lb) in a young adult male weighing 70 kg (154 lb).
Strength, which may be defined as the capacity to produce force against an external resistance, is related to the cross-sectional areas of the muscles producing the force. Muscle volume peaks at about 30 yr of age and then gradually decreases. The atrophy of skeletal muscle associated with disuse is much faster than any ageing-related atrophy. Muscle elasticity also decreases during ageing so that muscles are stiffer and less extensible. This is one contributing factor to the loss of joint range of motion described earlier.
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Training can increase both muscle strength and fiber contraction speed
Adaptations are common following injury and surgical treatment.
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Biomechanical Adaptations to Injury
Adaptations are common following injury and surgical treatment. One interesting example found in the orthopaedics literature is a phenomenon known as quadriceps avoidance gait. This particular adaptation occurs in ACL-deficient (ACLD) patients; that is, patients who have suffered a complete rupture of the ACL. ACLD patients usually have difficulty with movements involving either lateral thrusts (e.g., cutting from side to side) or twisting (i.e., rotation of the femur relative to the tibia in the transverse plane).
Gait analysis has been done on ACLD patients during various activities of daily living, including level walking, jogging, walking up and down stairs, and even running, cutting, and pivoting. Kinematic and force-plate data were used to estimate the net muscle moments exerted at the knee in all three planes of movement: flexion-extension, abduction-
adduction, and internal-external rotation. Interestingly, the greatest functional changes between ACLD patients and healthy subjects occurred while walking at preferred speeds on level ground. During the midstance portion of the gait cycle, the ACLD patients exhibited a net extensor moment that was significantly lower than normal (figure 10.9, solid line). One interpretation of these findings is that the ACLD patients walk by reducing the demand on their quadriceps during stance (i.e., the quadriceps muscles are activated less in the ACL-deficient patients, giving rise to a quadriceps-avoidance gait pattern).
It is important to realise that a lower net extensor moment at the knee does not necessarily mean that the moment exerted by quadriceps is lower. Because the net moment at the knee is a combination of quadriceps and hamstrings muscle action, a lower extensor moment could result from an increase in the flexor moment applied by hamstrings. However, muscle electromyography results tend to support the interpretation that quadriceps moment is lower because the activation level of the quadriceps is lower in the ACLD patients.
Gait analysis results have also shown that the ACLD patients reduce their net extensor moment by 25% during jogging compared with a 100% reduction in walking. However, the net extensor moment in stair climbing was the same in ACLD patients and normal subjects. When taken together, these results suggest that the observed quadriceps avoidance in ACLD patients is not a result of quadriceps weakness but rather is due to a change in neuromuscular control.
Dependence of Motor Performance on Changes in Muscle Properties
Strength and quickness are critical determinants of performance in explosive movements such as jumping, throwing, and sprinting. Together, these two factors determine the amount of power developed instantaneously during a task (i.e., muscle power is given by the product of muscle force and muscle contraction speed; see “In Focus: At What Speed Must a Muscle Shorten to Develop Maximum Power?”). As discussed earlier in this chapter, resistance strength training can alter both muscle force and fibre contraction speed. This section describes the effects that changes in the contractile properties of muscle have on motor performance, specifically, how biomechanical performance depends on muscle strength and muscle fibre contraction speed.
Vertical jumping is one of the most heavily studied motor tasks. Many scientists have studied jumping with the aim of learning more about the biomechanics and control of this task and, specifically, how performance may be influenced by training. In a recent jump-training program that incorporated stretching, plyometric exercises, and weight lifting, female high school volleyball players demonstrated a mean increase of 4 cm (1.5 in.) in jump height after training. This represented an almost 10% increase in jump height as a result of the 6 wk period of training. Even larger increases in vertical jumping performance have been documented in the literature. One extreme example is the 1984 United States Olympic gold medal volleyball team, which showed a 10 cm (4 in.) increase in jump height after 2 yr of jump training.
Using Computer Modelling to Study Vertical Jumping Performance
It is gratifying to learn that jumping performance can be increased significantly by strength and neuromuscular training. However, it is difficult, if not impossible, to explain why these increases occur. Noninvasive measurements of biomechanical performance cannot pinpoint the factor or factors responsible for the increase in jump height. The reason is that several properties of the neuromuscular and musculoskeletal systems change simultaneously during the training regimen. Alternatively, computer models may be used to study the relationships between training effects and performance; that is, a model of the neuromusculoskeletal system may be used to predict how changes to specific parameters affect the performance of a motor
task.
Models of the body similar to the one shown in figure 8.7 have been used to study how changes in muscle strength, muscle contraction speed, and motor unit recruitment affect jump height. The values of each of these parameters in the model were increased by amounts consistent with results obtained from strength-training programs. For example, the peak isometric strength and maximum shortening velocity of each muscle in the model were increased by 20% and the activation level of each muscle was increased by 10%. The three training effects were first applied to all muscles simultaneously and then to the ankle plantarflexors, knee extensors, and hip extensors separately. In this way, the modelling results were used to determine whether it is better to train all the leg muscles simultaneously or to isolate specific muscle groups such as the quadriceps (knee extensors) and gluteus maximus (hip
extensors).
Insights Into the Effects of Training Provided by Computer Models
Increasing the peak isometric force of all the leg muscles in the model by 20% produced the largest increase in jump height. In this case, jump height increased by 7 cm, or 5% of that obtained before the simulated training effect. Increasing the maximum shortening velocity of each muscle by 20% or the activation level of each muscle by 10% increased jump height by only 4 cm, or 3% of that obtained for the nominal (untrained) model. When all three training effects were introduced simultaneously, jumping performance increased by nearly 17 cm, or 12% of that calculated for the untrained model. Thus, training programs that increase strength, fibre contraction speed, and motor recruitment (i.e., activation level) of all the leg muscles simultaneously are most beneficial to overall jumping performance.
The modelling results also suggest that training the knee extensors is better than training either the ankle plantarflexors or the hip extensors. When peak isometric force and maximum shortening velocity of the quadriceps were increased by 20% and quadriceps activation was simultaneously increased by 10%, jump height increased by almost 10 cm, or 7% of the value calculated for the untrained model. The same changes made to either the ankle plantarflexors or the hip extensors produced increases in jump height of only 3 cm, or 2% of the value obtained for the untrained model.
The latter result is a little puzzling because the model calculations had previously shown that the quadriceps and gluteus maximus are the major energy producers—the prime movers—of the body in vertical jumping (see “In Focus: Which Muscles Are Most Important to Vertical Jumping Performance?” in chapter 8). It should be noted here that each time a change was introduced to the model, a new optimal pattern of muscle activations was found by re-solving the optimisation problem for a maximum-height jump. Thus, one interpretation of the result is that what matters most in terms of performance is the quadriceps:gluteus maximus muscle strength ratio and not the absolute strength of these muscles. In other words, jumping performance is most sensitive to a change in the knee-extensor:hip-extensor muscle strength ratio. Increasing quadriceps strength by 20% increases the knee-extensor:hip-extensor muscle strength ratio in the model, whereas increasing gluteus maximus muscle strength decreases this ratio. The same line of reasoning may be used to explain why increasing ankle plantarflexor muscle strength by 20% leads to an increase in jump height of just 3 cm in the model.
Training for strength is also better than training for quickness or speed. Figure 10.10 shows the effects of increasing muscle strength and muscle contraction speed on vertical jump height as predicted by the four-segment, eight-muscle, sagittal-plane model of the body described in chapter 8 (see figure 8.7). An increase in muscle strength was simulated in the model by simultaneously increasing body weight because muscle strength increases in proportion to muscle mass. Thus, changes in body strength:weight ratio are represented in figure 10.10 rather than changes in muscle strength alone. Also, jumping performance is normalised in these results by dividing by the value of jump height calculated for the untrained model. Similarly, body strength:weight ratio and muscle contraction speed are each normalised by dividing by the value of body strength:weight ratio and muscle contraction speed in the untrained model, respectively.
The simulation results show that the slope of the line predicted for changes in body strength:weight ratio is twice as large as that obtained when changes in muscle fibre contraction speed are made (compare solid and dashed lines in figure 10.10). Thus, muscle strength has a greater effect on vertical jump height than muscle fibre contraction speed, even when the accompanying increase in body mass is taken into account.
Another important lesson learned from the modelling studies is that musculoskeletal changes must be accompanied by appropriate changes in neuromuscular control; otherwise, the expected improvement in motor performance will not be seen. In vertical jumping, if the pattern of muscle activations remains unchanged subsequent to strength training, jump height actually decreases relative to the untrained state. Figure 10.11 shows a simulated jump in which the strength of the knee-extensor muscles has been increased by 20% but the control exerted over the joints is the same as that calculated for the untrained model before the training effect was introduced. When the pattern of muscle activations was not optimised to match the changes introduced to the neuromusculoskeletal model, the body left the ground prematurely (i.e., the centre of mass was at a lower height at liftoff than is optimal for a maximal jump).
One consequence of not optimising the pattern of muscle activations (i.e., the controls) is that a larger fraction of the total work produced by the muscles goes into rotating the body segments rather than accelerating the centre of mass upward. In a maximum-height jump, approximately 90% of the total work done by the leg muscles is used to propel the centre of mass upward. This number is closer to 80% when muscle coordination (i.e., the sequence and timing of muscle activations) is not optimal. So, even though jumping performance depends heavily on muscle strength, and to a lesser extent on muscle fibre contraction speed, optimal performance is also intimately related to neuromuscular control. For this reason, and as discussed at the beginning of this chapter, jump-training programs now focus on maneuvers that blend muscle strength with neural control.
Summary
Training can change both muscle strength and fibre contraction speed. Strength may be increased by increasing motor unit recruitment (net neural drive to the muscle) or by increasing muscle fibre size. Contraction speed may be altered by changing the shape of a muscle's force-velocity curve or by changing the value of its intrinsic maximum shortening velocity. Early in a training program (2-8 wk), increases in muscle strength are brought by neural adaptations rather than by increases in muscle size. Training can also change the way muscle action is coordinated during activity (i.e., neuromuscular control). Neuromuscular training programs are usually designed to improve stability (balance and coordination) and proprioception (joint position sense) in addition to muscle strength. Training is vital for improving biomechanical performance and for preventing injuries during sport.
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The three components of motivation affect exercise adherence
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
The purpose of this chapter is to examine the reciprocal links between psychology and exercise—namely, the effects of psychological functions, such as motivation, on exercise—and the effects of exercise on psychological factors such as feelings of well-being, mood states, and mental performance.
Effects of Psychological Factors on Exercise
The three components of motivation—direction, intensity, and persistence—mentioned in chapter 19 in relation to sport apply to exercise motivation as well. Direction clearly relates to whether an individual chooses the direction of the gym or the couch, the stairs or the elevator, or the pool or the bath. Once the direction of exercise has been chosen, intensity and persistence become important. For example, as a New Year's resolution two individuals may have both decided to join a gym. They have both chosen the direction of exercise. During their first day at the gym, one rides the bicycle for 30 min, tries the stepper and the rowing machine, completes multiple sets on the different weights available, and finishes off with an aerobics class. The other individual begins with just a few minutes on the bicycle and then completes just one set of each of the different weights exercises using light weights. The first person in this example exhibits high intensity and the second person exhibits low intensity. If we stop at this point in the example, you may conclude that the first person is more motivated than the second. Your opinion may change when the third component of motivation—persistence—is considered. Two days later the first individual is home in bed, too sore to move (thinking that exercise is painful and should be avoided whenever possible). The second individual, however, is once again at the gym adding a couple of minutes to his time on the bicycle and sticking to his light-weights workout.
Exercise Participation Motivation
Exercise participation motivation refers primarily to the direction component of motivation. Exercise participation motivation is the initiation of exercise. A variety of factors influence whether individuals initiate an exercise program.
Knowledge, attitudes, and beliefs about exercise influence motivation toward exercise participation. Individuals who understand the importance and value of regular exercise are more likely to initiate an exercise program than those who do not. Similarly, if people have positive attitudes about the value and importance of regular exercise they will have greater motivation to participate in exercise than will people with negative attitudes.
Valuing the importance of exercise, however, is not the only determinant of exercise participation. Beliefs about ourselves influence motivation as well. Even if individuals understand that exercise is important, they will be unlikely to begin an exercise program if they believe that they cannot succeed at it. If they believe that the exercise program is too difficult or that it requires more fitness, strength, coordination, or time than they have, it is doubtful that they will join the program. This confidence in one's ability to succeed at an exercise program is called exercise self-efficacy. It is logical that self-efficacy would influence behaviour. How likely would people be to do something they were convinced they could not do, particularly if they had to pay to do it? How likely would they be to invest any energy in pursuing that activity? Most people in this situation would not attempt the activity. People who have such feelings are described as having low self-efficacy. As self-efficacy, or one's belief in one's ability to succeed at a particular task, increases, so does the likelihood of undertaking that task.
How can we enhance exercise participation motivation? Educating people about the importance and value of exercise can be a valuable first step because individuals who understand the merit of exercise are more likely to adopt an exercise program. Unfortunately, imparting knowledge is not enough. A lot of us do things that we know are not good for us and, similarly, do not do things that we know are good for us. Enhancing the exercise self-efficacy of individuals will increase their motivation. Demonstrating how individuals can control their own activity is useful. Some people have low self-efficacy about exercise because they believe that they are too unfit to begin exercising. They may equate exercise with young, thin, Lycra-clad gym enthusiasts whom they see in the media. Programs that emphasise choice of activities, illustrate exercisers similar in age and fitness level to the potential exercisers, and reveal that exercising can be enjoyable may increase exercise participation motivation. Exercise programs that begin with activities that the individuals already know they are capable of doing, such as walking and climbing stairs, may also increase self-efficacy and thereby increase motivation. Imagery may also help individuals enhance their self-efficacy beliefs (see “Imagery, Exercise, and Self-Efficacy”).
Exercise Adherence Motivation
Although many people get motivated to begin an exercise program, many of the people who begin fail to continue. Approximately 50% of individuals who begin a regular physical-activity program drop out within the first 6 mo. These people had exercise participation motivation, but they lack exercise adherence motivation, the persistence component of motivation.
Biological, psychological, sensory, and situational factors all interact to influence exercise adherence. Biologically, body composition, aerobic fitness, and the presence of disease influence adherence. Unfortunately, it is usually the people who could gain the most from exercise who are the least likely to adhere. Overweight or obese, unfit, or chronically ill people are less likely to adhere to an exercise program than are thinner, fitter, and healthier people.
As is the case with exercise participation motivation, attitudes and beliefs influence exercise adherence motivation. Attitudes and beliefs about the importance of exercise play a role in adherence, but so too do individuals' expectations about the effects that exercise is having on them personally. For example, if individuals believe that major changes in fitness and body composition should occur after 6 wk of regular exercise and they do not perceive major improvement in their own bodies after 6 wk, they may believe that exercise does not do what it should and therefore quit. Even though it is unrealistic to believe that 6 wk of exercise can make up for 6 yr of inactivity, it is individuals' beliefs, not reality, that influence behaviour. Therefore, when introducing newcomers to exercise it is important that they have realistic expectations about the time and effort required and the anticipated effects of the proposed program.
In addition to attitudes and beliefs, other psychological factors influence exercise adherence motivation. Extroverts (people who are social and outgoing) tend to adhere to exercise programs better than do introverts. Exercise programs that are executed in the presence of other people are probably more comfortable for extroverts than for introverts. Extroverts tend to enjoy the interaction with class members and exercise partners, possibly encouraging their adherence. Introverts, on the other hand, may adhere better to individual, home-based exercise programs. The bulk of the research on exercise adherence has involved programs that take place on site at fitness facilities with other people. This setting may have led to the conclusion that extroverts are better adherers than introverts. If the research had been done on independent, home-based exercise programs, it might have been found that introverts were better adherers. Clearly efforts need to be made to match the social environment of the exercise program to the personality of the exerciser.
Individuals with high levels of self-motivation are more likely to adhere to exercise programs than are individuals with low levels of self-motivation. It is logical that highly self-motivated people have better adherence rates. The challenge is to help those individuals with low levels of self-motivation. One of the most effective methods of helping these individuals is to encourage their involvement in the goal-setting process.
Goal Setting
Setting goals can help enhance motivation for a number of reasons. Goal setting addresses all three components of motivation. Goals give direction by providing a target. Intensity and effort also can be enhanced because goals provide reasons for participating in the activity. We are all more likely to put in effort when we feel there is a reason for doing so. If individuals are given two jobs to do at work, one that has a particular target and objective and one that seems vague and purposeless, into which job are they more likely to put their effort? Having goals helps focus attention and effort. In addition, goals can augment persistence by fostering new strategies. If individuals have a goal to which they are committed and initial tactics appear unsuccessful, they will search for alternative strategies to achieve their aim. If the goal had not been set in the first place, instead of persisting with different plans of action, they would likely give up.
Goals are also beneficial because they reflect improvement. Too often people make short-term comparisons regarding their strength, fitness, flexibility, or weight. Because the positive effects of exercise take time to emerge, improvements being made are often not noticed when people use a short timeframe for comparison. If a goal is achieved, evidence of improvement exists.
Goal setting involves a number of steps:
- setting the goal,
- setting a target date by which to achieve the goal,
- determining strategies to achieve the goal, and
- evaluating the goal on a regular basis.
If a target date is not set (e.g., “One of these days I'll ride the exercise bike continuously for an hour”), the goal is really just a dream. For this idea to be a goal, the individual needs to set a specific date by which to achieve the behaviour. Goal setting usually involves both long-term and short-term goals. The long-term goal provides direction; the short-term goals provide the increase in intensity and effort. People often err by setting only long-term goals. They begin to work toward achieving the goal, but success seems so far away that they give up before they get there. If someone had decided to ride a bike for 1 h and was currently having trouble lasting 10 min, 1 h would seem virtually impossible. Achieving short-term goals along the way to the long-term goal boosts confidence and motivation because it is obvious that the effort is worthwhile because improvement is being made. Target dates are set for each short-term goal in a progressive order until the long-term goal is achieved. This pattern of goal setting can be considered as a staircase, where each short-term goal is a step on the way to the long-term goal (see figure 20.1).
For goals to be effective, however, they need to be properly set. Goals can be considered to be good if they meet certain criteria (table 20.1). Goals need to be challenging but realistic. If goals are not challenging, they probably are not requiring any real change in behaviour and therefore will have little effect. However, if goals are so challenging that they are unrealistic, people are setting themselves up for failure. Continued failure leads to lowered confidence and less motivation.
Goals also need to be specific and measurable. Saying “I want to be fitter” or “My goal is to be stronger” does not provide any way of knowing when success has been achieved. What is “fitter”? How strong is “stronger”? There needs to be some way of knowing whether the goal has been achieved when the target date arrives. The easiest way of making goals specific is to make goals numerical. Numbers can easily be used to quantify time spent exercising, distance travelled, repetitions accomplished, weight lifted, or exercise sessions attended.
In addition to being specific and measurable, goals need to be positive. If an exerciser makes it her goal not to recline and rest during the abdominals section of her aerobics class, she will be thinking about reclining and resting. If instead she makes it her goal to complete first 1 min and then 2 min of the abdominal exercises, she will be thinking about doing the exercises, thus increasing her chances of doing them. If we tell you not to think about pink elephants, what is the first thing you think about? Similarly, if you set a goal of not letting your back arch off the bench when you do bench presses, you will be thinking about your back arching. When you think about your back arching, your brain may be sending messages to the muscles that make your back arch (just like in the psychoneuromuscular theory of imagery discussed in chapter 19). By making your goal “not arching,” you may actually be increasing the likelihood of arching your back. A goal of keeping your back pushed flat against the bench would be much more effective. Goals should stipulate the desired behaviour. Positively worded goals help you think about, plan for, and prepare to do what it is you want to do.
Read more from Biophysical Foundations of Human Movement, Third Edition by Bruce Abernethy, Vaughan Kippers, Stephanie Hanrahan, Marcus Pandy, Ali McManus, and Laurel Mackinnon.