Advanced Analysis of Motor Development

Applying the Dynamic Systems Model to a Motor Development Problem

Perhaps the best way to appreciate how the characteristics of dynamic systems can be used to model motor development is to see how a research team used dynamic systems to address a particular developmental issue. Let's consider the case of the reflexes seen in infancy. Recall that a reflex is a particular automatic response to a specific stimulus. It is a response that is carried out without thought or intention. Developmentalists have long observed the existence of reflexes that disappear later on in infants.

Developmentalists have been particularly interested in the walking or stepping reflex for its similarity to voluntary walking (see table 3.1). The reflex is elicited by holding an infant upright and lowering the infant to a horizontal surface.

The reflexive response is stepping movements of the legs. The stepping reflex is typically observed up to 5 months of age, so it appears much earlier than voluntary walking and disappears months before voluntary walking.

Developmentalists have offered several explanations for the disappearance of the stepping reflex. The maturationists suggested that the disappearance reflects the maturation of the cortex. These researchers proposed that as higher brain centers mature they are able to inhibit the lower brain centers that mediate reflexes. With continued maturation, the centers control voluntary walking. The existence and disappearance of reflexes reflect the status of nervous system development.

Peiper (1963) suggested instead that reflexes have a purpose because they allow practice of movement patterns before the higher brain centers are mature enough to control those patterns. Zelazo et al. (Zelazo, 1983; Zelazo, Konner, Kolb, & Zelazo, 1974; Zelazo, Zelazo, & Kolb, 1972) offered yet another viewpoint. They purposely increased the number of times the stepping reflex was elicited in a small group of infants. Later, these infants were observed to start walking at a younger age than average. The researchers concluded that reflexes can be subsumed into voluntary movements. For example, the walking reflex is transformed into voluntary walking.

Unconvinced by these various hypotheses, Esther Thelen and her colleagues (Thelen & Fisher, 1982; Thelen, Fisher, & Ridley-Johnson, 1984) developed an ingenious series of experiments to examine different potential causes for the disappearance of the stepping reflex. They began with the notion that multiple systems—rather than just one system—play a role in the disappearance. They had observed that one of the changes in infants correlating with the disappearance of the stepping reflex is an increase in subcutaneous fat that occurs during infancy. Could this gain in fat and the resultant change in the strength required to move heavier legs be related to the disappearance of the stepping reflex? In order to test whether the muscle and adipose tissue systems are involved, the researchers had to find a way to make a light and stepping infant resemble a nonstepping and heavier infant and, just as importantly, to get a non-stepping, heavier infant to resemble a lighter, stepping infant. The only variable that could be changed was leg strength. In a first experiment, the researchers added tiny weights to the legs of lighter infants. The added weight was proportional to the amount of weight the infant would add between 4 and 6 weeks of age. If maturation of the nervous system were the only cause of reflex stepping, then the infants should continue to step regardless of the small weights. Actually, the infants reduced the number of reflex steps taken while wearing the weights.

The next experiment was designed to regress the leg weight of heavier infants to that of younger, lighter infants. The researchers achieved this by holding the infants upright in an aquarium filled with water up to their hips. The water reduced the pull of gravity on the legs and made them relatively lighter. The water was room temperature, and the researchers noted that often the infants did not respond in any way to being held in the water. When the infants' feet touched the bottom surface of the tank, though, the number of steps increased (Thelen & Fisher, 1982). If maturation of the nervous system is the only explanation
for the reflex disappearing, then nothing should have been able to increase the rate of stepping, as the higher brain centers would have inhibited it. Thus the researchers demonstrated that change in other systems plays a role in the disappearance of the stepping reflex.

Thelen et al. demonstrated that the systems approach can be used to examine the processes underlying development. While maturation of the nervous system is important in motor development, other subsystems are important to the change observed in the stepping reflex. As well as studying multiple subsystems, Thelen et al. demonstrated that change in a subsystem can function as a control variable. The role of a control variable in bringing about change is another characteristic of dynamic systems. More of Thelen's work will be discussed throughout the text.

Summary

The systems approach has much to offer researchers interested in understanding developmental change. Dynamic systems evolve over time and undergo change that is not linear. These characteristics describe the body as it grows and ages. Dynamic systems evolve by transitioning from times of stability, or attractor states, through times of instability to other attractor states. We observe this pattern in motor development, too. Dynamic systems are sensitive to the conditions initially in place when they begin to evolve. Movement scientists have long realized that models of movement control must allow for the same movement goal to be achieved from variable environmental conditions and different starting positions of the body and limbs. Most importantly, dynamic systems have been shown to self-organize and to use methods of control such as entrainment and the application of constraints. The demonstration of these possibilities is important, as they provide explanations for how the large number of degrees of freedom in movement can be reduced for the reasonable control of complex movements.

The systems approach provided a heuristic function for movement scientists by focusing researchers' attention on new ways to solve existing questions in motor development. There is value in examining multiple theories. Researchers using differing approaches all serve to gain from alternative views, as both theorists and empiricists are forced to question behaviors their theories can't account for and improve their models until a satisfactory explanation can be found (for example, see Schmidt & Lee, 2011 on generalized motor programs). Ultimately, these challenges and changes serve to enhance our understanding of motor development.

Development of Ballistic Skills

Movements can be classified by their EMG and force curve signatures. Three movement categories are (1) ramped movements in which force is gradually and steadily applied through continuously graded muscle firing; (2) discontinuous, slow movements that accelerate gradually, then decelerate, and then accelerate again, such as when tracking an object with a hand; and (3) ballistic movements that are rapid, all-out, forceful movements that reach peak acceleration within milliseconds of their initiation (Brooks, 1983; Desmedt, 1983).

In simple ballistic movements, such as a rapid reach for an object, the motion is started by an initial strong impulse in the agonist muscle. The agonist firing then stops, but the movement continues due to the momentum of the limb. A burst by the antagonist muscle subsequently causes the limb to decelerate. In targeting tasks, there may be a second burst in the agonist to correct the limb placement. This creates a signature triphasic pattern of muscle activation: agonist, antagonist, agonist (Palmer, Cafarelli, & Ashby, 1994).

In contrast to single-limb movements, ballistic skills involving the total body are quite complex. Typical sport skills are examples of ballistic skills. Of these, movements in which the performer projects an object have been of considerable interest to both biomechanists and developmentalists. Striking (as in batting and swinging a racket), kicking (as in football and soccer), and throwing (as in handball and baseball or softball) have all received study. These complex movements tend to begin proximally with action of the trunk muscles; energy is then passed from segment to segment of the open kinetic chain until it reaches the distal effector, usually the hand or foot. During striking, kicking, and throwing, this energy exchange is superimposed on a running or stepping base of support.

Another interesting commonality across these complex projectile skills is that they employ a backswing to place the body segments in position to move forward. In the advanced form of striking, throwing, and kicking, the backswing and forward swing partially overlap, and proximal segments begin to move forward while more distal segments are still moving backward. The advanced form of these movements also involves strong acceleration of the distal segment just milliseconds before contact or release, which yields high distal angular velocity. This acceleration seems to result partially from the inertia of each distal segment against the motion of adjacent proximal segments. The lagging segment stretches the agonist muscles of the distal segment, which may in turn excite reflexes that augment the muscular contractions in the distal segment (Roberts & Metcalfe, 1968). As we will see, lag is one aspect of object projection that develops only gradually over time. It is so important for effective ballistic movement that Southard (2002a, 2002b) used lag as a collective variable in his studies of throwing.

In this chapter we focus on the development of striking, throwing, and kicking. Most developmental studies on ballistic projectile skills have been based in developmental sequence theory, although a few studies are beginning to address issues from a dynamic systems perspective. At the close of this chapter we discuss future directions in this area.

Intertask Developmental Sequence

The only ballistic skills that have been studied in relation to their intertask development are overarm throwing and striking. The approach used in this research is quite different from the intertask approach used to study locomotion (see chapter 6). For locomotion, the developmental sequence is formed by ordering each locomotor skill according to its initial appearance in the movement repertoire. For throwing and striking, the two tasks are compared to see where participants are located within a component developmental sequence at the same point in time in each skill. Do the participants show the same developmental level in the two skills, or do they develop faster in one skill than they do in the other?

Langendorfer (1987a) compared the development of trunk and humerus action in overarm throwing with development of the same action in overarm striking. He asked 58 boys aged 1.5 to 10.3 years to throw a ball forcefully and to strike a tennis ball suspended 6 inches (15 cm) above their head forcefully with a lightweight racket. He then looked at side and rear film views to categorize the boys' movements in each skill. Figure 7.1 shows his results. From the cross-sectional data we can infer how the longitudinal data might look. The data on trunk action (see figure 7.1a) suggested that children advance in throwing before they do in striking: The boys started out at a primitive level in both skills and then advanced to level 2 throwing before achieving level 2 striking and advanced to level 3 throwing before reaching level 3 striking. The cross-sectional data on humerus action (see figure 7.1b) suggested that children begin at level 1 in both skills and then move ahead in either skill until they reach level 2 in both skills. Then, however, they progress to level 3 in throwing before they progress to level 3 in striking.

Langendorfer (1987a) learned from his data that he could not compare forearm action between striking and throwing. Rather, the action of the racket, which is the distal segment in striking, was more comparable to the forearm action of throwing. Figure 7.1c shows the developmental comparison for forearm action. While some children exhibited different relationships early on, more progressed in striking before they progressed in throwing. Then throwing took the developmental lead for more children.

The finding on forearm action demonstrates how equipment can affect a person's developmental level. Using Newell's notions of person constraints (see chapter 3), we can think of equipment as an extension of the body. Comparing hammering with a hammer with pounding with a fist demonstrates how an implement changes the inertial characteristics of the arm and hand. This analysis also applies to prosthetic limbs. In both cases the performer needs to learn how to incorporate the implement into the actual movement pattern. Langendorfer's (1987a) data suggest that at first the implement assists the child in achieving a higher developmental level in the distal component, probably because its greater inertia causes lag. Later, though, the handle length may hinder change to a higher developmental level by making it more difficult to delay that lag until the body has reached front facing (see tables 7.1 and 7.4).

The last component that Langendorfer (1987a) compared across the two skills was stepping. As we can see in figure 7.1d, the developmental relationship for stepping was less clear. Langendorfer attributed this result to a degree of unreliability in the component sequence: Stepping tended to be variable across trials in both throwing and striking, making intertask comparisons unreliable.

The arrows in figure 7.1 reflect Langendorfer's (1987a) speculation on the longitudinal course that these two tasks might display while developing. The only actual longitudinal information that we have regarding this question was reported by Roberton (1982). She compared the development of trunk action within sidearm striking (as opposed to overarm striking) and overarm throwing. Interestingly, the trunk action of four children did follow the developmental path that Langendorfer's cross-sectional overarm data suggested: The children progressed one level in their throwing trunk action and then caught up to that same level in their striking trunk action. Then they progressed again in throwing before catching up in striking.

Motor Development Differences in Children With Autism Spectrum Disorders

One of the great difficulties facing practitioners and researchers working with individuals with ASD is the degree to which variability supersedes universality in this population (Pope et al., 2010). That is, two individuals with the same diagnosis of ASD will act and react very differently from each other. These individual differences lead to significant challenges for researchers trying to identify and quantify motor development in this population. For example, individuals with ASD may respond differently to a controlled experimental design due to different sensitivity to sensory stimuli or a lack of understanding of the described procedures. Furthermore, each individual responds differently to different interventions. All in all, typical developmental trajectories are all but impossible to determine. However, researchers have identified some general characteristics related to movement and motor development that suggest that individuals with ASD move differently from their typically developing counterparts (Pope et al., 2010). The importance of determining motor differences, particularly in younger children, cannot be underscored enough: If early developing motor differences exist, then assessment of these deficits may aid in identifying children at risk for ASD at a younger age than is currently possible.

Primarily through retrospective video analysis, researchers have identified early motor differences in children with ASD. Kanner (1943), who first identified ASD, noted that the infants exhibited hypotonia, or low muscle tone (similar to infants with DS). Other researchers have also observed hypotonia (Adrien et al., 1993; Ming, Brimacombe, & Wagner, 2007). For example, Ming et al. (2007) reviewed the clinical records of 154 children diagnosed with ASD in an attempt to determine the types and prevalence of motor impairments in this population, particularly hypotonia, delayed motor milestones, coordination issues, and toe walking. Children ranged in age from 2 to 18 years, with a mean age of 6 years. Reflecting the gender differences within the ASD population, there were 126 males and 28 females within the study. All of the children were on the autism spectrum: 74 were diagnosed with autism, 70 with PDD-NOS, and 10 with AS. The researchers determined that 51% of the group demonstrated hypotonia as infants; the prevalence of hypotonia decreased over time to 38% in the age range of 7 to 18 years. Not all retrospective studies have indicated that infants with autism have hypotonia. Saint-Georges et al. (2010) performed an exhaustive research review using retrospective video analysis and determined that across the 41 studies reviewed, hypotonia was not a consistent finding.

Given that at least a percentage of children with ASD exhibit hypotonia, subsequent issues with the acquisition of motor milestones may exist (as in children with DS). Low muscle tone affects infants' ability to raise the head while in the prone position, which affects the ability to sit supported, and so on. In fact, another consistent finding among individuals with ASD is a delay in achieving motor milestones and fundamental motor skills as young children. Acknowledging the limitations of looking back at records after diagnosis, Landa and Garrett-Mayer (2006) performed a 2-year longitudinal prospective study in which they followed a group of 84 infants who were either high (n = 60) or low (n = 27) risk for ASD.

Infants were tested at 6, 14, and 24 months using the Mullen Scales of Early Learning (MSEL), a standardized developmental test for children aged 0 to 69 months. The MSEL consists of five subscales: gross motor, fine motor, visual reception, receptive language, and expressive language. At 2 years, all children were given the Autism Diagnostic Observation Schedule (ADOS), which is a semistructured, play-based interview that provides systematic probes for symptoms of autism in social interaction, communication, play, and repetitive behaviors. Of the 60 children deemed at risk, 21 met the ADOS algorithm criteria for ASD as well as received a clinical diagnosis of ASD; 2 children from the low-risk group met the criteria and were included in the high-risk group. Early on, no statistical differences existed between groups. However, by 24 months, the high-risk group showed delays in both gross and fine motor skills, and group members were significantly different from the typically developing group in all domains.

In related research, a group of investigators assessed motor development in children younger than 3 years of age (Provost, Lopez, & Heimerl, 2007). In this study, 19 children with ASD, ranging in age from 21 to 41 months, were examined with the Bayley Scales of Infant Development II (BSID II; Bayley, 1993) and the Peabody Developmental Motor Scales, Second Edition (PDMS-2; Folio & Fewell, 2000). The researchers determined that 84% of the children were significantly delayed on the BSID II and PDMS-2. In a similar study, Ozonoff et al. (2008) examined the early movement behavior of children later identified as ASD and reported a delay in the appearance of motor milestones, including lying in prone and supine position, rolling, sitting, crawling, and walking, with the most significant delay occurring in walking onset. Therefore, it appears that delays in motor milestones exist in many of the children diagnosed with
ASD.

Acknowledging that delays appear to exist in the ASD population, Ozonoff et al. followed strong inference (see chapter 4) and asked the next logical question: Can motor delays differentiate children with autism from children who have more general developmental delays? If motor delays could be used for differentiation, then practitioners could use motor delays to identify infants at risk for ASD. In the resulting study, Ozonoff et al. (2008) placed 103 children between 24 and 60 months of age in 1 of 3 groups: children with ASD, children with developmental delays (DD), or children with typical development (TD). Home videotapes of the participants were analyzed and coded with the Infant Motor Maturity and Atypicality Coding Scales (IMMACS), which rate six motor milestones (lying in prone and supine position, roll, sit, crawl, walk) as well as protective skills (e.g., righting the body after losing balance). What Ozonoff et al. (2008) found, in contrast to earlier reports and speculation, is that the only group that differed in terms of number of movement abnormalities and protective skills was the DD group; the ASD and TD groups were not significantly different. The ASD and DD groups both showed motor delays, but they were not significantly different from each other. Therefore, the notion that motor delays can be used to identify children at risk for ASD was not supported in this research. Obviously, a prospective study examining both qualitative and quantitative variables is necessary as a next step to confirm this finding.

Are individuals with ASD merely delayed in motor skill acquisition, or do they differ in the ways in which they move? Several research studies (see table 9.5) have found at least one consistent difference between children with ASD and children without ASD, and that is their walking patterns. Over three decades ago, researchers found that children with ASD demonstrate abnormal limb movements, shortened steps, and persistent toe walking (Damasio & Maurer, 1978; Vilensky, Damasio, & Maurer, 1981). More recently, Ming et al. (2007), in their retrospective study, determined that nearly 20% of children with ASD walked on their toes rather than on their entire foot. Esposito and Venturi (2008) used retrospective video analysis as well as an observational scale to examine 42 children with ASD who had been walking at least 6 months and also found differences in early walking patterns. Vernazza-Martin et al. (2005) found significant differences in gait when comparing 15 children aged 4 to 6 years with and without ASD, as did Woodward (2001) in a study of children with ASD aged 3 and 10 years. One consistent finding is that the gait cycle is slower and less consistent in children with ASD.

Dewey, Cantell, and Crawford (2007) compared the performance of children with different disabilities on the Bruininks-Oseretsky Test of Motor Proficiency-Short Form. Participants included 49 children with ASD, 46 children with developmental coordination disorder (DCD) and attention-deficit/hyperactivity disorder (ADHD), 38 children with DCD, 27 children with ADHD, and 78 children with typical development. The results indicated that although all the atypical groups displayed significant impairment of motor skills, children with ASD were significantly more impaired compared with their cohorts with specific motor skill deficits. They were also the only group to show impairment on gestural skills. Several researchers have found deficiencies in fine motor skills in children with ASD. These range from delays in manual dexterity (Miyahara et al., 1997) and graphomotor skills (Mayes & Calhoun, 2003) during early and middle childhood to motor control issues in prehension (Mari, Castiello, Marks, Marraffa, & Prior, 2003). Such fine motor skill deficits influence handwriting as well as many functional activities involving the hands and arms. Across these studies, it appears that individuals with autism do move differently when compared with their typically developing counterparts and that deficits range from fine to gross motor skills.

In sum, although motor deficits are not listed as part of the DSM-V diagnostic criteria for ASD, individuals with these disorders tend to exhibit motor skill deficiencies as well as differences in motor development. Infants with ASD may exhibit hypotonia (although this finding is not as robust as it is in infants with DS). In addition, delays may exist in the acquisition of motor milestones and fundamental motor skills. Differences in skills such as gait and manual dexterity persist into childhood.

Developmental Coordination Disorder

Another disorder with an unknown etiology that has garnered considerable empirical attention as of late is DCD (Wilson & Larkin, 2008). DCD, also known as developmental dyspraxia, is characterized by extreme lack of motor coordination and by other movement deficits in the absence of neurological defects. DCD has been calculated to affect as much as 6% of the elementary school population (American Psychiatric Association, 1994; Barnhart, Davenport, Epps, & Nordquist, 2003; Barnett, Kooistra, & Henderson, 1998). Although the disorder was described as early as 1937, it took until 1994 for a group of 43 experts in various movement disorder fields to come to a consensus on a name and description for DCD in order to facilitate both research and diagnosis (see the sidebar for the DSM-V description of DCD). Individuals characterized as having DCD fall into the 0 to 10th percentile of performance on the Movement Assessment Battery for Children (M-ABC), a standardized motor skills test used to assess motor proficiency in children with disabilities (Geuze, Jongmans, Schoemaker, & Smits-Engelsman, 2001; Henderson & Sugden, 1992; Miyahara & Möbs, 1995). Because DCD is categorized as a learning disability, it also may influence a child's ability to perform academically. Motor difficulties can range from gross to fine motor to balance skills and can include motor planning deficits and visual or spatial difficulties (Cermak & Larkin, 2002; de Castelnau, Albaret, Chaix, & Zanone, 2007; Kaplan, Wilson, Dewey, & Crawford, 1998; Przysucha & Taylor, 2004; Whitall et al., 2006). Table 9.6 provides examples of functional motor issues that children with DCD may have.

Applying the Dynamic Systems Model to a Motor Development Problem

Perhaps the best way to appreciate how the characteristics of dynamic systems can be used to model motor development is to see how a research team used dynamic systems to address a particular developmental issue. Let's consider the case of the reflexes seen in infancy. Recall that a reflex is a particular automatic response to a specific stimulus. It is a response that is carried out without thought or intention. Developmentalists have long observed the existence of reflexes that disappear later on in infants.

Developmentalists have been particularly interested in the walking or stepping reflex for its similarity to voluntary walking (see table 3.1). The reflex is elicited by holding an infant upright and lowering the infant to a horizontal surface.

The reflexive response is stepping movements of the legs. The stepping reflex is typically observed up to 5 months of age, so it appears much earlier than voluntary walking and disappears months before voluntary walking.

Developmentalists have offered several explanations for the disappearance of the stepping reflex. The maturationists suggested that the disappearance reflects the maturation of the cortex. These researchers proposed that as higher brain centers mature they are able to inhibit the lower brain centers that mediate reflexes. With continued maturation, the centers control voluntary walking. The existence and disappearance of reflexes reflect the status of nervous system development.

Peiper (1963) suggested instead that reflexes have a purpose because they allow practice of movement patterns before the higher brain centers are mature enough to control those patterns. Zelazo et al. (Zelazo, 1983; Zelazo, Konner, Kolb, & Zelazo, 1974; Zelazo, Zelazo, & Kolb, 1972) offered yet another viewpoint. They purposely increased the number of times the stepping reflex was elicited in a small group of infants. Later, these infants were observed to start walking at a younger age than average. The researchers concluded that reflexes can be subsumed into voluntary movements. For example, the walking reflex is transformed into voluntary walking.

Unconvinced by these various hypotheses, Esther Thelen and her colleagues (Thelen & Fisher, 1982; Thelen, Fisher, & Ridley-Johnson, 1984) developed an ingenious series of experiments to examine different potential causes for the disappearance of the stepping reflex. They began with the notion that multiple systems—rather than just one system—play a role in the disappearance. They had observed that one of the changes in infants correlating with the disappearance of the stepping reflex is an increase in subcutaneous fat that occurs during infancy. Could this gain in fat and the resultant change in the strength required to move heavier legs be related to the disappearance of the stepping reflex? In order to test whether the muscle and adipose tissue systems are involved, the researchers had to find a way to make a light and stepping infant resemble a nonstepping and heavier infant and, just as importantly, to get a non-stepping, heavier infant to resemble a lighter, stepping infant. The only variable that could be changed was leg strength. In a first experiment, the researchers added tiny weights to the legs of lighter infants. The added weight was proportional to the amount of weight the infant would add between 4 and 6 weeks of age. If maturation of the nervous system were the only cause of reflex stepping, then the infants should continue to step regardless of the small weights. Actually, the infants reduced the number of reflex steps taken while wearing the weights.

The next experiment was designed to regress the leg weight of heavier infants to that of younger, lighter infants. The researchers achieved this by holding the infants upright in an aquarium filled with water up to their hips. The water reduced the pull of gravity on the legs and made them relatively lighter. The water was room temperature, and the researchers noted that often the infants did not respond in any way to being held in the water. When the infants' feet touched the bottom surface of the tank, though, the number of steps increased (Thelen & Fisher, 1982). If maturation of the nervous system is the only explanation
for the reflex disappearing, then nothing should have been able to increase the rate of stepping, as the higher brain centers would have inhibited it. Thus the researchers demonstrated that change in other systems plays a role in the disappearance of the stepping reflex.

Thelen et al. demonstrated that the systems approach can be used to examine the processes underlying development. While maturation of the nervous system is important in motor development, other subsystems are important to the change observed in the stepping reflex. As well as studying multiple subsystems, Thelen et al. demonstrated that change in a subsystem can function as a control variable. The role of a control variable in bringing about change is another characteristic of dynamic systems. More of Thelen's work will be discussed throughout the text.

Summary

The systems approach has much to offer researchers interested in understanding developmental change. Dynamic systems evolve over time and undergo change that is not linear. These characteristics describe the body as it grows and ages. Dynamic systems evolve by transitioning from times of stability, or attractor states, through times of instability to other attractor states. We observe this pattern in motor development, too. Dynamic systems are sensitive to the conditions initially in place when they begin to evolve. Movement scientists have long realized that models of movement control must allow for the same movement goal to be achieved from variable environmental conditions and different starting positions of the body and limbs. Most importantly, dynamic systems have been shown to self-organize and to use methods of control such as entrainment and the application of constraints. The demonstration of these possibilities is important, as they provide explanations for how the large number of degrees of freedom in movement can be reduced for the reasonable control of complex movements.

The systems approach provided a heuristic function for movement scientists by focusing researchers' attention on new ways to solve existing questions in motor development. There is value in examining multiple theories. Researchers using differing approaches all serve to gain from alternative views, as both theorists and empiricists are forced to question behaviors their theories can't account for and improve their models until a satisfactory explanation can be found (for example, see Schmidt & Lee, 2011 on generalized motor programs). Ultimately, these challenges and changes serve to enhance our understanding of motor development.

Development of Ballistic Skills

Movements can be classified by their EMG and force curve signatures. Three movement categories are (1) ramped movements in which force is gradually and steadily applied through continuously graded muscle firing; (2) discontinuous, slow movements that accelerate gradually, then decelerate, and then accelerate again, such as when tracking an object with a hand; and (3) ballistic movements that are rapid, all-out, forceful movements that reach peak acceleration within milliseconds of their initiation (Brooks, 1983; Desmedt, 1983).

In simple ballistic movements, such as a rapid reach for an object, the motion is started by an initial strong impulse in the agonist muscle. The agonist firing then stops, but the movement continues due to the momentum of the limb. A burst by the antagonist muscle subsequently causes the limb to decelerate. In targeting tasks, there may be a second burst in the agonist to correct the limb placement. This creates a signature triphasic pattern of muscle activation: agonist, antagonist, agonist (Palmer, Cafarelli, & Ashby, 1994).

In contrast to single-limb movements, ballistic skills involving the total body are quite complex. Typical sport skills are examples of ballistic skills. Of these, movements in which the performer projects an object have been of considerable interest to both biomechanists and developmentalists. Striking (as in batting and swinging a racket), kicking (as in football and soccer), and throwing (as in handball and baseball or softball) have all received study. These complex movements tend to begin proximally with action of the trunk muscles; energy is then passed from segment to segment of the open kinetic chain until it reaches the distal effector, usually the hand or foot. During striking, kicking, and throwing, this energy exchange is superimposed on a running or stepping base of support.

Another interesting commonality across these complex projectile skills is that they employ a backswing to place the body segments in position to move forward. In the advanced form of striking, throwing, and kicking, the backswing and forward swing partially overlap, and proximal segments begin to move forward while more distal segments are still moving backward. The advanced form of these movements also involves strong acceleration of the distal segment just milliseconds before contact or release, which yields high distal angular velocity. This acceleration seems to result partially from the inertia of each distal segment against the motion of adjacent proximal segments. The lagging segment stretches the agonist muscles of the distal segment, which may in turn excite reflexes that augment the muscular contractions in the distal segment (Roberts & Metcalfe, 1968). As we will see, lag is one aspect of object projection that develops only gradually over time. It is so important for effective ballistic movement that Southard (2002a, 2002b) used lag as a collective variable in his studies of throwing.

In this chapter we focus on the development of striking, throwing, and kicking. Most developmental studies on ballistic projectile skills have been based in developmental sequence theory, although a few studies are beginning to address issues from a dynamic systems perspective. At the close of this chapter we discuss future directions in this area.

Intertask Developmental Sequence

The only ballistic skills that have been studied in relation to their intertask development are overarm throwing and striking. The approach used in this research is quite different from the intertask approach used to study locomotion (see chapter 6). For locomotion, the developmental sequence is formed by ordering each locomotor skill according to its initial appearance in the movement repertoire. For throwing and striking, the two tasks are compared to see where participants are located within a component developmental sequence at the same point in time in each skill. Do the participants show the same developmental level in the two skills, or do they develop faster in one skill than they do in the other?

Langendorfer (1987a) compared the development of trunk and humerus action in overarm throwing with development of the same action in overarm striking. He asked 58 boys aged 1.5 to 10.3 years to throw a ball forcefully and to strike a tennis ball suspended 6 inches (15 cm) above their head forcefully with a lightweight racket. He then looked at side and rear film views to categorize the boys' movements in each skill. Figure 7.1 shows his results. From the cross-sectional data we can infer how the longitudinal data might look. The data on trunk action (see figure 7.1a) suggested that children advance in throwing before they do in striking: The boys started out at a primitive level in both skills and then advanced to level 2 throwing before achieving level 2 striking and advanced to level 3 throwing before reaching level 3 striking. The cross-sectional data on humerus action (see figure 7.1b) suggested that children begin at level 1 in both skills and then move ahead in either skill until they reach level 2 in both skills. Then, however, they progress to level 3 in throwing before they progress to level 3 in striking.

Langendorfer (1987a) learned from his data that he could not compare forearm action between striking and throwing. Rather, the action of the racket, which is the distal segment in striking, was more comparable to the forearm action of throwing. Figure 7.1c shows the developmental comparison for forearm action. While some children exhibited different relationships early on, more progressed in striking before they progressed in throwing. Then throwing took the developmental lead for more children.

The finding on forearm action demonstrates how equipment can affect a person's developmental level. Using Newell's notions of person constraints (see chapter 3), we can think of equipment as an extension of the body. Comparing hammering with a hammer with pounding with a fist demonstrates how an implement changes the inertial characteristics of the arm and hand. This analysis also applies to prosthetic limbs. In both cases the performer needs to learn how to incorporate the implement into the actual movement pattern. Langendorfer's (1987a) data suggest that at first the implement assists the child in achieving a higher developmental level in the distal component, probably because its greater inertia causes lag. Later, though, the handle length may hinder change to a higher developmental level by making it more difficult to delay that lag until the body has reached front facing (see tables 7.1 and 7.4).

The last component that Langendorfer (1987a) compared across the two skills was stepping. As we can see in figure 7.1d, the developmental relationship for stepping was less clear. Langendorfer attributed this result to a degree of unreliability in the component sequence: Stepping tended to be variable across trials in both throwing and striking, making intertask comparisons unreliable.

The arrows in figure 7.1 reflect Langendorfer's (1987a) speculation on the longitudinal course that these two tasks might display while developing. The only actual longitudinal information that we have regarding this question was reported by Roberton (1982). She compared the development of trunk action within sidearm striking (as opposed to overarm striking) and overarm throwing. Interestingly, the trunk action of four children did follow the developmental path that Langendorfer's cross-sectional overarm data suggested: The children progressed one level in their throwing trunk action and then caught up to that same level in their striking trunk action. Then they progressed again in throwing before catching up in striking.

Motor Development Differences in Children With Autism Spectrum Disorders

One of the great difficulties facing practitioners and researchers working with individuals with ASD is the degree to which variability supersedes universality in this population (Pope et al., 2010). That is, two individuals with the same diagnosis of ASD will act and react very differently from each other. These individual differences lead to significant challenges for researchers trying to identify and quantify motor development in this population. For example, individuals with ASD may respond differently to a controlled experimental design due to different sensitivity to sensory stimuli or a lack of understanding of the described procedures. Furthermore, each individual responds differently to different interventions. All in all, typical developmental trajectories are all but impossible to determine. However, researchers have identified some general characteristics related to movement and motor development that suggest that individuals with ASD move differently from their typically developing counterparts (Pope et al., 2010). The importance of determining motor differences, particularly in younger children, cannot be underscored enough: If early developing motor differences exist, then assessment of these deficits may aid in identifying children at risk for ASD at a younger age than is currently possible.

Primarily through retrospective video analysis, researchers have identified early motor differences in children with ASD. Kanner (1943), who first identified ASD, noted that the infants exhibited hypotonia, or low muscle tone (similar to infants with DS). Other researchers have also observed hypotonia (Adrien et al., 1993; Ming, Brimacombe, & Wagner, 2007). For example, Ming et al. (2007) reviewed the clinical records of 154 children diagnosed with ASD in an attempt to determine the types and prevalence of motor impairments in this population, particularly hypotonia, delayed motor milestones, coordination issues, and toe walking. Children ranged in age from 2 to 18 years, with a mean age of 6 years. Reflecting the gender differences within the ASD population, there were 126 males and 28 females within the study. All of the children were on the autism spectrum: 74 were diagnosed with autism, 70 with PDD-NOS, and 10 with AS. The researchers determined that 51% of the group demonstrated hypotonia as infants; the prevalence of hypotonia decreased over time to 38% in the age range of 7 to 18 years. Not all retrospective studies have indicated that infants with autism have hypotonia. Saint-Georges et al. (2010) performed an exhaustive research review using retrospective video analysis and determined that across the 41 studies reviewed, hypotonia was not a consistent finding.

Given that at least a percentage of children with ASD exhibit hypotonia, subsequent issues with the acquisition of motor milestones may exist (as in children with DS). Low muscle tone affects infants' ability to raise the head while in the prone position, which affects the ability to sit supported, and so on. In fact, another consistent finding among individuals with ASD is a delay in achieving motor milestones and fundamental motor skills as young children. Acknowledging the limitations of looking back at records after diagnosis, Landa and Garrett-Mayer (2006) performed a 2-year longitudinal prospective study in which they followed a group of 84 infants who were either high (n = 60) or low (n = 27) risk for ASD.

Infants were tested at 6, 14, and 24 months using the Mullen Scales of Early Learning (MSEL), a standardized developmental test for children aged 0 to 69 months. The MSEL consists of five subscales: gross motor, fine motor, visual reception, receptive language, and expressive language. At 2 years, all children were given the Autism Diagnostic Observation Schedule (ADOS), which is a semistructured, play-based interview that provides systematic probes for symptoms of autism in social interaction, communication, play, and repetitive behaviors. Of the 60 children deemed at risk, 21 met the ADOS algorithm criteria for ASD as well as received a clinical diagnosis of ASD; 2 children from the low-risk group met the criteria and were included in the high-risk group. Early on, no statistical differences existed between groups. However, by 24 months, the high-risk group showed delays in both gross and fine motor skills, and group members were significantly different from the typically developing group in all domains.

In related research, a group of investigators assessed motor development in children younger than 3 years of age (Provost, Lopez, & Heimerl, 2007). In this study, 19 children with ASD, ranging in age from 21 to 41 months, were examined with the Bayley Scales of Infant Development II (BSID II; Bayley, 1993) and the Peabody Developmental Motor Scales, Second Edition (PDMS-2; Folio & Fewell, 2000). The researchers determined that 84% of the children were significantly delayed on the BSID II and PDMS-2. In a similar study, Ozonoff et al. (2008) examined the early movement behavior of children later identified as ASD and reported a delay in the appearance of motor milestones, including lying in prone and supine position, rolling, sitting, crawling, and walking, with the most significant delay occurring in walking onset. Therefore, it appears that delays in motor milestones exist in many of the children diagnosed with
ASD.

Acknowledging that delays appear to exist in the ASD population, Ozonoff et al. followed strong inference (see chapter 4) and asked the next logical question: Can motor delays differentiate children with autism from children who have more general developmental delays? If motor delays could be used for differentiation, then practitioners could use motor delays to identify infants at risk for ASD. In the resulting study, Ozonoff et al. (2008) placed 103 children between 24 and 60 months of age in 1 of 3 groups: children with ASD, children with developmental delays (DD), or children with typical development (TD). Home videotapes of the participants were analyzed and coded with the Infant Motor Maturity and Atypicality Coding Scales (IMMACS), which rate six motor milestones (lying in prone and supine position, roll, sit, crawl, walk) as well as protective skills (e.g., righting the body after losing balance). What Ozonoff et al. (2008) found, in contrast to earlier reports and speculation, is that the only group that differed in terms of number of movement abnormalities and protective skills was the DD group; the ASD and TD groups were not significantly different. The ASD and DD groups both showed motor delays, but they were not significantly different from each other. Therefore, the notion that motor delays can be used to identify children at risk for ASD was not supported in this research. Obviously, a prospective study examining both qualitative and quantitative variables is necessary as a next step to confirm this finding.

Are individuals with ASD merely delayed in motor skill acquisition, or do they differ in the ways in which they move? Several research studies (see table 9.5) have found at least one consistent difference between children with ASD and children without ASD, and that is their walking patterns. Over three decades ago, researchers found that children with ASD demonstrate abnormal limb movements, shortened steps, and persistent toe walking (Damasio & Maurer, 1978; Vilensky, Damasio, & Maurer, 1981). More recently, Ming et al. (2007), in their retrospective study, determined that nearly 20% of children with ASD walked on their toes rather than on their entire foot. Esposito and Venturi (2008) used retrospective video analysis as well as an observational scale to examine 42 children with ASD who had been walking at least 6 months and also found differences in early walking patterns. Vernazza-Martin et al. (2005) found significant differences in gait when comparing 15 children aged 4 to 6 years with and without ASD, as did Woodward (2001) in a study of children with ASD aged 3 and 10 years. One consistent finding is that the gait cycle is slower and less consistent in children with ASD.

Dewey, Cantell, and Crawford (2007) compared the performance of children with different disabilities on the Bruininks-Oseretsky Test of Motor Proficiency-Short Form. Participants included 49 children with ASD, 46 children with developmental coordination disorder (DCD) and attention-deficit/hyperactivity disorder (ADHD), 38 children with DCD, 27 children with ADHD, and 78 children with typical development. The results indicated that although all the atypical groups displayed significant impairment of motor skills, children with ASD were significantly more impaired compared with their cohorts with specific motor skill deficits. They were also the only group to show impairment on gestural skills. Several researchers have found deficiencies in fine motor skills in children with ASD. These range from delays in manual dexterity (Miyahara et al., 1997) and graphomotor skills (Mayes & Calhoun, 2003) during early and middle childhood to motor control issues in prehension (Mari, Castiello, Marks, Marraffa, & Prior, 2003). Such fine motor skill deficits influence handwriting as well as many functional activities involving the hands and arms. Across these studies, it appears that individuals with autism do move differently when compared with their typically developing counterparts and that deficits range from fine to gross motor skills.

In sum, although motor deficits are not listed as part of the DSM-V diagnostic criteria for ASD, individuals with these disorders tend to exhibit motor skill deficiencies as well as differences in motor development. Infants with ASD may exhibit hypotonia (although this finding is not as robust as it is in infants with DS). In addition, delays may exist in the acquisition of motor milestones and fundamental motor skills. Differences in skills such as gait and manual dexterity persist into childhood.

Developmental Coordination Disorder

Another disorder with an unknown etiology that has garnered considerable empirical attention as of late is DCD (Wilson & Larkin, 2008). DCD, also known as developmental dyspraxia, is characterized by extreme lack of motor coordination and by other movement deficits in the absence of neurological defects. DCD has been calculated to affect as much as 6% of the elementary school population (American Psychiatric Association, 1994; Barnhart, Davenport, Epps, & Nordquist, 2003; Barnett, Kooistra, & Henderson, 1998). Although the disorder was described as early as 1937, it took until 1994 for a group of 43 experts in various movement disorder fields to come to a consensus on a name and description for DCD in order to facilitate both research and diagnosis (see the sidebar for the DSM-V description of DCD). Individuals characterized as having DCD fall into the 0 to 10th percentile of performance on the Movement Assessment Battery for Children (M-ABC), a standardized motor skills test used to assess motor proficiency in children with disabilities (Geuze, Jongmans, Schoemaker, & Smits-Engelsman, 2001; Henderson & Sugden, 1992; Miyahara & Möbs, 1995). Because DCD is categorized as a learning disability, it also may influence a child's ability to perform academically. Motor difficulties can range from gross to fine motor to balance skills and can include motor planning deficits and visual or spatial difficulties (Cermak & Larkin, 2002; de Castelnau, Albaret, Chaix, & Zanone, 2007; Kaplan, Wilson, Dewey, & Crawford, 1998; Przysucha & Taylor, 2004; Whitall et al., 2006). Table 9.6 provides examples of functional motor issues that children with DCD may have.

Applying the Dynamic Systems Model to a Motor Development Problem

Perhaps the best way to appreciate how the characteristics of dynamic systems can be used to model motor development is to see how a research team used dynamic systems to address a particular developmental issue. Let's consider the case of the reflexes seen in infancy. Recall that a reflex is a particular automatic response to a specific stimulus. It is a response that is carried out without thought or intention. Developmentalists have long observed the existence of reflexes that disappear later on in infants.

Developmentalists have been particularly interested in the walking or stepping reflex for its similarity to voluntary walking (see table 3.1). The reflex is elicited by holding an infant upright and lowering the infant to a horizontal surface.

The reflexive response is stepping movements of the legs. The stepping reflex is typically observed up to 5 months of age, so it appears much earlier than voluntary walking and disappears months before voluntary walking.

Developmentalists have offered several explanations for the disappearance of the stepping reflex. The maturationists suggested that the disappearance reflects the maturation of the cortex. These researchers proposed that as higher brain centers mature they are able to inhibit the lower brain centers that mediate reflexes. With continued maturation, the centers control voluntary walking. The existence and disappearance of reflexes reflect the status of nervous system development.

Peiper (1963) suggested instead that reflexes have a purpose because they allow practice of movement patterns before the higher brain centers are mature enough to control those patterns. Zelazo et al. (Zelazo, 1983; Zelazo, Konner, Kolb, & Zelazo, 1974; Zelazo, Zelazo, & Kolb, 1972) offered yet another viewpoint. They purposely increased the number of times the stepping reflex was elicited in a small group of infants. Later, these infants were observed to start walking at a younger age than average. The researchers concluded that reflexes can be subsumed into voluntary movements. For example, the walking reflex is transformed into voluntary walking.

Unconvinced by these various hypotheses, Esther Thelen and her colleagues (Thelen & Fisher, 1982; Thelen, Fisher, & Ridley-Johnson, 1984) developed an ingenious series of experiments to examine different potential causes for the disappearance of the stepping reflex. They began with the notion that multiple systems—rather than just one system—play a role in the disappearance. They had observed that one of the changes in infants correlating with the disappearance of the stepping reflex is an increase in subcutaneous fat that occurs during infancy. Could this gain in fat and the resultant change in the strength required to move heavier legs be related to the disappearance of the stepping reflex? In order to test whether the muscle and adipose tissue systems are involved, the researchers had to find a way to make a light and stepping infant resemble a nonstepping and heavier infant and, just as importantly, to get a non-stepping, heavier infant to resemble a lighter, stepping infant. The only variable that could be changed was leg strength. In a first experiment, the researchers added tiny weights to the legs of lighter infants. The added weight was proportional to the amount of weight the infant would add between 4 and 6 weeks of age. If maturation of the nervous system were the only cause of reflex stepping, then the infants should continue to step regardless of the small weights. Actually, the infants reduced the number of reflex steps taken while wearing the weights.

The next experiment was designed to regress the leg weight of heavier infants to that of younger, lighter infants. The researchers achieved this by holding the infants upright in an aquarium filled with water up to their hips. The water reduced the pull of gravity on the legs and made them relatively lighter. The water was room temperature, and the researchers noted that often the infants did not respond in any way to being held in the water. When the infants' feet touched the bottom surface of the tank, though, the number of steps increased (Thelen & Fisher, 1982). If maturation of the nervous system is the only explanation
for the reflex disappearing, then nothing should have been able to increase the rate of stepping, as the higher brain centers would have inhibited it. Thus the researchers demonstrated that change in other systems plays a role in the disappearance of the stepping reflex.

Thelen et al. demonstrated that the systems approach can be used to examine the processes underlying development. While maturation of the nervous system is important in motor development, other subsystems are important to the change observed in the stepping reflex. As well as studying multiple subsystems, Thelen et al. demonstrated that change in a subsystem can function as a control variable. The role of a control variable in bringing about change is another characteristic of dynamic systems. More of Thelen's work will be discussed throughout the text.

Summary

The systems approach has much to offer researchers interested in understanding developmental change. Dynamic systems evolve over time and undergo change that is not linear. These characteristics describe the body as it grows and ages. Dynamic systems evolve by transitioning from times of stability, or attractor states, through times of instability to other attractor states. We observe this pattern in motor development, too. Dynamic systems are sensitive to the conditions initially in place when they begin to evolve. Movement scientists have long realized that models of movement control must allow for the same movement goal to be achieved from variable environmental conditions and different starting positions of the body and limbs. Most importantly, dynamic systems have been shown to self-organize and to use methods of control such as entrainment and the application of constraints. The demonstration of these possibilities is important, as they provide explanations for how the large number of degrees of freedom in movement can be reduced for the reasonable control of complex movements.

The systems approach provided a heuristic function for movement scientists by focusing researchers' attention on new ways to solve existing questions in motor development. There is value in examining multiple theories. Researchers using differing approaches all serve to gain from alternative views, as both theorists and empiricists are forced to question behaviors their theories can't account for and improve their models until a satisfactory explanation can be found (for example, see Schmidt & Lee, 2011 on generalized motor programs). Ultimately, these challenges and changes serve to enhance our understanding of motor development.

Development of Ballistic Skills

Movements can be classified by their EMG and force curve signatures. Three movement categories are (1) ramped movements in which force is gradually and steadily applied through continuously graded muscle firing; (2) discontinuous, slow movements that accelerate gradually, then decelerate, and then accelerate again, such as when tracking an object with a hand; and (3) ballistic movements that are rapid, all-out, forceful movements that reach peak acceleration within milliseconds of their initiation (Brooks, 1983; Desmedt, 1983).

In simple ballistic movements, such as a rapid reach for an object, the motion is started by an initial strong impulse in the agonist muscle. The agonist firing then stops, but the movement continues due to the momentum of the limb. A burst by the antagonist muscle subsequently causes the limb to decelerate. In targeting tasks, there may be a second burst in the agonist to correct the limb placement. This creates a signature triphasic pattern of muscle activation: agonist, antagonist, agonist (Palmer, Cafarelli, & Ashby, 1994).

In contrast to single-limb movements, ballistic skills involving the total body are quite complex. Typical sport skills are examples of ballistic skills. Of these, movements in which the performer projects an object have been of considerable interest to both biomechanists and developmentalists. Striking (as in batting and swinging a racket), kicking (as in football and soccer), and throwing (as in handball and baseball or softball) have all received study. These complex movements tend to begin proximally with action of the trunk muscles; energy is then passed from segment to segment of the open kinetic chain until it reaches the distal effector, usually the hand or foot. During striking, kicking, and throwing, this energy exchange is superimposed on a running or stepping base of support.

Another interesting commonality across these complex projectile skills is that they employ a backswing to place the body segments in position to move forward. In the advanced form of striking, throwing, and kicking, the backswing and forward swing partially overlap, and proximal segments begin to move forward while more distal segments are still moving backward. The advanced form of these movements also involves strong acceleration of the distal segment just milliseconds before contact or release, which yields high distal angular velocity. This acceleration seems to result partially from the inertia of each distal segment against the motion of adjacent proximal segments. The lagging segment stretches the agonist muscles of the distal segment, which may in turn excite reflexes that augment the muscular contractions in the distal segment (Roberts & Metcalfe, 1968). As we will see, lag is one aspect of object projection that develops only gradually over time. It is so important for effective ballistic movement that Southard (2002a, 2002b) used lag as a collective variable in his studies of throwing.

In this chapter we focus on the development of striking, throwing, and kicking. Most developmental studies on ballistic projectile skills have been based in developmental sequence theory, although a few studies are beginning to address issues from a dynamic systems perspective. At the close of this chapter we discuss future directions in this area.

Intertask Developmental Sequence

The only ballistic skills that have been studied in relation to their intertask development are overarm throwing and striking. The approach used in this research is quite different from the intertask approach used to study locomotion (see chapter 6). For locomotion, the developmental sequence is formed by ordering each locomotor skill according to its initial appearance in the movement repertoire. For throwing and striking, the two tasks are compared to see where participants are located within a component developmental sequence at the same point in time in each skill. Do the participants show the same developmental level in the two skills, or do they develop faster in one skill than they do in the other?

Langendorfer (1987a) compared the development of trunk and humerus action in overarm throwing with development of the same action in overarm striking. He asked 58 boys aged 1.5 to 10.3 years to throw a ball forcefully and to strike a tennis ball suspended 6 inches (15 cm) above their head forcefully with a lightweight racket. He then looked at side and rear film views to categorize the boys' movements in each skill. Figure 7.1 shows his results. From the cross-sectional data we can infer how the longitudinal data might look. The data on trunk action (see figure 7.1a) suggested that children advance in throwing before they do in striking: The boys started out at a primitive level in both skills and then advanced to level 2 throwing before achieving level 2 striking and advanced to level 3 throwing before reaching level 3 striking. The cross-sectional data on humerus action (see figure 7.1b) suggested that children begin at level 1 in both skills and then move ahead in either skill until they reach level 2 in both skills. Then, however, they progress to level 3 in throwing before they progress to level 3 in striking.

Langendorfer (1987a) learned from his data that he could not compare forearm action between striking and throwing. Rather, the action of the racket, which is the distal segment in striking, was more comparable to the forearm action of throwing. Figure 7.1c shows the developmental comparison for forearm action. While some children exhibited different relationships early on, more progressed in striking before they progressed in throwing. Then throwing took the developmental lead for more children.

The finding on forearm action demonstrates how equipment can affect a person's developmental level. Using Newell's notions of person constraints (see chapter 3), we can think of equipment as an extension of the body. Comparing hammering with a hammer with pounding with a fist demonstrates how an implement changes the inertial characteristics of the arm and hand. This analysis also applies to prosthetic limbs. In both cases the performer needs to learn how to incorporate the implement into the actual movement pattern. Langendorfer's (1987a) data suggest that at first the implement assists the child in achieving a higher developmental level in the distal component, probably because its greater inertia causes lag. Later, though, the handle length may hinder change to a higher developmental level by making it more difficult to delay that lag until the body has reached front facing (see tables 7.1 and 7.4).

The last component that Langendorfer (1987a) compared across the two skills was stepping. As we can see in figure 7.1d, the developmental relationship for stepping was less clear. Langendorfer attributed this result to a degree of unreliability in the component sequence: Stepping tended to be variable across trials in both throwing and striking, making intertask comparisons unreliable.

The arrows in figure 7.1 reflect Langendorfer's (1987a) speculation on the longitudinal course that these two tasks might display while developing. The only actual longitudinal information that we have regarding this question was reported by Roberton (1982). She compared the development of trunk action within sidearm striking (as opposed to overarm striking) and overarm throwing. Interestingly, the trunk action of four children did follow the developmental path that Langendorfer's cross-sectional overarm data suggested: The children progressed one level in their throwing trunk action and then caught up to that same level in their striking trunk action. Then they progressed again in throwing before catching up in striking.

Motor Development Differences in Children With Autism Spectrum Disorders

One of the great difficulties facing practitioners and researchers working with individuals with ASD is the degree to which variability supersedes universality in this population (Pope et al., 2010). That is, two individuals with the same diagnosis of ASD will act and react very differently from each other. These individual differences lead to significant challenges for researchers trying to identify and quantify motor development in this population. For example, individuals with ASD may respond differently to a controlled experimental design due to different sensitivity to sensory stimuli or a lack of understanding of the described procedures. Furthermore, each individual responds differently to different interventions. All in all, typical developmental trajectories are all but impossible to determine. However, researchers have identified some general characteristics related to movement and motor development that suggest that individuals with ASD move differently from their typically developing counterparts (Pope et al., 2010). The importance of determining motor differences, particularly in younger children, cannot be underscored enough: If early developing motor differences exist, then assessment of these deficits may aid in identifying children at risk for ASD at a younger age than is currently possible.

Primarily through retrospective video analysis, researchers have identified early motor differences in children with ASD. Kanner (1943), who first identified ASD, noted that the infants exhibited hypotonia, or low muscle tone (similar to infants with DS). Other researchers have also observed hypotonia (Adrien et al., 1993; Ming, Brimacombe, & Wagner, 2007). For example, Ming et al. (2007) reviewed the clinical records of 154 children diagnosed with ASD in an attempt to determine the types and prevalence of motor impairments in this population, particularly hypotonia, delayed motor milestones, coordination issues, and toe walking. Children ranged in age from 2 to 18 years, with a mean age of 6 years. Reflecting the gender differences within the ASD population, there were 126 males and 28 females within the study. All of the children were on the autism spectrum: 74 were diagnosed with autism, 70 with PDD-NOS, and 10 with AS. The researchers determined that 51% of the group demonstrated hypotonia as infants; the prevalence of hypotonia decreased over time to 38% in the age range of 7 to 18 years. Not all retrospective studies have indicated that infants with autism have hypotonia. Saint-Georges et al. (2010) performed an exhaustive research review using retrospective video analysis and determined that across the 41 studies reviewed, hypotonia was not a consistent finding.

Given that at least a percentage of children with ASD exhibit hypotonia, subsequent issues with the acquisition of motor milestones may exist (as in children with DS). Low muscle tone affects infants' ability to raise the head while in the prone position, which affects the ability to sit supported, and so on. In fact, another consistent finding among individuals with ASD is a delay in achieving motor milestones and fundamental motor skills as young children. Acknowledging the limitations of looking back at records after diagnosis, Landa and Garrett-Mayer (2006) performed a 2-year longitudinal prospective study in which they followed a group of 84 infants who were either high (n = 60) or low (n = 27) risk for ASD.

Infants were tested at 6, 14, and 24 months using the Mullen Scales of Early Learning (MSEL), a standardized developmental test for children aged 0 to 69 months. The MSEL consists of five subscales: gross motor, fine motor, visual reception, receptive language, and expressive language. At 2 years, all children were given the Autism Diagnostic Observation Schedule (ADOS), which is a semistructured, play-based interview that provides systematic probes for symptoms of autism in social interaction, communication, play, and repetitive behaviors. Of the 60 children deemed at risk, 21 met the ADOS algorithm criteria for ASD as well as received a clinical diagnosis of ASD; 2 children from the low-risk group met the criteria and were included in the high-risk group. Early on, no statistical differences existed between groups. However, by 24 months, the high-risk group showed delays in both gross and fine motor skills, and group members were significantly different from the typically developing group in all domains.

In related research, a group of investigators assessed motor development in children younger than 3 years of age (Provost, Lopez, & Heimerl, 2007). In this study, 19 children with ASD, ranging in age from 21 to 41 months, were examined with the Bayley Scales of Infant Development II (BSID II; Bayley, 1993) and the Peabody Developmental Motor Scales, Second Edition (PDMS-2; Folio & Fewell, 2000). The researchers determined that 84% of the children were significantly delayed on the BSID II and PDMS-2. In a similar study, Ozonoff et al. (2008) examined the early movement behavior of children later identified as ASD and reported a delay in the appearance of motor milestones, including lying in prone and supine position, rolling, sitting, crawling, and walking, with the most significant delay occurring in walking onset. Therefore, it appears that delays in motor milestones exist in many of the children diagnosed with
ASD.

Acknowledging that delays appear to exist in the ASD population, Ozonoff et al. followed strong inference (see chapter 4) and asked the next logical question: Can motor delays differentiate children with autism from children who have more general developmental delays? If motor delays could be used for differentiation, then practitioners could use motor delays to identify infants at risk for ASD. In the resulting study, Ozonoff et al. (2008) placed 103 children between 24 and 60 months of age in 1 of 3 groups: children with ASD, children with developmental delays (DD), or children with typical development (TD). Home videotapes of the participants were analyzed and coded with the Infant Motor Maturity and Atypicality Coding Scales (IMMACS), which rate six motor milestones (lying in prone and supine position, roll, sit, crawl, walk) as well as protective skills (e.g., righting the body after losing balance). What Ozonoff et al. (2008) found, in contrast to earlier reports and speculation, is that the only group that differed in terms of number of movement abnormalities and protective skills was the DD group; the ASD and TD groups were not significantly different. The ASD and DD groups both showed motor delays, but they were not significantly different from each other. Therefore, the notion that motor delays can be used to identify children at risk for ASD was not supported in this research. Obviously, a prospective study examining both qualitative and quantitative variables is necessary as a next step to confirm this finding.

Are individuals with ASD merely delayed in motor skill acquisition, or do they differ in the ways in which they move? Several research studies (see table 9.5) have found at least one consistent difference between children with ASD and children without ASD, and that is their walking patterns. Over three decades ago, researchers found that children with ASD demonstrate abnormal limb movements, shortened steps, and persistent toe walking (Damasio & Maurer, 1978; Vilensky, Damasio, & Maurer, 1981). More recently, Ming et al. (2007), in their retrospective study, determined that nearly 20% of children with ASD walked on their toes rather than on their entire foot. Esposito and Venturi (2008) used retrospective video analysis as well as an observational scale to examine 42 children with ASD who had been walking at least 6 months and also found differences in early walking patterns. Vernazza-Martin et al. (2005) found significant differences in gait when comparing 15 children aged 4 to 6 years with and without ASD, as did Woodward (2001) in a study of children with ASD aged 3 and 10 years. One consistent finding is that the gait cycle is slower and less consistent in children with ASD.

Dewey, Cantell, and Crawford (2007) compared the performance of children with different disabilities on the Bruininks-Oseretsky Test of Motor Proficiency-Short Form. Participants included 49 children with ASD, 46 children with developmental coordination disorder (DCD) and attention-deficit/hyperactivity disorder (ADHD), 38 children with DCD, 27 children with ADHD, and 78 children with typical development. The results indicated that although all the atypical groups displayed significant impairment of motor skills, children with ASD were significantly more impaired compared with their cohorts with specific motor skill deficits. They were also the only group to show impairment on gestural skills. Several researchers have found deficiencies in fine motor skills in children with ASD. These range from delays in manual dexterity (Miyahara et al., 1997) and graphomotor skills (Mayes & Calhoun, 2003) during early and middle childhood to motor control issues in prehension (Mari, Castiello, Marks, Marraffa, & Prior, 2003). Such fine motor skill deficits influence handwriting as well as many functional activities involving the hands and arms. Across these studies, it appears that individuals with autism do move differently when compared with their typically developing counterparts and that deficits range from fine to gross motor skills.

In sum, although motor deficits are not listed as part of the DSM-V diagnostic criteria for ASD, individuals with these disorders tend to exhibit motor skill deficiencies as well as differences in motor development. Infants with ASD may exhibit hypotonia (although this finding is not as robust as it is in infants with DS). In addition, delays may exist in the acquisition of motor milestones and fundamental motor skills. Differences in skills such as gait and manual dexterity persist into childhood.

Developmental Coordination Disorder

Another disorder with an unknown etiology that has garnered considerable empirical attention as of late is DCD (Wilson & Larkin, 2008). DCD, also known as developmental dyspraxia, is characterized by extreme lack of motor coordination and by other movement deficits in the absence of neurological defects. DCD has been calculated to affect as much as 6% of the elementary school population (American Psychiatric Association, 1994; Barnhart, Davenport, Epps, & Nordquist, 2003; Barnett, Kooistra, & Henderson, 1998). Although the disorder was described as early as 1937, it took until 1994 for a group of 43 experts in various movement disorder fields to come to a consensus on a name and description for DCD in order to facilitate both research and diagnosis (see the sidebar for the DSM-V description of DCD). Individuals characterized as having DCD fall into the 0 to 10th percentile of performance on the Movement Assessment Battery for Children (M-ABC), a standardized motor skills test used to assess motor proficiency in children with disabilities (Geuze, Jongmans, Schoemaker, & Smits-Engelsman, 2001; Henderson & Sugden, 1992; Miyahara & Möbs, 1995). Because DCD is categorized as a learning disability, it also may influence a child's ability to perform academically. Motor difficulties can range from gross to fine motor to balance skills and can include motor planning deficits and visual or spatial difficulties (Cermak & Larkin, 2002; de Castelnau, Albaret, Chaix, & Zanone, 2007; Kaplan, Wilson, Dewey, & Crawford, 1998; Przysucha & Taylor, 2004; Whitall et al., 2006). Table 9.6 provides examples of functional motor issues that children with DCD may have.

Applying the Dynamic Systems Model to a Motor Development Problem

Perhaps the best way to appreciate how the characteristics of dynamic systems can be used to model motor development is to see how a research team used dynamic systems to address a particular developmental issue. Let's consider the case of the reflexes seen in infancy. Recall that a reflex is a particular automatic response to a specific stimulus. It is a response that is carried out without thought or intention. Developmentalists have long observed the existence of reflexes that disappear later on in infants.

Developmentalists have been particularly interested in the walking or stepping reflex for its similarity to voluntary walking (see table 3.1). The reflex is elicited by holding an infant upright and lowering the infant to a horizontal surface.

The reflexive response is stepping movements of the legs. The stepping reflex is typically observed up to 5 months of age, so it appears much earlier than voluntary walking and disappears months before voluntary walking.

Developmentalists have offered several explanations for the disappearance of the stepping reflex. The maturationists suggested that the disappearance reflects the maturation of the cortex. These researchers proposed that as higher brain centers mature they are able to inhibit the lower brain centers that mediate reflexes. With continued maturation, the centers control voluntary walking. The existence and disappearance of reflexes reflect the status of nervous system development.

Peiper (1963) suggested instead that reflexes have a purpose because they allow practice of movement patterns before the higher brain centers are mature enough to control those patterns. Zelazo et al. (Zelazo, 1983; Zelazo, Konner, Kolb, & Zelazo, 1974; Zelazo, Zelazo, & Kolb, 1972) offered yet another viewpoint. They purposely increased the number of times the stepping reflex was elicited in a small group of infants. Later, these infants were observed to start walking at a younger age than average. The researchers concluded that reflexes can be subsumed into voluntary movements. For example, the walking reflex is transformed into voluntary walking.

Unconvinced by these various hypotheses, Esther Thelen and her colleagues (Thelen & Fisher, 1982; Thelen, Fisher, & Ridley-Johnson, 1984) developed an ingenious series of experiments to examine different potential causes for the disappearance of the stepping reflex. They began with the notion that multiple systems—rather than just one system—play a role in the disappearance. They had observed that one of the changes in infants correlating with the disappearance of the stepping reflex is an increase in subcutaneous fat that occurs during infancy. Could this gain in fat and the resultant change in the strength required to move heavier legs be related to the disappearance of the stepping reflex? In order to test whether the muscle and adipose tissue systems are involved, the researchers had to find a way to make a light and stepping infant resemble a nonstepping and heavier infant and, just as importantly, to get a non-stepping, heavier infant to resemble a lighter, stepping infant. The only variable that could be changed was leg strength. In a first experiment, the researchers added tiny weights to the legs of lighter infants. The added weight was proportional to the amount of weight the infant would add between 4 and 6 weeks of age. If maturation of the nervous system were the only cause of reflex stepping, then the infants should continue to step regardless of the small weights. Actually, the infants reduced the number of reflex steps taken while wearing the weights.

The next experiment was designed to regress the leg weight of heavier infants to that of younger, lighter infants. The researchers achieved this by holding the infants upright in an aquarium filled with water up to their hips. The water reduced the pull of gravity on the legs and made them relatively lighter. The water was room temperature, and the researchers noted that often the infants did not respond in any way to being held in the water. When the infants' feet touched the bottom surface of the tank, though, the number of steps increased (Thelen & Fisher, 1982). If maturation of the nervous system is the only explanation
for the reflex disappearing, then nothing should have been able to increase the rate of stepping, as the higher brain centers would have inhibited it. Thus the researchers demonstrated that change in other systems plays a role in the disappearance of the stepping reflex.

Thelen et al. demonstrated that the systems approach can be used to examine the processes underlying development. While maturation of the nervous system is important in motor development, other subsystems are important to the change observed in the stepping reflex. As well as studying multiple subsystems, Thelen et al. demonstrated that change in a subsystem can function as a control variable. The role of a control variable in bringing about change is another characteristic of dynamic systems. More of Thelen's work will be discussed throughout the text.

Summary

The systems approach has much to offer researchers interested in understanding developmental change. Dynamic systems evolve over time and undergo change that is not linear. These characteristics describe the body as it grows and ages. Dynamic systems evolve by transitioning from times of stability, or attractor states, through times of instability to other attractor states. We observe this pattern in motor development, too. Dynamic systems are sensitive to the conditions initially in place when they begin to evolve. Movement scientists have long realized that models of movement control must allow for the same movement goal to be achieved from variable environmental conditions and different starting positions of the body and limbs. Most importantly, dynamic systems have been shown to self-organize and to use methods of control such as entrainment and the application of constraints. The demonstration of these possibilities is important, as they provide explanations for how the large number of degrees of freedom in movement can be reduced for the reasonable control of complex movements.

The systems approach provided a heuristic function for movement scientists by focusing researchers' attention on new ways to solve existing questions in motor development. There is value in examining multiple theories. Researchers using differing approaches all serve to gain from alternative views, as both theorists and empiricists are forced to question behaviors their theories can't account for and improve their models until a satisfactory explanation can be found (for example, see Schmidt & Lee, 2011 on generalized motor programs). Ultimately, these challenges and changes serve to enhance our understanding of motor development.

Development of Ballistic Skills

Movements can be classified by their EMG and force curve signatures. Three movement categories are (1) ramped movements in which force is gradually and steadily applied through continuously graded muscle firing; (2) discontinuous, slow movements that accelerate gradually, then decelerate, and then accelerate again, such as when tracking an object with a hand; and (3) ballistic movements that are rapid, all-out, forceful movements that reach peak acceleration within milliseconds of their initiation (Brooks, 1983; Desmedt, 1983).

In simple ballistic movements, such as a rapid reach for an object, the motion is started by an initial strong impulse in the agonist muscle. The agonist firing then stops, but the movement continues due to the momentum of the limb. A burst by the antagonist muscle subsequently causes the limb to decelerate. In targeting tasks, there may be a second burst in the agonist to correct the limb placement. This creates a signature triphasic pattern of muscle activation: agonist, antagonist, agonist (Palmer, Cafarelli, & Ashby, 1994).

In contrast to single-limb movements, ballistic skills involving the total body are quite complex. Typical sport skills are examples of ballistic skills. Of these, movements in which the performer projects an object have been of considerable interest to both biomechanists and developmentalists. Striking (as in batting and swinging a racket), kicking (as in football and soccer), and throwing (as in handball and baseball or softball) have all received study. These complex movements tend to begin proximally with action of the trunk muscles; energy is then passed from segment to segment of the open kinetic chain until it reaches the distal effector, usually the hand or foot. During striking, kicking, and throwing, this energy exchange is superimposed on a running or stepping base of support.

Another interesting commonality across these complex projectile skills is that they employ a backswing to place the body segments in position to move forward. In the advanced form of striking, throwing, and kicking, the backswing and forward swing partially overlap, and proximal segments begin to move forward while more distal segments are still moving backward. The advanced form of these movements also involves strong acceleration of the distal segment just milliseconds before contact or release, which yields high distal angular velocity. This acceleration seems to result partially from the inertia of each distal segment against the motion of adjacent proximal segments. The lagging segment stretches the agonist muscles of the distal segment, which may in turn excite reflexes that augment the muscular contractions in the distal segment (Roberts & Metcalfe, 1968). As we will see, lag is one aspect of object projection that develops only gradually over time. It is so important for effective ballistic movement that Southard (2002a, 2002b) used lag as a collective variable in his studies of throwing.

In this chapter we focus on the development of striking, throwing, and kicking. Most developmental studies on ballistic projectile skills have been based in developmental sequence theory, although a few studies are beginning to address issues from a dynamic systems perspective. At the close of this chapter we discuss future directions in this area.

Intertask Developmental Sequence

The only ballistic skills that have been studied in relation to their intertask development are overarm throwing and striking. The approach used in this research is quite different from the intertask approach used to study locomotion (see chapter 6). For locomotion, the developmental sequence is formed by ordering each locomotor skill according to its initial appearance in the movement repertoire. For throwing and striking, the two tasks are compared to see where participants are located within a component developmental sequence at the same point in time in each skill. Do the participants show the same developmental level in the two skills, or do they develop faster in one skill than they do in the other?

Langendorfer (1987a) compared the development of trunk and humerus action in overarm throwing with development of the same action in overarm striking. He asked 58 boys aged 1.5 to 10.3 years to throw a ball forcefully and to strike a tennis ball suspended 6 inches (15 cm) above their head forcefully with a lightweight racket. He then looked at side and rear film views to categorize the boys' movements in each skill. Figure 7.1 shows his results. From the cross-sectional data we can infer how the longitudinal data might look. The data on trunk action (see figure 7.1a) suggested that children advance in throwing before they do in striking: The boys started out at a primitive level in both skills and then advanced to level 2 throwing before achieving level 2 striking and advanced to level 3 throwing before reaching level 3 striking. The cross-sectional data on humerus action (see figure 7.1b) suggested that children begin at level 1 in both skills and then move ahead in either skill until they reach level 2 in both skills. Then, however, they progress to level 3 in throwing before they progress to level 3 in striking.

Langendorfer (1987a) learned from his data that he could not compare forearm action between striking and throwing. Rather, the action of the racket, which is the distal segment in striking, was more comparable to the forearm action of throwing. Figure 7.1c shows the developmental comparison for forearm action. While some children exhibited different relationships early on, more progressed in striking before they progressed in throwing. Then throwing took the developmental lead for more children.

The finding on forearm action demonstrates how equipment can affect a person's developmental level. Using Newell's notions of person constraints (see chapter 3), we can think of equipment as an extension of the body. Comparing hammering with a hammer with pounding with a fist demonstrates how an implement changes the inertial characteristics of the arm and hand. This analysis also applies to prosthetic limbs. In both cases the performer needs to learn how to incorporate the implement into the actual movement pattern. Langendorfer's (1987a) data suggest that at first the implement assists the child in achieving a higher developmental level in the distal component, probably because its greater inertia causes lag. Later, though, the handle length may hinder change to a higher developmental level by making it more difficult to delay that lag until the body has reached front facing (see tables 7.1 and 7.4).

The last component that Langendorfer (1987a) compared across the two skills was stepping. As we can see in figure 7.1d, the developmental relationship for stepping was less clear. Langendorfer attributed this result to a degree of unreliability in the component sequence: Stepping tended to be variable across trials in both throwing and striking, making intertask comparisons unreliable.

The arrows in figure 7.1 reflect Langendorfer's (1987a) speculation on the longitudinal course that these two tasks might display while developing. The only actual longitudinal information that we have regarding this question was reported by Roberton (1982). She compared the development of trunk action within sidearm striking (as opposed to overarm striking) and overarm throwing. Interestingly, the trunk action of four children did follow the developmental path that Langendorfer's cross-sectional overarm data suggested: The children progressed one level in their throwing trunk action and then caught up to that same level in their striking trunk action. Then they progressed again in throwing before catching up in striking.

Motor Development Differences in Children With Autism Spectrum Disorders

One of the great difficulties facing practitioners and researchers working with individuals with ASD is the degree to which variability supersedes universality in this population (Pope et al., 2010). That is, two individuals with the same diagnosis of ASD will act and react very differently from each other. These individual differences lead to significant challenges for researchers trying to identify and quantify motor development in this population. For example, individuals with ASD may respond differently to a controlled experimental design due to different sensitivity to sensory stimuli or a lack of understanding of the described procedures. Furthermore, each individual responds differently to different interventions. All in all, typical developmental trajectories are all but impossible to determine. However, researchers have identified some general characteristics related to movement and motor development that suggest that individuals with ASD move differently from their typically developing counterparts (Pope et al., 2010). The importance of determining motor differences, particularly in younger children, cannot be underscored enough: If early developing motor differences exist, then assessment of these deficits may aid in identifying children at risk for ASD at a younger age than is currently possible.

Primarily through retrospective video analysis, researchers have identified early motor differences in children with ASD. Kanner (1943), who first identified ASD, noted that the infants exhibited hypotonia, or low muscle tone (similar to infants with DS). Other researchers have also observed hypotonia (Adrien et al., 1993; Ming, Brimacombe, & Wagner, 2007). For example, Ming et al. (2007) reviewed the clinical records of 154 children diagnosed with ASD in an attempt to determine the types and prevalence of motor impairments in this population, particularly hypotonia, delayed motor milestones, coordination issues, and toe walking. Children ranged in age from 2 to 18 years, with a mean age of 6 years. Reflecting the gender differences within the ASD population, there were 126 males and 28 females within the study. All of the children were on the autism spectrum: 74 were diagnosed with autism, 70 with PDD-NOS, and 10 with AS. The researchers determined that 51% of the group demonstrated hypotonia as infants; the prevalence of hypotonia decreased over time to 38% in the age range of 7 to 18 years. Not all retrospective studies have indicated that infants with autism have hypotonia. Saint-Georges et al. (2010) performed an exhaustive research review using retrospective video analysis and determined that across the 41 studies reviewed, hypotonia was not a consistent finding.

Given that at least a percentage of children with ASD exhibit hypotonia, subsequent issues with the acquisition of motor milestones may exist (as in children with DS). Low muscle tone affects infants' ability to raise the head while in the prone position, which affects the ability to sit supported, and so on. In fact, another consistent finding among individuals with ASD is a delay in achieving motor milestones and fundamental motor skills as young children. Acknowledging the limitations of looking back at records after diagnosis, Landa and Garrett-Mayer (2006) performed a 2-year longitudinal prospective study in which they followed a group of 84 infants who were either high (n = 60) or low (n = 27) risk for ASD.

Infants were tested at 6, 14, and 24 months using the Mullen Scales of Early Learning (MSEL), a standardized developmental test for children aged 0 to 69 months. The MSEL consists of five subscales: gross motor, fine motor, visual reception, receptive language, and expressive language. At 2 years, all children were given the Autism Diagnostic Observation Schedule (ADOS), which is a semistructured, play-based interview that provides systematic probes for symptoms of autism in social interaction, communication, play, and repetitive behaviors. Of the 60 children deemed at risk, 21 met the ADOS algorithm criteria for ASD as well as received a clinical diagnosis of ASD; 2 children from the low-risk group met the criteria and were included in the high-risk group. Early on, no statistical differences existed between groups. However, by 24 months, the high-risk group showed delays in both gross and fine motor skills, and group members were significantly different from the typically developing group in all domains.

In related research, a group of investigators assessed motor development in children younger than 3 years of age (Provost, Lopez, & Heimerl, 2007). In this study, 19 children with ASD, ranging in age from 21 to 41 months, were examined with the Bayley Scales of Infant Development II (BSID II; Bayley, 1993) and the Peabody Developmental Motor Scales, Second Edition (PDMS-2; Folio & Fewell, 2000). The researchers determined that 84% of the children were significantly delayed on the BSID II and PDMS-2. In a similar study, Ozonoff et al. (2008) examined the early movement behavior of children later identified as ASD and reported a delay in the appearance of motor milestones, including lying in prone and supine position, rolling, sitting, crawling, and walking, with the most significant delay occurring in walking onset. Therefore, it appears that delays in motor milestones exist in many of the children diagnosed with
ASD.

Acknowledging that delays appear to exist in the ASD population, Ozonoff et al. followed strong inference (see chapter 4) and asked the next logical question: Can motor delays differentiate children with autism from children who have more general developmental delays? If motor delays could be used for differentiation, then practitioners could use motor delays to identify infants at risk for ASD. In the resulting study, Ozonoff et al. (2008) placed 103 children between 24 and 60 months of age in 1 of 3 groups: children with ASD, children with developmental delays (DD), or children with typical development (TD). Home videotapes of the participants were analyzed and coded with the Infant Motor Maturity and Atypicality Coding Scales (IMMACS), which rate six motor milestones (lying in prone and supine position, roll, sit, crawl, walk) as well as protective skills (e.g., righting the body after losing balance). What Ozonoff et al. (2008) found, in contrast to earlier reports and speculation, is that the only group that differed in terms of number of movement abnormalities and protective skills was the DD group; the ASD and TD groups were not significantly different. The ASD and DD groups both showed motor delays, but they were not significantly different from each other. Therefore, the notion that motor delays can be used to identify children at risk for ASD was not supported in this research. Obviously, a prospective study examining both qualitative and quantitative variables is necessary as a next step to confirm this finding.

Are individuals with ASD merely delayed in motor skill acquisition, or do they differ in the ways in which they move? Several research studies (see table 9.5) have found at least one consistent difference between children with ASD and children without ASD, and that is their walking patterns. Over three decades ago, researchers found that children with ASD demonstrate abnormal limb movements, shortened steps, and persistent toe walking (Damasio & Maurer, 1978; Vilensky, Damasio, & Maurer, 1981). More recently, Ming et al. (2007), in their retrospective study, determined that nearly 20% of children with ASD walked on their toes rather than on their entire foot. Esposito and Venturi (2008) used retrospective video analysis as well as an observational scale to examine 42 children with ASD who had been walking at least 6 months and also found differences in early walking patterns. Vernazza-Martin et al. (2005) found significant differences in gait when comparing 15 children aged 4 to 6 years with and without ASD, as did Woodward (2001) in a study of children with ASD aged 3 and 10 years. One consistent finding is that the gait cycle is slower and less consistent in children with ASD.

Dewey, Cantell, and Crawford (2007) compared the performance of children with different disabilities on the Bruininks-Oseretsky Test of Motor Proficiency-Short Form. Participants included 49 children with ASD, 46 children with developmental coordination disorder (DCD) and attention-deficit/hyperactivity disorder (ADHD), 38 children with DCD, 27 children with ADHD, and 78 children with typical development. The results indicated that although all the atypical groups displayed significant impairment of motor skills, children with ASD were significantly more impaired compared with their cohorts with specific motor skill deficits. They were also the only group to show impairment on gestural skills. Several researchers have found deficiencies in fine motor skills in children with ASD. These range from delays in manual dexterity (Miyahara et al., 1997) and graphomotor skills (Mayes & Calhoun, 2003) during early and middle childhood to motor control issues in prehension (Mari, Castiello, Marks, Marraffa, & Prior, 2003). Such fine motor skill deficits influence handwriting as well as many functional activities involving the hands and arms. Across these studies, it appears that individuals with autism do move differently when compared with their typically developing counterparts and that deficits range from fine to gross motor skills.

In sum, although motor deficits are not listed as part of the DSM-V diagnostic criteria for ASD, individuals with these disorders tend to exhibit motor skill deficiencies as well as differences in motor development. Infants with ASD may exhibit hypotonia (although this finding is not as robust as it is in infants with DS). In addition, delays may exist in the acquisition of motor milestones and fundamental motor skills. Differences in skills such as gait and manual dexterity persist into childhood.

Developmental Coordination Disorder

Another disorder with an unknown etiology that has garnered considerable empirical attention as of late is DCD (Wilson & Larkin, 2008). DCD, also known as developmental dyspraxia, is characterized by extreme lack of motor coordination and by other movement deficits in the absence of neurological defects. DCD has been calculated to affect as much as 6% of the elementary school population (American Psychiatric Association, 1994; Barnhart, Davenport, Epps, & Nordquist, 2003; Barnett, Kooistra, & Henderson, 1998). Although the disorder was described as early as 1937, it took until 1994 for a group of 43 experts in various movement disorder fields to come to a consensus on a name and description for DCD in order to facilitate both research and diagnosis (see the sidebar for the DSM-V description of DCD). Individuals characterized as having DCD fall into the 0 to 10th percentile of performance on the Movement Assessment Battery for Children (M-ABC), a standardized motor skills test used to assess motor proficiency in children with disabilities (Geuze, Jongmans, Schoemaker, & Smits-Engelsman, 2001; Henderson & Sugden, 1992; Miyahara & Möbs, 1995). Because DCD is categorized as a learning disability, it also may influence a child's ability to perform academically. Motor difficulties can range from gross to fine motor to balance skills and can include motor planning deficits and visual or spatial difficulties (Cermak & Larkin, 2002; de Castelnau, Albaret, Chaix, & Zanone, 2007; Kaplan, Wilson, Dewey, & Crawford, 1998; Przysucha & Taylor, 2004; Whitall et al., 2006). Table 9.6 provides examples of functional motor issues that children with DCD may have.

Applying the Dynamic Systems Model to a Motor Development Problem

Perhaps the best way to appreciate how the characteristics of dynamic systems can be used to model motor development is to see how a research team used dynamic systems to address a particular developmental issue. Let's consider the case of the reflexes seen in infancy. Recall that a reflex is a particular automatic response to a specific stimulus. It is a response that is carried out without thought or intention. Developmentalists have long observed the existence of reflexes that disappear later on in infants.

Developmentalists have been particularly interested in the walking or stepping reflex for its similarity to voluntary walking (see table 3.1). The reflex is elicited by holding an infant upright and lowering the infant to a horizontal surface.

The reflexive response is stepping movements of the legs. The stepping reflex is typically observed up to 5 months of age, so it appears much earlier than voluntary walking and disappears months before voluntary walking.

Developmentalists have offered several explanations for the disappearance of the stepping reflex. The maturationists suggested that the disappearance reflects the maturation of the cortex. These researchers proposed that as higher brain centers mature they are able to inhibit the lower brain centers that mediate reflexes. With continued maturation, the centers control voluntary walking. The existence and disappearance of reflexes reflect the status of nervous system development.

Peiper (1963) suggested instead that reflexes have a purpose because they allow practice of movement patterns before the higher brain centers are mature enough to control those patterns. Zelazo et al. (Zelazo, 1983; Zelazo, Konner, Kolb, & Zelazo, 1974; Zelazo, Zelazo, & Kolb, 1972) offered yet another viewpoint. They purposely increased the number of times the stepping reflex was elicited in a small group of infants. Later, these infants were observed to start walking at a younger age than average. The researchers concluded that reflexes can be subsumed into voluntary movements. For example, the walking reflex is transformed into voluntary walking.

Unconvinced by these various hypotheses, Esther Thelen and her colleagues (Thelen & Fisher, 1982; Thelen, Fisher, & Ridley-Johnson, 1984) developed an ingenious series of experiments to examine different potential causes for the disappearance of the stepping reflex. They began with the notion that multiple systems—rather than just one system—play a role in the disappearance. They had observed that one of the changes in infants correlating with the disappearance of the stepping reflex is an increase in subcutaneous fat that occurs during infancy. Could this gain in fat and the resultant change in the strength required to move heavier legs be related to the disappearance of the stepping reflex? In order to test whether the muscle and adipose tissue systems are involved, the researchers had to find a way to make a light and stepping infant resemble a nonstepping and heavier infant and, just as importantly, to get a non-stepping, heavier infant to resemble a lighter, stepping infant. The only variable that could be changed was leg strength. In a first experiment, the researchers added tiny weights to the legs of lighter infants. The added weight was proportional to the amount of weight the infant would add between 4 and 6 weeks of age. If maturation of the nervous system were the only cause of reflex stepping, then the infants should continue to step regardless of the small weights. Actually, the infants reduced the number of reflex steps taken while wearing the weights.

The next experiment was designed to regress the leg weight of heavier infants to that of younger, lighter infants. The researchers achieved this by holding the infants upright in an aquarium filled with water up to their hips. The water reduced the pull of gravity on the legs and made them relatively lighter. The water was room temperature, and the researchers noted that often the infants did not respond in any way to being held in the water. When the infants' feet touched the bottom surface of the tank, though, the number of steps increased (Thelen & Fisher, 1982). If maturation of the nervous system is the only explanation
for the reflex disappearing, then nothing should have been able to increase the rate of stepping, as the higher brain centers would have inhibited it. Thus the researchers demonstrated that change in other systems plays a role in the disappearance of the stepping reflex.

Thelen et al. demonstrated that the systems approach can be used to examine the processes underlying development. While maturation of the nervous system is important in motor development, other subsystems are important to the change observed in the stepping reflex. As well as studying multiple subsystems, Thelen et al. demonstrated that change in a subsystem can function as a control variable. The role of a control variable in bringing about change is another characteristic of dynamic systems. More of Thelen's work will be discussed throughout the text.

Summary

The systems approach has much to offer researchers interested in understanding developmental change. Dynamic systems evolve over time and undergo change that is not linear. These characteristics describe the body as it grows and ages. Dynamic systems evolve by transitioning from times of stability, or attractor states, through times of instability to other attractor states. We observe this pattern in motor development, too. Dynamic systems are sensitive to the conditions initially in place when they begin to evolve. Movement scientists have long realized that models of movement control must allow for the same movement goal to be achieved from variable environmental conditions and different starting positions of the body and limbs. Most importantly, dynamic systems have been shown to self-organize and to use methods of control such as entrainment and the application of constraints. The demonstration of these possibilities is important, as they provide explanations for how the large number of degrees of freedom in movement can be reduced for the reasonable control of complex movements.

The systems approach provided a heuristic function for movement scientists by focusing researchers' attention on new ways to solve existing questions in motor development. There is value in examining multiple theories. Researchers using differing approaches all serve to gain from alternative views, as both theorists and empiricists are forced to question behaviors their theories can't account for and improve their models until a satisfactory explanation can be found (for example, see Schmidt & Lee, 2011 on generalized motor programs). Ultimately, these challenges and changes serve to enhance our understanding of motor development.

Development of Ballistic Skills

Movements can be classified by their EMG and force curve signatures. Three movement categories are (1) ramped movements in which force is gradually and steadily applied through continuously graded muscle firing; (2) discontinuous, slow movements that accelerate gradually, then decelerate, and then accelerate again, such as when tracking an object with a hand; and (3) ballistic movements that are rapid, all-out, forceful movements that reach peak acceleration within milliseconds of their initiation (Brooks, 1983; Desmedt, 1983).

In simple ballistic movements, such as a rapid reach for an object, the motion is started by an initial strong impulse in the agonist muscle. The agonist firing then stops, but the movement continues due to the momentum of the limb. A burst by the antagonist muscle subsequently causes the limb to decelerate. In targeting tasks, there may be a second burst in the agonist to correct the limb placement. This creates a signature triphasic pattern of muscle activation: agonist, antagonist, agonist (Palmer, Cafarelli, & Ashby, 1994).

In contrast to single-limb movements, ballistic skills involving the total body are quite complex. Typical sport skills are examples of ballistic skills. Of these, movements in which the performer projects an object have been of considerable interest to both biomechanists and developmentalists. Striking (as in batting and swinging a racket), kicking (as in football and soccer), and throwing (as in handball and baseball or softball) have all received study. These complex movements tend to begin proximally with action of the trunk muscles; energy is then passed from segment to segment of the open kinetic chain until it reaches the distal effector, usually the hand or foot. During striking, kicking, and throwing, this energy exchange is superimposed on a running or stepping base of support.

Another interesting commonality across these complex projectile skills is that they employ a backswing to place the body segments in position to move forward. In the advanced form of striking, throwing, and kicking, the backswing and forward swing partially overlap, and proximal segments begin to move forward while more distal segments are still moving backward. The advanced form of these movements also involves strong acceleration of the distal segment just milliseconds before contact or release, which yields high distal angular velocity. This acceleration seems to result partially from the inertia of each distal segment against the motion of adjacent proximal segments. The lagging segment stretches the agonist muscles of the distal segment, which may in turn excite reflexes that augment the muscular contractions in the distal segment (Roberts & Metcalfe, 1968). As we will see, lag is one aspect of object projection that develops only gradually over time. It is so important for effective ballistic movement that Southard (2002a, 2002b) used lag as a collective variable in his studies of throwing.

In this chapter we focus on the development of striking, throwing, and kicking. Most developmental studies on ballistic projectile skills have been based in developmental sequence theory, although a few studies are beginning to address issues from a dynamic systems perspective. At the close of this chapter we discuss future directions in this area.

Intertask Developmental Sequence

The only ballistic skills that have been studied in relation to their intertask development are overarm throwing and striking. The approach used in this research is quite different from the intertask approach used to study locomotion (see chapter 6). For locomotion, the developmental sequence is formed by ordering each locomotor skill according to its initial appearance in the movement repertoire. For throwing and striking, the two tasks are compared to see where participants are located within a component developmental sequence at the same point in time in each skill. Do the participants show the same developmental level in the two skills, or do they develop faster in one skill than they do in the other?

Langendorfer (1987a) compared the development of trunk and humerus action in overarm throwing with development of the same action in overarm striking. He asked 58 boys aged 1.5 to 10.3 years to throw a ball forcefully and to strike a tennis ball suspended 6 inches (15 cm) above their head forcefully with a lightweight racket. He then looked at side and rear film views to categorize the boys' movements in each skill. Figure 7.1 shows his results. From the cross-sectional data we can infer how the longitudinal data might look. The data on trunk action (see figure 7.1a) suggested that children advance in throwing before they do in striking: The boys started out at a primitive level in both skills and then advanced to level 2 throwing before achieving level 2 striking and advanced to level 3 throwing before reaching level 3 striking. The cross-sectional data on humerus action (see figure 7.1b) suggested that children begin at level 1 in both skills and then move ahead in either skill until they reach level 2 in both skills. Then, however, they progress to level 3 in throwing before they progress to level 3 in striking.

Langendorfer (1987a) learned from his data that he could not compare forearm action between striking and throwing. Rather, the action of the racket, which is the distal segment in striking, was more comparable to the forearm action of throwing. Figure 7.1c shows the developmental comparison for forearm action. While some children exhibited different relationships early on, more progressed in striking before they progressed in throwing. Then throwing took the developmental lead for more children.

The finding on forearm action demonstrates how equipment can affect a person's developmental level. Using Newell's notions of person constraints (see chapter 3), we can think of equipment as an extension of the body. Comparing hammering with a hammer with pounding with a fist demonstrates how an implement changes the inertial characteristics of the arm and hand. This analysis also applies to prosthetic limbs. In both cases the performer needs to learn how to incorporate the implement into the actual movement pattern. Langendorfer's (1987a) data suggest that at first the implement assists the child in achieving a higher developmental level in the distal component, probably because its greater inertia causes lag. Later, though, the handle length may hinder change to a higher developmental level by making it more difficult to delay that lag until the body has reached front facing (see tables 7.1 and 7.4).

The last component that Langendorfer (1987a) compared across the two skills was stepping. As we can see in figure 7.1d, the developmental relationship for stepping was less clear. Langendorfer attributed this result to a degree of unreliability in the component sequence: Stepping tended to be variable across trials in both throwing and striking, making intertask comparisons unreliable.

The arrows in figure 7.1 reflect Langendorfer's (1987a) speculation on the longitudinal course that these two tasks might display while developing. The only actual longitudinal information that we have regarding this question was reported by Roberton (1982). She compared the development of trunk action within sidearm striking (as opposed to overarm striking) and overarm throwing. Interestingly, the trunk action of four children did follow the developmental path that Langendorfer's cross-sectional overarm data suggested: The children progressed one level in their throwing trunk action and then caught up to that same level in their striking trunk action. Then they progressed again in throwing before catching up in striking.

Motor Development Differences in Children With Autism Spectrum Disorders

One of the great difficulties facing practitioners and researchers working with individuals with ASD is the degree to which variability supersedes universality in this population (Pope et al., 2010). That is, two individuals with the same diagnosis of ASD will act and react very differently from each other. These individual differences lead to significant challenges for researchers trying to identify and quantify motor development in this population. For example, individuals with ASD may respond differently to a controlled experimental design due to different sensitivity to sensory stimuli or a lack of understanding of the described procedures. Furthermore, each individual responds differently to different interventions. All in all, typical developmental trajectories are all but impossible to determine. However, researchers have identified some general characteristics related to movement and motor development that suggest that individuals with ASD move differently from their typically developing counterparts (Pope et al., 2010). The importance of determining motor differences, particularly in younger children, cannot be underscored enough: If early developing motor differences exist, then assessment of these deficits may aid in identifying children at risk for ASD at a younger age than is currently possible.

Primarily through retrospective video analysis, researchers have identified early motor differences in children with ASD. Kanner (1943), who first identified ASD, noted that the infants exhibited hypotonia, or low muscle tone (similar to infants with DS). Other researchers have also observed hypotonia (Adrien et al., 1993; Ming, Brimacombe, & Wagner, 2007). For example, Ming et al. (2007) reviewed the clinical records of 154 children diagnosed with ASD in an attempt to determine the types and prevalence of motor impairments in this population, particularly hypotonia, delayed motor milestones, coordination issues, and toe walking. Children ranged in age from 2 to 18 years, with a mean age of 6 years. Reflecting the gender differences within the ASD population, there were 126 males and 28 females within the study. All of the children were on the autism spectrum: 74 were diagnosed with autism, 70 with PDD-NOS, and 10 with AS. The researchers determined that 51% of the group demonstrated hypotonia as infants; the prevalence of hypotonia decreased over time to 38% in the age range of 7 to 18 years. Not all retrospective studies have indicated that infants with autism have hypotonia. Saint-Georges et al. (2010) performed an exhaustive research review using retrospective video analysis and determined that across the 41 studies reviewed, hypotonia was not a consistent finding.

Given that at least a percentage of children with ASD exhibit hypotonia, subsequent issues with the acquisition of motor milestones may exist (as in children with DS). Low muscle tone affects infants' ability to raise the head while in the prone position, which affects the ability to sit supported, and so on. In fact, another consistent finding among individuals with ASD is a delay in achieving motor milestones and fundamental motor skills as young children. Acknowledging the limitations of looking back at records after diagnosis, Landa and Garrett-Mayer (2006) performed a 2-year longitudinal prospective study in which they followed a group of 84 infants who were either high (n = 60) or low (n = 27) risk for ASD.

Infants were tested at 6, 14, and 24 months using the Mullen Scales of Early Learning (MSEL), a standardized developmental test for children aged 0 to 69 months. The MSEL consists of five subscales: gross motor, fine motor, visual reception, receptive language, and expressive language. At 2 years, all children were given the Autism Diagnostic Observation Schedule (ADOS), which is a semistructured, play-based interview that provides systematic probes for symptoms of autism in social interaction, communication, play, and repetitive behaviors. Of the 60 children deemed at risk, 21 met the ADOS algorithm criteria for ASD as well as received a clinical diagnosis of ASD; 2 children from the low-risk group met the criteria and were included in the high-risk group. Early on, no statistical differences existed between groups. However, by 24 months, the high-risk group showed delays in both gross and fine motor skills, and group members were significantly different from the typically developing group in all domains.

In related research, a group of investigators assessed motor development in children younger than 3 years of age (Provost, Lopez, & Heimerl, 2007). In this study, 19 children with ASD, ranging in age from 21 to 41 months, were examined with the Bayley Scales of Infant Development II (BSID II; Bayley, 1993) and the Peabody Developmental Motor Scales, Second Edition (PDMS-2; Folio & Fewell, 2000). The researchers determined that 84% of the children were significantly delayed on the BSID II and PDMS-2. In a similar study, Ozonoff et al. (2008) examined the early movement behavior of children later identified as ASD and reported a delay in the appearance of motor milestones, including lying in prone and supine position, rolling, sitting, crawling, and walking, with the most significant delay occurring in walking onset. Therefore, it appears that delays in motor milestones exist in many of the children diagnosed with
ASD.

Acknowledging that delays appear to exist in the ASD population, Ozonoff et al. followed strong inference (see chapter 4) and asked the next logical question: Can motor delays differentiate children with autism from children who have more general developmental delays? If motor delays could be used for differentiation, then practitioners could use motor delays to identify infants at risk for ASD. In the resulting study, Ozonoff et al. (2008) placed 103 children between 24 and 60 months of age in 1 of 3 groups: children with ASD, children with developmental delays (DD), or children with typical development (TD). Home videotapes of the participants were analyzed and coded with the Infant Motor Maturity and Atypicality Coding Scales (IMMACS), which rate six motor milestones (lying in prone and supine position, roll, sit, crawl, walk) as well as protective skills (e.g., righting the body after losing balance). What Ozonoff et al. (2008) found, in contrast to earlier reports and speculation, is that the only group that differed in terms of number of movement abnormalities and protective skills was the DD group; the ASD and TD groups were not significantly different. The ASD and DD groups both showed motor delays, but they were not significantly different from each other. Therefore, the notion that motor delays can be used to identify children at risk for ASD was not supported in this research. Obviously, a prospective study examining both qualitative and quantitative variables is necessary as a next step to confirm this finding.

Are individuals with ASD merely delayed in motor skill acquisition, or do they differ in the ways in which they move? Several research studies (see table 9.5) have found at least one consistent difference between children with ASD and children without ASD, and that is their walking patterns. Over three decades ago, researchers found that children with ASD demonstrate abnormal limb movements, shortened steps, and persistent toe walking (Damasio & Maurer, 1978; Vilensky, Damasio, & Maurer, 1981). More recently, Ming et al. (2007), in their retrospective study, determined that nearly 20% of children with ASD walked on their toes rather than on their entire foot. Esposito and Venturi (2008) used retrospective video analysis as well as an observational scale to examine 42 children with ASD who had been walking at least 6 months and also found differences in early walking patterns. Vernazza-Martin et al. (2005) found significant differences in gait when comparing 15 children aged 4 to 6 years with and without ASD, as did Woodward (2001) in a study of children with ASD aged 3 and 10 years. One consistent finding is that the gait cycle is slower and less consistent in children with ASD.

Dewey, Cantell, and Crawford (2007) compared the performance of children with different disabilities on the Bruininks-Oseretsky Test of Motor Proficiency-Short Form. Participants included 49 children with ASD, 46 children with developmental coordination disorder (DCD) and attention-deficit/hyperactivity disorder (ADHD), 38 children with DCD, 27 children with ADHD, and 78 children with typical development. The results indicated that although all the atypical groups displayed significant impairment of motor skills, children with ASD were significantly more impaired compared with their cohorts with specific motor skill deficits. They were also the only group to show impairment on gestural skills. Several researchers have found deficiencies in fine motor skills in children with ASD. These range from delays in manual dexterity (Miyahara et al., 1997) and graphomotor skills (Mayes & Calhoun, 2003) during early and middle childhood to motor control issues in prehension (Mari, Castiello, Marks, Marraffa, & Prior, 2003). Such fine motor skill deficits influence handwriting as well as many functional activities involving the hands and arms. Across these studies, it appears that individuals with autism do move differently when compared with their typically developing counterparts and that deficits range from fine to gross motor skills.

In sum, although motor deficits are not listed as part of the DSM-V diagnostic criteria for ASD, individuals with these disorders tend to exhibit motor skill deficiencies as well as differences in motor development. Infants with ASD may exhibit hypotonia (although this finding is not as robust as it is in infants with DS). In addition, delays may exist in the acquisition of motor milestones and fundamental motor skills. Differences in skills such as gait and manual dexterity persist into childhood.

Developmental Coordination Disorder

Another disorder with an unknown etiology that has garnered considerable empirical attention as of late is DCD (Wilson & Larkin, 2008). DCD, also known as developmental dyspraxia, is characterized by extreme lack of motor coordination and by other movement deficits in the absence of neurological defects. DCD has been calculated to affect as much as 6% of the elementary school population (American Psychiatric Association, 1994; Barnhart, Davenport, Epps, & Nordquist, 2003; Barnett, Kooistra, & Henderson, 1998). Although the disorder was described as early as 1937, it took until 1994 for a group of 43 experts in various movement disorder fields to come to a consensus on a name and description for DCD in order to facilitate both research and diagnosis (see the sidebar for the DSM-V description of DCD). Individuals characterized as having DCD fall into the 0 to 10th percentile of performance on the Movement Assessment Battery for Children (M-ABC), a standardized motor skills test used to assess motor proficiency in children with disabilities (Geuze, Jongmans, Schoemaker, & Smits-Engelsman, 2001; Henderson & Sugden, 1992; Miyahara & Möbs, 1995). Because DCD is categorized as a learning disability, it also may influence a child's ability to perform academically. Motor difficulties can range from gross to fine motor to balance skills and can include motor planning deficits and visual or spatial difficulties (Cermak & Larkin, 2002; de Castelnau, Albaret, Chaix, & Zanone, 2007; Kaplan, Wilson, Dewey, & Crawford, 1998; Przysucha & Taylor, 2004; Whitall et al., 2006). Table 9.6 provides examples of functional motor issues that children with DCD may have.

Applying the Dynamic Systems Model to a Motor Development Problem

Perhaps the best way to appreciate how the characteristics of dynamic systems can be used to model motor development is to see how a research team used dynamic systems to address a particular developmental issue. Let's consider the case of the reflexes seen in infancy. Recall that a reflex is a particular automatic response to a specific stimulus. It is a response that is carried out without thought or intention. Developmentalists have long observed the existence of reflexes that disappear later on in infants.

Developmentalists have been particularly interested in the walking or stepping reflex for its similarity to voluntary walking (see table 3.1). The reflex is elicited by holding an infant upright and lowering the infant to a horizontal surface.

The reflexive response is stepping movements of the legs. The stepping reflex is typically observed up to 5 months of age, so it appears much earlier than voluntary walking and disappears months before voluntary walking.

Developmentalists have offered several explanations for the disappearance of the stepping reflex. The maturationists suggested that the disappearance reflects the maturation of the cortex. These researchers proposed that as higher brain centers mature they are able to inhibit the lower brain centers that mediate reflexes. With continued maturation, the centers control voluntary walking. The existence and disappearance of reflexes reflect the status of nervous system development.

Peiper (1963) suggested instead that reflexes have a purpose because they allow practice of movement patterns before the higher brain centers are mature enough to control those patterns. Zelazo et al. (Zelazo, 1983; Zelazo, Konner, Kolb, & Zelazo, 1974; Zelazo, Zelazo, & Kolb, 1972) offered yet another viewpoint. They purposely increased the number of times the stepping reflex was elicited in a small group of infants. Later, these infants were observed to start walking at a younger age than average. The researchers concluded that reflexes can be subsumed into voluntary movements. For example, the walking reflex is transformed into voluntary walking.

Unconvinced by these various hypotheses, Esther Thelen and her colleagues (Thelen & Fisher, 1982; Thelen, Fisher, & Ridley-Johnson, 1984) developed an ingenious series of experiments to examine different potential causes for the disappearance of the stepping reflex. They began with the notion that multiple systems—rather than just one system—play a role in the disappearance. They had observed that one of the changes in infants correlating with the disappearance of the stepping reflex is an increase in subcutaneous fat that occurs during infancy. Could this gain in fat and the resultant change in the strength required to move heavier legs be related to the disappearance of the stepping reflex? In order to test whether the muscle and adipose tissue systems are involved, the researchers had to find a way to make a light and stepping infant resemble a nonstepping and heavier infant and, just as importantly, to get a non-stepping, heavier infant to resemble a lighter, stepping infant. The only variable that could be changed was leg strength. In a first experiment, the researchers added tiny weights to the legs of lighter infants. The added weight was proportional to the amount of weight the infant would add between 4 and 6 weeks of age. If maturation of the nervous system were the only cause of reflex stepping, then the infants should continue to step regardless of the small weights. Actually, the infants reduced the number of reflex steps taken while wearing the weights.

The next experiment was designed to regress the leg weight of heavier infants to that of younger, lighter infants. The researchers achieved this by holding the infants upright in an aquarium filled with water up to their hips. The water reduced the pull of gravity on the legs and made them relatively lighter. The water was room temperature, and the researchers noted that often the infants did not respond in any way to being held in the water. When the infants' feet touched the bottom surface of the tank, though, the number of steps increased (Thelen & Fisher, 1982). If maturation of the nervous system is the only explanation
for the reflex disappearing, then nothing should have been able to increase the rate of stepping, as the higher brain centers would have inhibited it. Thus the researchers demonstrated that change in other systems plays a role in the disappearance of the stepping reflex.

Thelen et al. demonstrated that the systems approach can be used to examine the processes underlying development. While maturation of the nervous system is important in motor development, other subsystems are important to the change observed in the stepping reflex. As well as studying multiple subsystems, Thelen et al. demonstrated that change in a subsystem can function as a control variable. The role of a control variable in bringing about change is another characteristic of dynamic systems. More of Thelen's work will be discussed throughout the text.

Summary

The systems approach has much to offer researchers interested in understanding developmental change. Dynamic systems evolve over time and undergo change that is not linear. These characteristics describe the body as it grows and ages. Dynamic systems evolve by transitioning from times of stability, or attractor states, through times of instability to other attractor states. We observe this pattern in motor development, too. Dynamic systems are sensitive to the conditions initially in place when they begin to evolve. Movement scientists have long realized that models of movement control must allow for the same movement goal to be achieved from variable environmental conditions and different starting positions of the body and limbs. Most importantly, dynamic systems have been shown to self-organize and to use methods of control such as entrainment and the application of constraints. The demonstration of these possibilities is important, as they provide explanations for how the large number of degrees of freedom in movement can be reduced for the reasonable control of complex movements.

The systems approach provided a heuristic function for movement scientists by focusing researchers' attention on new ways to solve existing questions in motor development. There is value in examining multiple theories. Researchers using differing approaches all serve to gain from alternative views, as both theorists and empiricists are forced to question behaviors their theories can't account for and improve their models until a satisfactory explanation can be found (for example, see Schmidt & Lee, 2011 on generalized motor programs). Ultimately, these challenges and changes serve to enhance our understanding of motor development.

Development of Ballistic Skills

Movements can be classified by their EMG and force curve signatures. Three movement categories are (1) ramped movements in which force is gradually and steadily applied through continuously graded muscle firing; (2) discontinuous, slow movements that accelerate gradually, then decelerate, and then accelerate again, such as when tracking an object with a hand; and (3) ballistic movements that are rapid, all-out, forceful movements that reach peak acceleration within milliseconds of their initiation (Brooks, 1983; Desmedt, 1983).

In simple ballistic movements, such as a rapid reach for an object, the motion is started by an initial strong impulse in the agonist muscle. The agonist firing then stops, but the movement continues due to the momentum of the limb. A burst by the antagonist muscle subsequently causes the limb to decelerate. In targeting tasks, there may be a second burst in the agonist to correct the limb placement. This creates a signature triphasic pattern of muscle activation: agonist, antagonist, agonist (Palmer, Cafarelli, & Ashby, 1994).

In contrast to single-limb movements, ballistic skills involving the total body are quite complex. Typical sport skills are examples of ballistic skills. Of these, movements in which the performer projects an object have been of considerable interest to both biomechanists and developmentalists. Striking (as in batting and swinging a racket), kicking (as in football and soccer), and throwing (as in handball and baseball or softball) have all received study. These complex movements tend to begin proximally with action of the trunk muscles; energy is then passed from segment to segment of the open kinetic chain until it reaches the distal effector, usually the hand or foot. During striking, kicking, and throwing, this energy exchange is superimposed on a running or stepping base of support.

Another interesting commonality across these complex projectile skills is that they employ a backswing to place the body segments in position to move forward. In the advanced form of striking, throwing, and kicking, the backswing and forward swing partially overlap, and proximal segments begin to move forward while more distal segments are still moving backward. The advanced form of these movements also involves strong acceleration of the distal segment just milliseconds before contact or release, which yields high distal angular velocity. This acceleration seems to result partially from the inertia of each distal segment against the motion of adjacent proximal segments. The lagging segment stretches the agonist muscles of the distal segment, which may in turn excite reflexes that augment the muscular contractions in the distal segment (Roberts & Metcalfe, 1968). As we will see, lag is one aspect of object projection that develops only gradually over time. It is so important for effective ballistic movement that Southard (2002a, 2002b) used lag as a collective variable in his studies of throwing.

In this chapter we focus on the development of striking, throwing, and kicking. Most developmental studies on ballistic projectile skills have been based in developmental sequence theory, although a few studies are beginning to address issues from a dynamic systems perspective. At the close of this chapter we discuss future directions in this area.

Intertask Developmental Sequence

The only ballistic skills that have been studied in relation to their intertask development are overarm throwing and striking. The approach used in this research is quite different from the intertask approach used to study locomotion (see chapter 6). For locomotion, the developmental sequence is formed by ordering each locomotor skill according to its initial appearance in the movement repertoire. For throwing and striking, the two tasks are compared to see where participants are located within a component developmental sequence at the same point in time in each skill. Do the participants show the same developmental level in the two skills, or do they develop faster in one skill than they do in the other?

Langendorfer (1987a) compared the development of trunk and humerus action in overarm throwing with development of the same action in overarm striking. He asked 58 boys aged 1.5 to 10.3 years to throw a ball forcefully and to strike a tennis ball suspended 6 inches (15 cm) above their head forcefully with a lightweight racket. He then looked at side and rear film views to categorize the boys' movements in each skill. Figure 7.1 shows his results. From the cross-sectional data we can infer how the longitudinal data might look. The data on trunk action (see figure 7.1a) suggested that children advance in throwing before they do in striking: The boys started out at a primitive level in both skills and then advanced to level 2 throwing before achieving level 2 striking and advanced to level 3 throwing before reaching level 3 striking. The cross-sectional data on humerus action (see figure 7.1b) suggested that children begin at level 1 in both skills and then move ahead in either skill until they reach level 2 in both skills. Then, however, they progress to level 3 in throwing before they progress to level 3 in striking.

Langendorfer (1987a) learned from his data that he could not compare forearm action between striking and throwing. Rather, the action of the racket, which is the distal segment in striking, was more comparable to the forearm action of throwing. Figure 7.1c shows the developmental comparison for forearm action. While some children exhibited different relationships early on, more progressed in striking before they progressed in throwing. Then throwing took the developmental lead for more children.

The finding on forearm action demonstrates how equipment can affect a person's developmental level. Using Newell's notions of person constraints (see chapter 3), we can think of equipment as an extension of the body. Comparing hammering with a hammer with pounding with a fist demonstrates how an implement changes the inertial characteristics of the arm and hand. This analysis also applies to prosthetic limbs. In both cases the performer needs to learn how to incorporate the implement into the actual movement pattern. Langendorfer's (1987a) data suggest that at first the implement assists the child in achieving a higher developmental level in the distal component, probably because its greater inertia causes lag. Later, though, the handle length may hinder change to a higher developmental level by making it more difficult to delay that lag until the body has reached front facing (see tables 7.1 and 7.4).

The last component that Langendorfer (1987a) compared across the two skills was stepping. As we can see in figure 7.1d, the developmental relationship for stepping was less clear. Langendorfer attributed this result to a degree of unreliability in the component sequence: Stepping tended to be variable across trials in both throwing and striking, making intertask comparisons unreliable.

The arrows in figure 7.1 reflect Langendorfer's (1987a) speculation on the longitudinal course that these two tasks might display while developing. The only actual longitudinal information that we have regarding this question was reported by Roberton (1982). She compared the development of trunk action within sidearm striking (as opposed to overarm striking) and overarm throwing. Interestingly, the trunk action of four children did follow the developmental path that Langendorfer's cross-sectional overarm data suggested: The children progressed one level in their throwing trunk action and then caught up to that same level in their striking trunk action. Then they progressed again in throwing before catching up in striking.

Motor Development Differences in Children With Autism Spectrum Disorders

One of the great difficulties facing practitioners and researchers working with individuals with ASD is the degree to which variability supersedes universality in this population (Pope et al., 2010). That is, two individuals with the same diagnosis of ASD will act and react very differently from each other. These individual differences lead to significant challenges for researchers trying to identify and quantify motor development in this population. For example, individuals with ASD may respond differently to a controlled experimental design due to different sensitivity to sensory stimuli or a lack of understanding of the described procedures. Furthermore, each individual responds differently to different interventions. All in all, typical developmental trajectories are all but impossible to determine. However, researchers have identified some general characteristics related to movement and motor development that suggest that individuals with ASD move differently from their typically developing counterparts (Pope et al., 2010). The importance of determining motor differences, particularly in younger children, cannot be underscored enough: If early developing motor differences exist, then assessment of these deficits may aid in identifying children at risk for ASD at a younger age than is currently possible.

Primarily through retrospective video analysis, researchers have identified early motor differences in children with ASD. Kanner (1943), who first identified ASD, noted that the infants exhibited hypotonia, or low muscle tone (similar to infants with DS). Other researchers have also observed hypotonia (Adrien et al., 1993; Ming, Brimacombe, & Wagner, 2007). For example, Ming et al. (2007) reviewed the clinical records of 154 children diagnosed with ASD in an attempt to determine the types and prevalence of motor impairments in this population, particularly hypotonia, delayed motor milestones, coordination issues, and toe walking. Children ranged in age from 2 to 18 years, with a mean age of 6 years. Reflecting the gender differences within the ASD population, there were 126 males and 28 females within the study. All of the children were on the autism spectrum: 74 were diagnosed with autism, 70 with PDD-NOS, and 10 with AS. The researchers determined that 51% of the group demonstrated hypotonia as infants; the prevalence of hypotonia decreased over time to 38% in the age range of 7 to 18 years. Not all retrospective studies have indicated that infants with autism have hypotonia. Saint-Georges et al. (2010) performed an exhaustive research review using retrospective video analysis and determined that across the 41 studies reviewed, hypotonia was not a consistent finding.

Given that at least a percentage of children with ASD exhibit hypotonia, subsequent issues with the acquisition of motor milestones may exist (as in children with DS). Low muscle tone affects infants' ability to raise the head while in the prone position, which affects the ability to sit supported, and so on. In fact, another consistent finding among individuals with ASD is a delay in achieving motor milestones and fundamental motor skills as young children. Acknowledging the limitations of looking back at records after diagnosis, Landa and Garrett-Mayer (2006) performed a 2-year longitudinal prospective study in which they followed a group of 84 infants who were either high (n = 60) or low (n = 27) risk for ASD.

Infants were tested at 6, 14, and 24 months using the Mullen Scales of Early Learning (MSEL), a standardized developmental test for children aged 0 to 69 months. The MSEL consists of five subscales: gross motor, fine motor, visual reception, receptive language, and expressive language. At 2 years, all children were given the Autism Diagnostic Observation Schedule (ADOS), which is a semistructured, play-based interview that provides systematic probes for symptoms of autism in social interaction, communication, play, and repetitive behaviors. Of the 60 children deemed at risk, 21 met the ADOS algorithm criteria for ASD as well as received a clinical diagnosis of ASD; 2 children from the low-risk group met the criteria and were included in the high-risk group. Early on, no statistical differences existed between groups. However, by 24 months, the high-risk group showed delays in both gross and fine motor skills, and group members were significantly different from the typically developing group in all domains.

In related research, a group of investigators assessed motor development in children younger than 3 years of age (Provost, Lopez, & Heimerl, 2007). In this study, 19 children with ASD, ranging in age from 21 to 41 months, were examined with the Bayley Scales of Infant Development II (BSID II; Bayley, 1993) and the Peabody Developmental Motor Scales, Second Edition (PDMS-2; Folio & Fewell, 2000). The researchers determined that 84% of the children were significantly delayed on the BSID II and PDMS-2. In a similar study, Ozonoff et al. (2008) examined the early movement behavior of children later identified as ASD and reported a delay in the appearance of motor milestones, including lying in prone and supine position, rolling, sitting, crawling, and walking, with the most significant delay occurring in walking onset. Therefore, it appears that delays in motor milestones exist in many of the children diagnosed with
ASD.

Acknowledging that delays appear to exist in the ASD population, Ozonoff et al. followed strong inference (see chapter 4) and asked the next logical question: Can motor delays differentiate children with autism from children who have more general developmental delays? If motor delays could be used for differentiation, then practitioners could use motor delays to identify infants at risk for ASD. In the resulting study, Ozonoff et al. (2008) placed 103 children between 24 and 60 months of age in 1 of 3 groups: children with ASD, children with developmental delays (DD), or children with typical development (TD). Home videotapes of the participants were analyzed and coded with the Infant Motor Maturity and Atypicality Coding Scales (IMMACS), which rate six motor milestones (lying in prone and supine position, roll, sit, crawl, walk) as well as protective skills (e.g., righting the body after losing balance). What Ozonoff et al. (2008) found, in contrast to earlier reports and speculation, is that the only group that differed in terms of number of movement abnormalities and protective skills was the DD group; the ASD and TD groups were not significantly different. The ASD and DD groups both showed motor delays, but they were not significantly different from each other. Therefore, the notion that motor delays can be used to identify children at risk for ASD was not supported in this research. Obviously, a prospective study examining both qualitative and quantitative variables is necessary as a next step to confirm this finding.

Are individuals with ASD merely delayed in motor skill acquisition, or do they differ in the ways in which they move? Several research studies (see table 9.5) have found at least one consistent difference between children with ASD and children without ASD, and that is their walking patterns. Over three decades ago, researchers found that children with ASD demonstrate abnormal limb movements, shortened steps, and persistent toe walking (Damasio & Maurer, 1978; Vilensky, Damasio, & Maurer, 1981). More recently, Ming et al. (2007), in their retrospective study, determined that nearly 20% of children with ASD walked on their toes rather than on their entire foot. Esposito and Venturi (2008) used retrospective video analysis as well as an observational scale to examine 42 children with ASD who had been walking at least 6 months and also found differences in early walking patterns. Vernazza-Martin et al. (2005) found significant differences in gait when comparing 15 children aged 4 to 6 years with and without ASD, as did Woodward (2001) in a study of children with ASD aged 3 and 10 years. One consistent finding is that the gait cycle is slower and less consistent in children with ASD.

Dewey, Cantell, and Crawford (2007) compared the performance of children with different disabilities on the Bruininks-Oseretsky Test of Motor Proficiency-Short Form. Participants included 49 children with ASD, 46 children with developmental coordination disorder (DCD) and attention-deficit/hyperactivity disorder (ADHD), 38 children with DCD, 27 children with ADHD, and 78 children with typical development. The results indicated that although all the atypical groups displayed significant impairment of motor skills, children with ASD were significantly more impaired compared with their cohorts with specific motor skill deficits. They were also the only group to show impairment on gestural skills. Several researchers have found deficiencies in fine motor skills in children with ASD. These range from delays in manual dexterity (Miyahara et al., 1997) and graphomotor skills (Mayes & Calhoun, 2003) during early and middle childhood to motor control issues in prehension (Mari, Castiello, Marks, Marraffa, & Prior, 2003). Such fine motor skill deficits influence handwriting as well as many functional activities involving the hands and arms. Across these studies, it appears that individuals with autism do move differently when compared with their typically developing counterparts and that deficits range from fine to gross motor skills.

In sum, although motor deficits are not listed as part of the DSM-V diagnostic criteria for ASD, individuals with these disorders tend to exhibit motor skill deficiencies as well as differences in motor development. Infants with ASD may exhibit hypotonia (although this finding is not as robust as it is in infants with DS). In addition, delays may exist in the acquisition of motor milestones and fundamental motor skills. Differences in skills such as gait and manual dexterity persist into childhood.

Developmental Coordination Disorder

Another disorder with an unknown etiology that has garnered considerable empirical attention as of late is DCD (Wilson & Larkin, 2008). DCD, also known as developmental dyspraxia, is characterized by extreme lack of motor coordination and by other movement deficits in the absence of neurological defects. DCD has been calculated to affect as much as 6% of the elementary school population (American Psychiatric Association, 1994; Barnhart, Davenport, Epps, & Nordquist, 2003; Barnett, Kooistra, & Henderson, 1998). Although the disorder was described as early as 1937, it took until 1994 for a group of 43 experts in various movement disorder fields to come to a consensus on a name and description for DCD in order to facilitate both research and diagnosis (see the sidebar for the DSM-V description of DCD). Individuals characterized as having DCD fall into the 0 to 10th percentile of performance on the Movement Assessment Battery for Children (M-ABC), a standardized motor skills test used to assess motor proficiency in children with disabilities (Geuze, Jongmans, Schoemaker, & Smits-Engelsman, 2001; Henderson & Sugden, 1992; Miyahara & Möbs, 1995). Because DCD is categorized as a learning disability, it also may influence a child's ability to perform academically. Motor difficulties can range from gross to fine motor to balance skills and can include motor planning deficits and visual or spatial difficulties (Cermak & Larkin, 2002; de Castelnau, Albaret, Chaix, & Zanone, 2007; Kaplan, Wilson, Dewey, & Crawford, 1998; Przysucha & Taylor, 2004; Whitall et al., 2006). Table 9.6 provides examples of functional motor issues that children with DCD may have.

Applying the Dynamic Systems Model to a Motor Development Problem

Perhaps the best way to appreciate how the characteristics of dynamic systems can be used to model motor development is to see how a research team used dynamic systems to address a particular developmental issue. Let's consider the case of the reflexes seen in infancy. Recall that a reflex is a particular automatic response to a specific stimulus. It is a response that is carried out without thought or intention. Developmentalists have long observed the existence of reflexes that disappear later on in infants.

Developmentalists have been particularly interested in the walking or stepping reflex for its similarity to voluntary walking (see table 3.1). The reflex is elicited by holding an infant upright and lowering the infant to a horizontal surface.

The reflexive response is stepping movements of the legs. The stepping reflex is typically observed up to 5 months of age, so it appears much earlier than voluntary walking and disappears months before voluntary walking.

Developmentalists have offered several explanations for the disappearance of the stepping reflex. The maturationists suggested that the disappearance reflects the maturation of the cortex. These researchers proposed that as higher brain centers mature they are able to inhibit the lower brain centers that mediate reflexes. With continued maturation, the centers control voluntary walking. The existence and disappearance of reflexes reflect the status of nervous system development.

Peiper (1963) suggested instead that reflexes have a purpose because they allow practice of movement patterns before the higher brain centers are mature enough to control those patterns. Zelazo et al. (Zelazo, 1983; Zelazo, Konner, Kolb, & Zelazo, 1974; Zelazo, Zelazo, & Kolb, 1972) offered yet another viewpoint. They purposely increased the number of times the stepping reflex was elicited in a small group of infants. Later, these infants were observed to start walking at a younger age than average. The researchers concluded that reflexes can be subsumed into voluntary movements. For example, the walking reflex is transformed into voluntary walking.

Unconvinced by these various hypotheses, Esther Thelen and her colleagues (Thelen & Fisher, 1982; Thelen, Fisher, & Ridley-Johnson, 1984) developed an ingenious series of experiments to examine different potential causes for the disappearance of the stepping reflex. They began with the notion that multiple systems—rather than just one system—play a role in the disappearance. They had observed that one of the changes in infants correlating with the disappearance of the stepping reflex is an increase in subcutaneous fat that occurs during infancy. Could this gain in fat and the resultant change in the strength required to move heavier legs be related to the disappearance of the stepping reflex? In order to test whether the muscle and adipose tissue systems are involved, the researchers had to find a way to make a light and stepping infant resemble a nonstepping and heavier infant and, just as importantly, to get a non-stepping, heavier infant to resemble a lighter, stepping infant. The only variable that could be changed was leg strength. In a first experiment, the researchers added tiny weights to the legs of lighter infants. The added weight was proportional to the amount of weight the infant would add between 4 and 6 weeks of age. If maturation of the nervous system were the only cause of reflex stepping, then the infants should continue to step regardless of the small weights. Actually, the infants reduced the number of reflex steps taken while wearing the weights.

The next experiment was designed to regress the leg weight of heavier infants to that of younger, lighter infants. The researchers achieved this by holding the infants upright in an aquarium filled with water up to their hips. The water reduced the pull of gravity on the legs and made them relatively lighter. The water was room temperature, and the researchers noted that often the infants did not respond in any way to being held in the water. When the infants' feet touched the bottom surface of the tank, though, the number of steps increased (Thelen & Fisher, 1982). If maturation of the nervous system is the only explanation
for the reflex disappearing, then nothing should have been able to increase the rate of stepping, as the higher brain centers would have inhibited it. Thus the researchers demonstrated that change in other systems plays a role in the disappearance of the stepping reflex.

Thelen et al. demonstrated that the systems approach can be used to examine the processes underlying development. While maturation of the nervous system is important in motor development, other subsystems are important to the change observed in the stepping reflex. As well as studying multiple subsystems, Thelen et al. demonstrated that change in a subsystem can function as a control variable. The role of a control variable in bringing about change is another characteristic of dynamic systems. More of Thelen's work will be discussed throughout the text.

Summary

The systems approach has much to offer researchers interested in understanding developmental change. Dynamic systems evolve over time and undergo change that is not linear. These characteristics describe the body as it grows and ages. Dynamic systems evolve by transitioning from times of stability, or attractor states, through times of instability to other attractor states. We observe this pattern in motor development, too. Dynamic systems are sensitive to the conditions initially in place when they begin to evolve. Movement scientists have long realized that models of movement control must allow for the same movement goal to be achieved from variable environmental conditions and different starting positions of the body and limbs. Most importantly, dynamic systems have been shown to self-organize and to use methods of control such as entrainment and the application of constraints. The demonstration of these possibilities is important, as they provide explanations for how the large number of degrees of freedom in movement can be reduced for the reasonable control of complex movements.

The systems approach provided a heuristic function for movement scientists by focusing researchers' attention on new ways to solve existing questions in motor development. There is value in examining multiple theories. Researchers using differing approaches all serve to gain from alternative views, as both theorists and empiricists are forced to question behaviors their theories can't account for and improve their models until a satisfactory explanation can be found (for example, see Schmidt & Lee, 2011 on generalized motor programs). Ultimately, these challenges and changes serve to enhance our understanding of motor development.

Development of Ballistic Skills

Movements can be classified by their EMG and force curve signatures. Three movement categories are (1) ramped movements in which force is gradually and steadily applied through continuously graded muscle firing; (2) discontinuous, slow movements that accelerate gradually, then decelerate, and then accelerate again, such as when tracking an object with a hand; and (3) ballistic movements that are rapid, all-out, forceful movements that reach peak acceleration within milliseconds of their initiation (Brooks, 1983; Desmedt, 1983).

In simple ballistic movements, such as a rapid reach for an object, the motion is started by an initial strong impulse in the agonist muscle. The agonist firing then stops, but the movement continues due to the momentum of the limb. A burst by the antagonist muscle subsequently causes the limb to decelerate. In targeting tasks, there may be a second burst in the agonist to correct the limb placement. This creates a signature triphasic pattern of muscle activation: agonist, antagonist, agonist (Palmer, Cafarelli, & Ashby, 1994).

In contrast to single-limb movements, ballistic skills involving the total body are quite complex. Typical sport skills are examples of ballistic skills. Of these, movements in which the performer projects an object have been of considerable interest to both biomechanists and developmentalists. Striking (as in batting and swinging a racket), kicking (as in football and soccer), and throwing (as in handball and baseball or softball) have all received study. These complex movements tend to begin proximally with action of the trunk muscles; energy is then passed from segment to segment of the open kinetic chain until it reaches the distal effector, usually the hand or foot. During striking, kicking, and throwing, this energy exchange is superimposed on a running or stepping base of support.

Another interesting commonality across these complex projectile skills is that they employ a backswing to place the body segments in position to move forward. In the advanced form of striking, throwing, and kicking, the backswing and forward swing partially overlap, and proximal segments begin to move forward while more distal segments are still moving backward. The advanced form of these movements also involves strong acceleration of the distal segment just milliseconds before contact or release, which yields high distal angular velocity. This acceleration seems to result partially from the inertia of each distal segment against the motion of adjacent proximal segments. The lagging segment stretches the agonist muscles of the distal segment, which may in turn excite reflexes that augment the muscular contractions in the distal segment (Roberts & Metcalfe, 1968). As we will see, lag is one aspect of object projection that develops only gradually over time. It is so important for effective ballistic movement that Southard (2002a, 2002b) used lag as a collective variable in his studies of throwing.

In this chapter we focus on the development of striking, throwing, and kicking. Most developmental studies on ballistic projectile skills have been based in developmental sequence theory, although a few studies are beginning to address issues from a dynamic systems perspective. At the close of this chapter we discuss future directions in this area.

Intertask Developmental Sequence

The only ballistic skills that have been studied in relation to their intertask development are overarm throwing and striking. The approach used in this research is quite different from the intertask approach used to study locomotion (see chapter 6). For locomotion, the developmental sequence is formed by ordering each locomotor skill according to its initial appearance in the movement repertoire. For throwing and striking, the two tasks are compared to see where participants are located within a component developmental sequence at the same point in time in each skill. Do the participants show the same developmental level in the two skills, or do they develop faster in one skill than they do in the other?

Langendorfer (1987a) compared the development of trunk and humerus action in overarm throwing with development of the same action in overarm striking. He asked 58 boys aged 1.5 to 10.3 years to throw a ball forcefully and to strike a tennis ball suspended 6 inches (15 cm) above their head forcefully with a lightweight racket. He then looked at side and rear film views to categorize the boys' movements in each skill. Figure 7.1 shows his results. From the cross-sectional data we can infer how the longitudinal data might look. The data on trunk action (see figure 7.1a) suggested that children advance in throwing before they do in striking: The boys started out at a primitive level in both skills and then advanced to level 2 throwing before achieving level 2 striking and advanced to level 3 throwing before reaching level 3 striking. The cross-sectional data on humerus action (see figure 7.1b) suggested that children begin at level 1 in both skills and then move ahead in either skill until they reach level 2 in both skills. Then, however, they progress to level 3 in throwing before they progress to level 3 in striking.

Langendorfer (1987a) learned from his data that he could not compare forearm action between striking and throwing. Rather, the action of the racket, which is the distal segment in striking, was more comparable to the forearm action of throwing. Figure 7.1c shows the developmental comparison for forearm action. While some children exhibited different relationships early on, more progressed in striking before they progressed in throwing. Then throwing took the developmental lead for more children.

The finding on forearm action demonstrates how equipment can affect a person's developmental level. Using Newell's notions of person constraints (see chapter 3), we can think of equipment as an extension of the body. Comparing hammering with a hammer with pounding with a fist demonstrates how an implement changes the inertial characteristics of the arm and hand. This analysis also applies to prosthetic limbs. In both cases the performer needs to learn how to incorporate the implement into the actual movement pattern. Langendorfer's (1987a) data suggest that at first the implement assists the child in achieving a higher developmental level in the distal component, probably because its greater inertia causes lag. Later, though, the handle length may hinder change to a higher developmental level by making it more difficult to delay that lag until the body has reached front facing (see tables 7.1 and 7.4).

The last component that Langendorfer (1987a) compared across the two skills was stepping. As we can see in figure 7.1d, the developmental relationship for stepping was less clear. Langendorfer attributed this result to a degree of unreliability in the component sequence: Stepping tended to be variable across trials in both throwing and striking, making intertask comparisons unreliable.

The arrows in figure 7.1 reflect Langendorfer's (1987a) speculation on the longitudinal course that these two tasks might display while developing. The only actual longitudinal information that we have regarding this question was reported by Roberton (1982). She compared the development of trunk action within sidearm striking (as opposed to overarm striking) and overarm throwing. Interestingly, the trunk action of four children did follow the developmental path that Langendorfer's cross-sectional overarm data suggested: The children progressed one level in their throwing trunk action and then caught up to that same level in their striking trunk action. Then they progressed again in throwing before catching up in striking.

Motor Development Differences in Children With Autism Spectrum Disorders

One of the great difficulties facing practitioners and researchers working with individuals with ASD is the degree to which variability supersedes universality in this population (Pope et al., 2010). That is, two individuals with the same diagnosis of ASD will act and react very differently from each other. These individual differences lead to significant challenges for researchers trying to identify and quantify motor development in this population. For example, individuals with ASD may respond differently to a controlled experimental design due to different sensitivity to sensory stimuli or a lack of understanding of the described procedures. Furthermore, each individual responds differently to different interventions. All in all, typical developmental trajectories are all but impossible to determine. However, researchers have identified some general characteristics related to movement and motor development that suggest that individuals with ASD move differently from their typically developing counterparts (Pope et al., 2010). The importance of determining motor differences, particularly in younger children, cannot be underscored enough: If early developing motor differences exist, then assessment of these deficits may aid in identifying children at risk for ASD at a younger age than is currently possible.

Primarily through retrospective video analysis, researchers have identified early motor differences in children with ASD. Kanner (1943), who first identified ASD, noted that the infants exhibited hypotonia, or low muscle tone (similar to infants with DS). Other researchers have also observed hypotonia (Adrien et al., 1993; Ming, Brimacombe, & Wagner, 2007). For example, Ming et al. (2007) reviewed the clinical records of 154 children diagnosed with ASD in an attempt to determine the types and prevalence of motor impairments in this population, particularly hypotonia, delayed motor milestones, coordination issues, and toe walking. Children ranged in age from 2 to 18 years, with a mean age of 6 years. Reflecting the gender differences within the ASD population, there were 126 males and 28 females within the study. All of the children were on the autism spectrum: 74 were diagnosed with autism, 70 with PDD-NOS, and 10 with AS. The researchers determined that 51% of the group demonstrated hypotonia as infants; the prevalence of hypotonia decreased over time to 38% in the age range of 7 to 18 years. Not all retrospective studies have indicated that infants with autism have hypotonia. Saint-Georges et al. (2010) performed an exhaustive research review using retrospective video analysis and determined that across the 41 studies reviewed, hypotonia was not a consistent finding.

Given that at least a percentage of children with ASD exhibit hypotonia, subsequent issues with the acquisition of motor milestones may exist (as in children with DS). Low muscle tone affects infants' ability to raise the head while in the prone position, which affects the ability to sit supported, and so on. In fact, another consistent finding among individuals with ASD is a delay in achieving motor milestones and fundamental motor skills as young children. Acknowledging the limitations of looking back at records after diagnosis, Landa and Garrett-Mayer (2006) performed a 2-year longitudinal prospective study in which they followed a group of 84 infants who were either high (n = 60) or low (n = 27) risk for ASD.

Infants were tested at 6, 14, and 24 months using the Mullen Scales of Early Learning (MSEL), a standardized developmental test for children aged 0 to 69 months. The MSEL consists of five subscales: gross motor, fine motor, visual reception, receptive language, and expressive language. At 2 years, all children were given the Autism Diagnostic Observation Schedule (ADOS), which is a semistructured, play-based interview that provides systematic probes for symptoms of autism in social interaction, communication, play, and repetitive behaviors. Of the 60 children deemed at risk, 21 met the ADOS algorithm criteria for ASD as well as received a clinical diagnosis of ASD; 2 children from the low-risk group met the criteria and were included in the high-risk group. Early on, no statistical differences existed between groups. However, by 24 months, the high-risk group showed delays in both gross and fine motor skills, and group members were significantly different from the typically developing group in all domains.

In related research, a group of investigators assessed motor development in children younger than 3 years of age (Provost, Lopez, & Heimerl, 2007). In this study, 19 children with ASD, ranging in age from 21 to 41 months, were examined with the Bayley Scales of Infant Development II (BSID II; Bayley, 1993) and the Peabody Developmental Motor Scales, Second Edition (PDMS-2; Folio & Fewell, 2000). The researchers determined that 84% of the children were significantly delayed on the BSID II and PDMS-2. In a similar study, Ozonoff et al. (2008) examined the early movement behavior of children later identified as ASD and reported a delay in the appearance of motor milestones, including lying in prone and supine position, rolling, sitting, crawling, and walking, with the most significant delay occurring in walking onset. Therefore, it appears that delays in motor milestones exist in many of the children diagnosed with
ASD.

Acknowledging that delays appear to exist in the ASD population, Ozonoff et al. followed strong inference (see chapter 4) and asked the next logical question: Can motor delays differentiate children with autism from children who have more general developmental delays? If motor delays could be used for differentiation, then practitioners could use motor delays to identify infants at risk for ASD. In the resulting study, Ozonoff et al. (2008) placed 103 children between 24 and 60 months of age in 1 of 3 groups: children with ASD, children with developmental delays (DD), or children with typical development (TD). Home videotapes of the participants were analyzed and coded with the Infant Motor Maturity and Atypicality Coding Scales (IMMACS), which rate six motor milestones (lying in prone and supine position, roll, sit, crawl, walk) as well as protective skills (e.g., righting the body after losing balance). What Ozonoff et al. (2008) found, in contrast to earlier reports and speculation, is that the only group that differed in terms of number of movement abnormalities and protective skills was the DD group; the ASD and TD groups were not significantly different. The ASD and DD groups both showed motor delays, but they were not significantly different from each other. Therefore, the notion that motor delays can be used to identify children at risk for ASD was not supported in this research. Obviously, a prospective study examining both qualitative and quantitative variables is necessary as a next step to confirm this finding.

Are individuals with ASD merely delayed in motor skill acquisition, or do they differ in the ways in which they move? Several research studies (see table 9.5) have found at least one consistent difference between children with ASD and children without ASD, and that is their walking patterns. Over three decades ago, researchers found that children with ASD demonstrate abnormal limb movements, shortened steps, and persistent toe walking (Damasio & Maurer, 1978; Vilensky, Damasio, & Maurer, 1981). More recently, Ming et al. (2007), in their retrospective study, determined that nearly 20% of children with ASD walked on their toes rather than on their entire foot. Esposito and Venturi (2008) used retrospective video analysis as well as an observational scale to examine 42 children with ASD who had been walking at least 6 months and also found differences in early walking patterns. Vernazza-Martin et al. (2005) found significant differences in gait when comparing 15 children aged 4 to 6 years with and without ASD, as did Woodward (2001) in a study of children with ASD aged 3 and 10 years. One consistent finding is that the gait cycle is slower and less consistent in children with ASD.

Dewey, Cantell, and Crawford (2007) compared the performance of children with different disabilities on the Bruininks-Oseretsky Test of Motor Proficiency-Short Form. Participants included 49 children with ASD, 46 children with developmental coordination disorder (DCD) and attention-deficit/hyperactivity disorder (ADHD), 38 children with DCD, 27 children with ADHD, and 78 children with typical development. The results indicated that although all the atypical groups displayed significant impairment of motor skills, children with ASD were significantly more impaired compared with their cohorts with specific motor skill deficits. They were also the only group to show impairment on gestural skills. Several researchers have found deficiencies in fine motor skills in children with ASD. These range from delays in manual dexterity (Miyahara et al., 1997) and graphomotor skills (Mayes & Calhoun, 2003) during early and middle childhood to motor control issues in prehension (Mari, Castiello, Marks, Marraffa, & Prior, 2003). Such fine motor skill deficits influence handwriting as well as many functional activities involving the hands and arms. Across these studies, it appears that individuals with autism do move differently when compared with their typically developing counterparts and that deficits range from fine to gross motor skills.

In sum, although motor deficits are not listed as part of the DSM-V diagnostic criteria for ASD, individuals with these disorders tend to exhibit motor skill deficiencies as well as differences in motor development. Infants with ASD may exhibit hypotonia (although this finding is not as robust as it is in infants with DS). In addition, delays may exist in the acquisition of motor milestones and fundamental motor skills. Differences in skills such as gait and manual dexterity persist into childhood.

Developmental Coordination Disorder

Another disorder with an unknown etiology that has garnered considerable empirical attention as of late is DCD (Wilson & Larkin, 2008). DCD, also known as developmental dyspraxia, is characterized by extreme lack of motor coordination and by other movement deficits in the absence of neurological defects. DCD has been calculated to affect as much as 6% of the elementary school population (American Psychiatric Association, 1994; Barnhart, Davenport, Epps, & Nordquist, 2003; Barnett, Kooistra, & Henderson, 1998). Although the disorder was described as early as 1937, it took until 1994 for a group of 43 experts in various movement disorder fields to come to a consensus on a name and description for DCD in order to facilitate both research and diagnosis (see the sidebar for the DSM-V description of DCD). Individuals characterized as having DCD fall into the 0 to 10th percentile of performance on the Movement Assessment Battery for Children (M-ABC), a standardized motor skills test used to assess motor proficiency in children with disabilities (Geuze, Jongmans, Schoemaker, & Smits-Engelsman, 2001; Henderson & Sugden, 1992; Miyahara & Möbs, 1995). Because DCD is categorized as a learning disability, it also may influence a child's ability to perform academically. Motor difficulties can range from gross to fine motor to balance skills and can include motor planning deficits and visual or spatial difficulties (Cermak & Larkin, 2002; de Castelnau, Albaret, Chaix, & Zanone, 2007; Kaplan, Wilson, Dewey, & Crawford, 1998; Przysucha & Taylor, 2004; Whitall et al., 2006). Table 9.6 provides examples of functional motor issues that children with DCD may have.

Applying the Dynamic Systems Model to a Motor Development Problem

Perhaps the best way to appreciate how the characteristics of dynamic systems can be used to model motor development is to see how a research team used dynamic systems to address a particular developmental issue. Let's consider the case of the reflexes seen in infancy. Recall that a reflex is a particular automatic response to a specific stimulus. It is a response that is carried out without thought or intention. Developmentalists have long observed the existence of reflexes that disappear later on in infants.

Developmentalists have been particularly interested in the walking or stepping reflex for its similarity to voluntary walking (see table 3.1). The reflex is elicited by holding an infant upright and lowering the infant to a horizontal surface.

The reflexive response is stepping movements of the legs. The stepping reflex is typically observed up to 5 months of age, so it appears much earlier than voluntary walking and disappears months before voluntary walking.

Developmentalists have offered several explanations for the disappearance of the stepping reflex. The maturationists suggested that the disappearance reflects the maturation of the cortex. These researchers proposed that as higher brain centers mature they are able to inhibit the lower brain centers that mediate reflexes. With continued maturation, the centers control voluntary walking. The existence and disappearance of reflexes reflect the status of nervous system development.

Peiper (1963) suggested instead that reflexes have a purpose because they allow practice of movement patterns before the higher brain centers are mature enough to control those patterns. Zelazo et al. (Zelazo, 1983; Zelazo, Konner, Kolb, & Zelazo, 1974; Zelazo, Zelazo, & Kolb, 1972) offered yet another viewpoint. They purposely increased the number of times the stepping reflex was elicited in a small group of infants. Later, these infants were observed to start walking at a younger age than average. The researchers concluded that reflexes can be subsumed into voluntary movements. For example, the walking reflex is transformed into voluntary walking.

Unconvinced by these various hypotheses, Esther Thelen and her colleagues (Thelen & Fisher, 1982; Thelen, Fisher, & Ridley-Johnson, 1984) developed an ingenious series of experiments to examine different potential causes for the disappearance of the stepping reflex. They began with the notion that multiple systems—rather than just one system—play a role in the disappearance. They had observed that one of the changes in infants correlating with the disappearance of the stepping reflex is an increase in subcutaneous fat that occurs during infancy. Could this gain in fat and the resultant change in the strength required to move heavier legs be related to the disappearance of the stepping reflex? In order to test whether the muscle and adipose tissue systems are involved, the researchers had to find a way to make a light and stepping infant resemble a nonstepping and heavier infant and, just as importantly, to get a non-stepping, heavier infant to resemble a lighter, stepping infant. The only variable that could be changed was leg strength. In a first experiment, the researchers added tiny weights to the legs of lighter infants. The added weight was proportional to the amount of weight the infant would add between 4 and 6 weeks of age. If maturation of the nervous system were the only cause of reflex stepping, then the infants should continue to step regardless of the small weights. Actually, the infants reduced the number of reflex steps taken while wearing the weights.

The next experiment was designed to regress the leg weight of heavier infants to that of younger, lighter infants. The researchers achieved this by holding the infants upright in an aquarium filled with water up to their hips. The water reduced the pull of gravity on the legs and made them relatively lighter. The water was room temperature, and the researchers noted that often the infants did not respond in any way to being held in the water. When the infants' feet touched the bottom surface of the tank, though, the number of steps increased (Thelen & Fisher, 1982). If maturation of the nervous system is the only explanation
for the reflex disappearing, then nothing should have been able to increase the rate of stepping, as the higher brain centers would have inhibited it. Thus the researchers demonstrated that change in other systems plays a role in the disappearance of the stepping reflex.

Thelen et al. demonstrated that the systems approach can be used to examine the processes underlying development. While maturation of the nervous system is important in motor development, other subsystems are important to the change observed in the stepping reflex. As well as studying multiple subsystems, Thelen et al. demonstrated that change in a subsystem can function as a control variable. The role of a control variable in bringing about change is another characteristic of dynamic systems. More of Thelen's work will be discussed throughout the text.

Summary

The systems approach has much to offer researchers interested in understanding developmental change. Dynamic systems evolve over time and undergo change that is not linear. These characteristics describe the body as it grows and ages. Dynamic systems evolve by transitioning from times of stability, or attractor states, through times of instability to other attractor states. We observe this pattern in motor development, too. Dynamic systems are sensitive to the conditions initially in place when they begin to evolve. Movement scientists have long realized that models of movement control must allow for the same movement goal to be achieved from variable environmental conditions and different starting positions of the body and limbs. Most importantly, dynamic systems have been shown to self-organize and to use methods of control such as entrainment and the application of constraints. The demonstration of these possibilities is important, as they provide explanations for how the large number of degrees of freedom in movement can be reduced for the reasonable control of complex movements.

The systems approach provided a heuristic function for movement scientists by focusing researchers' attention on new ways to solve existing questions in motor development. There is value in examining multiple theories. Researchers using differing approaches all serve to gain from alternative views, as both theorists and empiricists are forced to question behaviors their theories can't account for and improve their models until a satisfactory explanation can be found (for example, see Schmidt & Lee, 2011 on generalized motor programs). Ultimately, these challenges and changes serve to enhance our understanding of motor development.

Development of Ballistic Skills

Movements can be classified by their EMG and force curve signatures. Three movement categories are (1) ramped movements in which force is gradually and steadily applied through continuously graded muscle firing; (2) discontinuous, slow movements that accelerate gradually, then decelerate, and then accelerate again, such as when tracking an object with a hand; and (3) ballistic movements that are rapid, all-out, forceful movements that reach peak acceleration within milliseconds of their initiation (Brooks, 1983; Desmedt, 1983).

In simple ballistic movements, such as a rapid reach for an object, the motion is started by an initial strong impulse in the agonist muscle. The agonist firing then stops, but the movement continues due to the momentum of the limb. A burst by the antagonist muscle subsequently causes the limb to decelerate. In targeting tasks, there may be a second burst in the agonist to correct the limb placement. This creates a signature triphasic pattern of muscle activation: agonist, antagonist, agonist (Palmer, Cafarelli, & Ashby, 1994).

In contrast to single-limb movements, ballistic skills involving the total body are quite complex. Typical sport skills are examples of ballistic skills. Of these, movements in which the performer projects an object have been of considerable interest to both biomechanists and developmentalists. Striking (as in batting and swinging a racket), kicking (as in football and soccer), and throwing (as in handball and baseball or softball) have all received study. These complex movements tend to begin proximally with action of the trunk muscles; energy is then passed from segment to segment of the open kinetic chain until it reaches the distal effector, usually the hand or foot. During striking, kicking, and throwing, this energy exchange is superimposed on a running or stepping base of support.

Another interesting commonality across these complex projectile skills is that they employ a backswing to place the body segments in position to move forward. In the advanced form of striking, throwing, and kicking, the backswing and forward swing partially overlap, and proximal segments begin to move forward while more distal segments are still moving backward. The advanced form of these movements also involves strong acceleration of the distal segment just milliseconds before contact or release, which yields high distal angular velocity. This acceleration seems to result partially from the inertia of each distal segment against the motion of adjacent proximal segments. The lagging segment stretches the agonist muscles of the distal segment, which may in turn excite reflexes that augment the muscular contractions in the distal segment (Roberts & Metcalfe, 1968). As we will see, lag is one aspect of object projection that develops only gradually over time. It is so important for effective ballistic movement that Southard (2002a, 2002b) used lag as a collective variable in his studies of throwing.

In this chapter we focus on the development of striking, throwing, and kicking. Most developmental studies on ballistic projectile skills have been based in developmental sequence theory, although a few studies are beginning to address issues from a dynamic systems perspective. At the close of this chapter we discuss future directions in this area.

Intertask Developmental Sequence

The only ballistic skills that have been studied in relation to their intertask development are overarm throwing and striking. The approach used in this research is quite different from the intertask approach used to study locomotion (see chapter 6). For locomotion, the developmental sequence is formed by ordering each locomotor skill according to its initial appearance in the movement repertoire. For throwing and striking, the two tasks are compared to see where participants are located within a component developmental sequence at the same point in time in each skill. Do the participants show the same developmental level in the two skills, or do they develop faster in one skill than they do in the other?

Langendorfer (1987a) compared the development of trunk and humerus action in overarm throwing with development of the same action in overarm striking. He asked 58 boys aged 1.5 to 10.3 years to throw a ball forcefully and to strike a tennis ball suspended 6 inches (15 cm) above their head forcefully with a lightweight racket. He then looked at side and rear film views to categorize the boys' movements in each skill. Figure 7.1 shows his results. From the cross-sectional data we can infer how the longitudinal data might look. The data on trunk action (see figure 7.1a) suggested that children advance in throwing before they do in striking: The boys started out at a primitive level in both skills and then advanced to level 2 throwing before achieving level 2 striking and advanced to level 3 throwing before reaching level 3 striking. The cross-sectional data on humerus action (see figure 7.1b) suggested that children begin at level 1 in both skills and then move ahead in either skill until they reach level 2 in both skills. Then, however, they progress to level 3 in throwing before they progress to level 3 in striking.

Langendorfer (1987a) learned from his data that he could not compare forearm action between striking and throwing. Rather, the action of the racket, which is the distal segment in striking, was more comparable to the forearm action of throwing. Figure 7.1c shows the developmental comparison for forearm action. While some children exhibited different relationships early on, more progressed in striking before they progressed in throwing. Then throwing took the developmental lead for more children.

The finding on forearm action demonstrates how equipment can affect a person's developmental level. Using Newell's notions of person constraints (see chapter 3), we can think of equipment as an extension of the body. Comparing hammering with a hammer with pounding with a fist demonstrates how an implement changes the inertial characteristics of the arm and hand. This analysis also applies to prosthetic limbs. In both cases the performer needs to learn how to incorporate the implement into the actual movement pattern. Langendorfer's (1987a) data suggest that at first the implement assists the child in achieving a higher developmental level in the distal component, probably because its greater inertia causes lag. Later, though, the handle length may hinder change to a higher developmental level by making it more difficult to delay that lag until the body has reached front facing (see tables 7.1 and 7.4).

The last component that Langendorfer (1987a) compared across the two skills was stepping. As we can see in figure 7.1d, the developmental relationship for stepping was less clear. Langendorfer attributed this result to a degree of unreliability in the component sequence: Stepping tended to be variable across trials in both throwing and striking, making intertask comparisons unreliable.

The arrows in figure 7.1 reflect Langendorfer's (1987a) speculation on the longitudinal course that these two tasks might display while developing. The only actual longitudinal information that we have regarding this question was reported by Roberton (1982). She compared the development of trunk action within sidearm striking (as opposed to overarm striking) and overarm throwing. Interestingly, the trunk action of four children did follow the developmental path that Langendorfer's cross-sectional overarm data suggested: The children progressed one level in their throwing trunk action and then caught up to that same level in their striking trunk action. Then they progressed again in throwing before catching up in striking.

Motor Development Differences in Children With Autism Spectrum Disorders

One of the great difficulties facing practitioners and researchers working with individuals with ASD is the degree to which variability supersedes universality in this population (Pope et al., 2010). That is, two individuals with the same diagnosis of ASD will act and react very differently from each other. These individual differences lead to significant challenges for researchers trying to identify and quantify motor development in this population. For example, individuals with ASD may respond differently to a controlled experimental design due to different sensitivity to sensory stimuli or a lack of understanding of the described procedures. Furthermore, each individual responds differently to different interventions. All in all, typical developmental trajectories are all but impossible to determine. However, researchers have identified some general characteristics related to movement and motor development that suggest that individuals with ASD move differently from their typically developing counterparts (Pope et al., 2010). The importance of determining motor differences, particularly in younger children, cannot be underscored enough: If early developing motor differences exist, then assessment of these deficits may aid in identifying children at risk for ASD at a younger age than is currently possible.

Primarily through retrospective video analysis, researchers have identified early motor differences in children with ASD. Kanner (1943), who first identified ASD, noted that the infants exhibited hypotonia, or low muscle tone (similar to infants with DS). Other researchers have also observed hypotonia (Adrien et al., 1993; Ming, Brimacombe, & Wagner, 2007). For example, Ming et al. (2007) reviewed the clinical records of 154 children diagnosed with ASD in an attempt to determine the types and prevalence of motor impairments in this population, particularly hypotonia, delayed motor milestones, coordination issues, and toe walking. Children ranged in age from 2 to 18 years, with a mean age of 6 years. Reflecting the gender differences within the ASD population, there were 126 males and 28 females within the study. All of the children were on the autism spectrum: 74 were diagnosed with autism, 70 with PDD-NOS, and 10 with AS. The researchers determined that 51% of the group demonstrated hypotonia as infants; the prevalence of hypotonia decreased over time to 38% in the age range of 7 to 18 years. Not all retrospective studies have indicated that infants with autism have hypotonia. Saint-Georges et al. (2010) performed an exhaustive research review using retrospective video analysis and determined that across the 41 studies reviewed, hypotonia was not a consistent finding.

Given that at least a percentage of children with ASD exhibit hypotonia, subsequent issues with the acquisition of motor milestones may exist (as in children with DS). Low muscle tone affects infants' ability to raise the head while in the prone position, which affects the ability to sit supported, and so on. In fact, another consistent finding among individuals with ASD is a delay in achieving motor milestones and fundamental motor skills as young children. Acknowledging the limitations of looking back at records after diagnosis, Landa and Garrett-Mayer (2006) performed a 2-year longitudinal prospective study in which they followed a group of 84 infants who were either high (n = 60) or low (n = 27) risk for ASD.

Infants were tested at 6, 14, and 24 months using the Mullen Scales of Early Learning (MSEL), a standardized developmental test for children aged 0 to 69 months. The MSEL consists of five subscales: gross motor, fine motor, visual reception, receptive language, and expressive language. At 2 years, all children were given the Autism Diagnostic Observation Schedule (ADOS), which is a semistructured, play-based interview that provides systematic probes for symptoms of autism in social interaction, communication, play, and repetitive behaviors. Of the 60 children deemed at risk, 21 met the ADOS algorithm criteria for ASD as well as received a clinical diagnosis of ASD; 2 children from the low-risk group met the criteria and were included in the high-risk group. Early on, no statistical differences existed between groups. However, by 24 months, the high-risk group showed delays in both gross and fine motor skills, and group members were significantly different from the typically developing group in all domains.

In related research, a group of investigators assessed motor development in children younger than 3 years of age (Provost, Lopez, & Heimerl, 2007). In this study, 19 children with ASD, ranging in age from 21 to 41 months, were examined with the Bayley Scales of Infant Development II (BSID II; Bayley, 1993) and the Peabody Developmental Motor Scales, Second Edition (PDMS-2; Folio & Fewell, 2000). The researchers determined that 84% of the children were significantly delayed on the BSID II and PDMS-2. In a similar study, Ozonoff et al. (2008) examined the early movement behavior of children later identified as ASD and reported a delay in the appearance of motor milestones, including lying in prone and supine position, rolling, sitting, crawling, and walking, with the most significant delay occurring in walking onset. Therefore, it appears that delays in motor milestones exist in many of the children diagnosed with
ASD.

Acknowledging that delays appear to exist in the ASD population, Ozonoff et al. followed strong inference (see chapter 4) and asked the next logical question: Can motor delays differentiate children with autism from children who have more general developmental delays? If motor delays could be used for differentiation, then practitioners could use motor delays to identify infants at risk for ASD. In the resulting study, Ozonoff et al. (2008) placed 103 children between 24 and 60 months of age in 1 of 3 groups: children with ASD, children with developmental delays (DD), or children with typical development (TD). Home videotapes of the participants were analyzed and coded with the Infant Motor Maturity and Atypicality Coding Scales (IMMACS), which rate six motor milestones (lying in prone and supine position, roll, sit, crawl, walk) as well as protective skills (e.g., righting the body after losing balance). What Ozonoff et al. (2008) found, in contrast to earlier reports and speculation, is that the only group that differed in terms of number of movement abnormalities and protective skills was the DD group; the ASD and TD groups were not significantly different. The ASD and DD groups both showed motor delays, but they were not significantly different from each other. Therefore, the notion that motor delays can be used to identify children at risk for ASD was not supported in this research. Obviously, a prospective study examining both qualitative and quantitative variables is necessary as a next step to confirm this finding.

Are individuals with ASD merely delayed in motor skill acquisition, or do they differ in the ways in which they move? Several research studies (see table 9.5) have found at least one consistent difference between children with ASD and children without ASD, and that is their walking patterns. Over three decades ago, researchers found that children with ASD demonstrate abnormal limb movements, shortened steps, and persistent toe walking (Damasio & Maurer, 1978; Vilensky, Damasio, & Maurer, 1981). More recently, Ming et al. (2007), in their retrospective study, determined that nearly 20% of children with ASD walked on their toes rather than on their entire foot. Esposito and Venturi (2008) used retrospective video analysis as well as an observational scale to examine 42 children with ASD who had been walking at least 6 months and also found differences in early walking patterns. Vernazza-Martin et al. (2005) found significant differences in gait when comparing 15 children aged 4 to 6 years with and without ASD, as did Woodward (2001) in a study of children with ASD aged 3 and 10 years. One consistent finding is that the gait cycle is slower and less consistent in children with ASD.

Dewey, Cantell, and Crawford (2007) compared the performance of children with different disabilities on the Bruininks-Oseretsky Test of Motor Proficiency-Short Form. Participants included 49 children with ASD, 46 children with developmental coordination disorder (DCD) and attention-deficit/hyperactivity disorder (ADHD), 38 children with DCD, 27 children with ADHD, and 78 children with typical development. The results indicated that although all the atypical groups displayed significant impairment of motor skills, children with ASD were significantly more impaired compared with their cohorts with specific motor skill deficits. They were also the only group to show impairment on gestural skills. Several researchers have found deficiencies in fine motor skills in children with ASD. These range from delays in manual dexterity (Miyahara et al., 1997) and graphomotor skills (Mayes & Calhoun, 2003) during early and middle childhood to motor control issues in prehension (Mari, Castiello, Marks, Marraffa, & Prior, 2003). Such fine motor skill deficits influence handwriting as well as many functional activities involving the hands and arms. Across these studies, it appears that individuals with autism do move differently when compared with their typically developing counterparts and that deficits range from fine to gross motor skills.

In sum, although motor deficits are not listed as part of the DSM-V diagnostic criteria for ASD, individuals with these disorders tend to exhibit motor skill deficiencies as well as differences in motor development. Infants with ASD may exhibit hypotonia (although this finding is not as robust as it is in infants with DS). In addition, delays may exist in the acquisition of motor milestones and fundamental motor skills. Differences in skills such as gait and manual dexterity persist into childhood.

Developmental Coordination Disorder

Another disorder with an unknown etiology that has garnered considerable empirical attention as of late is DCD (Wilson & Larkin, 2008). DCD, also known as developmental dyspraxia, is characterized by extreme lack of motor coordination and by other movement deficits in the absence of neurological defects. DCD has been calculated to affect as much as 6% of the elementary school population (American Psychiatric Association, 1994; Barnhart, Davenport, Epps, & Nordquist, 2003; Barnett, Kooistra, & Henderson, 1998). Although the disorder was described as early as 1937, it took until 1994 for a group of 43 experts in various movement disorder fields to come to a consensus on a name and description for DCD in order to facilitate both research and diagnosis (see the sidebar for the DSM-V description of DCD). Individuals characterized as having DCD fall into the 0 to 10th percentile of performance on the Movement Assessment Battery for Children (M-ABC), a standardized motor skills test used to assess motor proficiency in children with disabilities (Geuze, Jongmans, Schoemaker, & Smits-Engelsman, 2001; Henderson & Sugden, 1992; Miyahara & Möbs, 1995). Because DCD is categorized as a learning disability, it also may influence a child's ability to perform academically. Motor difficulties can range from gross to fine motor to balance skills and can include motor planning deficits and visual or spatial difficulties (Cermak & Larkin, 2002; de Castelnau, Albaret, Chaix, & Zanone, 2007; Kaplan, Wilson, Dewey, & Crawford, 1998; Przysucha & Taylor, 2004; Whitall et al., 2006). Table 9.6 provides examples of functional motor issues that children with DCD may have.

Applying the Dynamic Systems Model to a Motor Development Problem

Perhaps the best way to appreciate how the characteristics of dynamic systems can be used to model motor development is to see how a research team used dynamic systems to address a particular developmental issue. Let's consider the case of the reflexes seen in infancy. Recall that a reflex is a particular automatic response to a specific stimulus. It is a response that is carried out without thought or intention. Developmentalists have long observed the existence of reflexes that disappear later on in infants.

Developmentalists have been particularly interested in the walking or stepping reflex for its similarity to voluntary walking (see table 3.1). The reflex is elicited by holding an infant upright and lowering the infant to a horizontal surface.

The reflexive response is stepping movements of the legs. The stepping reflex is typically observed up to 5 months of age, so it appears much earlier than voluntary walking and disappears months before voluntary walking.

Developmentalists have offered several explanations for the disappearance of the stepping reflex. The maturationists suggested that the disappearance reflects the maturation of the cortex. These researchers proposed that as higher brain centers mature they are able to inhibit the lower brain centers that mediate reflexes. With continued maturation, the centers control voluntary walking. The existence and disappearance of reflexes reflect the status of nervous system development.

Peiper (1963) suggested instead that reflexes have a purpose because they allow practice of movement patterns before the higher brain centers are mature enough to control those patterns. Zelazo et al. (Zelazo, 1983; Zelazo, Konner, Kolb, & Zelazo, 1974; Zelazo, Zelazo, & Kolb, 1972) offered yet another viewpoint. They purposely increased the number of times the stepping reflex was elicited in a small group of infants. Later, these infants were observed to start walking at a younger age than average. The researchers concluded that reflexes can be subsumed into voluntary movements. For example, the walking reflex is transformed into voluntary walking.

Unconvinced by these various hypotheses, Esther Thelen and her colleagues (Thelen & Fisher, 1982; Thelen, Fisher, & Ridley-Johnson, 1984) developed an ingenious series of experiments to examine different potential causes for the disappearance of the stepping reflex. They began with the notion that multiple systems—rather than just one system—play a role in the disappearance. They had observed that one of the changes in infants correlating with the disappearance of the stepping reflex is an increase in subcutaneous fat that occurs during infancy. Could this gain in fat and the resultant change in the strength required to move heavier legs be related to the disappearance of the stepping reflex? In order to test whether the muscle and adipose tissue systems are involved, the researchers had to find a way to make a light and stepping infant resemble a nonstepping and heavier infant and, just as importantly, to get a non-stepping, heavier infant to resemble a lighter, stepping infant. The only variable that could be changed was leg strength. In a first experiment, the researchers added tiny weights to the legs of lighter infants. The added weight was proportional to the amount of weight the infant would add between 4 and 6 weeks of age. If maturation of the nervous system were the only cause of reflex stepping, then the infants should continue to step regardless of the small weights. Actually, the infants reduced the number of reflex steps taken while wearing the weights.

The next experiment was designed to regress the leg weight of heavier infants to that of younger, lighter infants. The researchers achieved this by holding the infants upright in an aquarium filled with water up to their hips. The water reduced the pull of gravity on the legs and made them relatively lighter. The water was room temperature, and the researchers noted that often the infants did not respond in any way to being held in the water. When the infants' feet touched the bottom surface of the tank, though, the number of steps increased (Thelen & Fisher, 1982). If maturation of the nervous system is the only explanation
for the reflex disappearing, then nothing should have been able to increase the rate of stepping, as the higher brain centers would have inhibited it. Thus the researchers demonstrated that change in other systems plays a role in the disappearance of the stepping reflex.

Thelen et al. demonstrated that the systems approach can be used to examine the processes underlying development. While maturation of the nervous system is important in motor development, other subsystems are important to the change observed in the stepping reflex. As well as studying multiple subsystems, Thelen et al. demonstrated that change in a subsystem can function as a control variable. The role of a control variable in bringing about change is another characteristic of dynamic systems. More of Thelen's work will be discussed throughout the text.

Summary

The systems approach has much to offer researchers interested in understanding developmental change. Dynamic systems evolve over time and undergo change that is not linear. These characteristics describe the body as it grows and ages. Dynamic systems evolve by transitioning from times of stability, or attractor states, through times of instability to other attractor states. We observe this pattern in motor development, too. Dynamic systems are sensitive to the conditions initially in place when they begin to evolve. Movement scientists have long realized that models of movement control must allow for the same movement goal to be achieved from variable environmental conditions and different starting positions of the body and limbs. Most importantly, dynamic systems have been shown to self-organize and to use methods of control such as entrainment and the application of constraints. The demonstration of these possibilities is important, as they provide explanations for how the large number of degrees of freedom in movement can be reduced for the reasonable control of complex movements.

The systems approach provided a heuristic function for movement scientists by focusing researchers' attention on new ways to solve existing questions in motor development. There is value in examining multiple theories. Researchers using differing approaches all serve to gain from alternative views, as both theorists and empiricists are forced to question behaviors their theories can't account for and improve their models until a satisfactory explanation can be found (for example, see Schmidt & Lee, 2011 on generalized motor programs). Ultimately, these challenges and changes serve to enhance our understanding of motor development.

Development of Ballistic Skills

Movements can be classified by their EMG and force curve signatures. Three movement categories are (1) ramped movements in which force is gradually and steadily applied through continuously graded muscle firing; (2) discontinuous, slow movements that accelerate gradually, then decelerate, and then accelerate again, such as when tracking an object with a hand; and (3) ballistic movements that are rapid, all-out, forceful movements that reach peak acceleration within milliseconds of their initiation (Brooks, 1983; Desmedt, 1983).

In simple ballistic movements, such as a rapid reach for an object, the motion is started by an initial strong impulse in the agonist muscle. The agonist firing then stops, but the movement continues due to the momentum of the limb. A burst by the antagonist muscle subsequently causes the limb to decelerate. In targeting tasks, there may be a second burst in the agonist to correct the limb placement. This creates a signature triphasic pattern of muscle activation: agonist, antagonist, agonist (Palmer, Cafarelli, & Ashby, 1994).

In contrast to single-limb movements, ballistic skills involving the total body are quite complex. Typical sport skills are examples of ballistic skills. Of these, movements in which the performer projects an object have been of considerable interest to both biomechanists and developmentalists. Striking (as in batting and swinging a racket), kicking (as in football and soccer), and throwing (as in handball and baseball or softball) have all received study. These complex movements tend to begin proximally with action of the trunk muscles; energy is then passed from segment to segment of the open kinetic chain until it reaches the distal effector, usually the hand or foot. During striking, kicking, and throwing, this energy exchange is superimposed on a running or stepping base of support.

Another interesting commonality across these complex projectile skills is that they employ a backswing to place the body segments in position to move forward. In the advanced form of striking, throwing, and kicking, the backswing and forward swing partially overlap, and proximal segments begin to move forward while more distal segments are still moving backward. The advanced form of these movements also involves strong acceleration of the distal segment just milliseconds before contact or release, which yields high distal angular velocity. This acceleration seems to result partially from the inertia of each distal segment against the motion of adjacent proximal segments. The lagging segment stretches the agonist muscles of the distal segment, which may in turn excite reflexes that augment the muscular contractions in the distal segment (Roberts & Metcalfe, 1968). As we will see, lag is one aspect of object projection that develops only gradually over time. It is so important for effective ballistic movement that Southard (2002a, 2002b) used lag as a collective variable in his studies of throwing.

In this chapter we focus on the development of striking, throwing, and kicking. Most developmental studies on ballistic projectile skills have been based in developmental sequence theory, although a few studies are beginning to address issues from a dynamic systems perspective. At the close of this chapter we discuss future directions in this area.

Intertask Developmental Sequence

The only ballistic skills that have been studied in relation to their intertask development are overarm throwing and striking. The approach used in this research is quite different from the intertask approach used to study locomotion (see chapter 6). For locomotion, the developmental sequence is formed by ordering each locomotor skill according to its initial appearance in the movement repertoire. For throwing and striking, the two tasks are compared to see where participants are located within a component developmental sequence at the same point in time in each skill. Do the participants show the same developmental level in the two skills, or do they develop faster in one skill than they do in the other?

Langendorfer (1987a) compared the development of trunk and humerus action in overarm throwing with development of the same action in overarm striking. He asked 58 boys aged 1.5 to 10.3 years to throw a ball forcefully and to strike a tennis ball suspended 6 inches (15 cm) above their head forcefully with a lightweight racket. He then looked at side and rear film views to categorize the boys' movements in each skill. Figure 7.1 shows his results. From the cross-sectional data we can infer how the longitudinal data might look. The data on trunk action (see figure 7.1a) suggested that children advance in throwing before they do in striking: The boys started out at a primitive level in both skills and then advanced to level 2 throwing before achieving level 2 striking and advanced to level 3 throwing before reaching level 3 striking. The cross-sectional data on humerus action (see figure 7.1b) suggested that children begin at level 1 in both skills and then move ahead in either skill until they reach level 2 in both skills. Then, however, they progress to level 3 in throwing before they progress to level 3 in striking.

Langendorfer (1987a) learned from his data that he could not compare forearm action between striking and throwing. Rather, the action of the racket, which is the distal segment in striking, was more comparable to the forearm action of throwing. Figure 7.1c shows the developmental comparison for forearm action. While some children exhibited different relationships early on, more progressed in striking before they progressed in throwing. Then throwing took the developmental lead for more children.

The finding on forearm action demonstrates how equipment can affect a person's developmental level. Using Newell's notions of person constraints (see chapter 3), we can think of equipment as an extension of the body. Comparing hammering with a hammer with pounding with a fist demonstrates how an implement changes the inertial characteristics of the arm and hand. This analysis also applies to prosthetic limbs. In both cases the performer needs to learn how to incorporate the implement into the actual movement pattern. Langendorfer's (1987a) data suggest that at first the implement assists the child in achieving a higher developmental level in the distal component, probably because its greater inertia causes lag. Later, though, the handle length may hinder change to a higher developmental level by making it more difficult to delay that lag until the body has reached front facing (see tables 7.1 and 7.4).

The last component that Langendorfer (1987a) compared across the two skills was stepping. As we can see in figure 7.1d, the developmental relationship for stepping was less clear. Langendorfer attributed this result to a degree of unreliability in the component sequence: Stepping tended to be variable across trials in both throwing and striking, making intertask comparisons unreliable.

The arrows in figure 7.1 reflect Langendorfer's (1987a) speculation on the longitudinal course that these two tasks might display while developing. The only actual longitudinal information that we have regarding this question was reported by Roberton (1982). She compared the development of trunk action within sidearm striking (as opposed to overarm striking) and overarm throwing. Interestingly, the trunk action of four children did follow the developmental path that Langendorfer's cross-sectional overarm data suggested: The children progressed one level in their throwing trunk action and then caught up to that same level in their striking trunk action. Then they progressed again in throwing before catching up in striking.

Motor Development Differences in Children With Autism Spectrum Disorders

One of the great difficulties facing practitioners and researchers working with individuals with ASD is the degree to which variability supersedes universality in this population (Pope et al., 2010). That is, two individuals with the same diagnosis of ASD will act and react very differently from each other. These individual differences lead to significant challenges for researchers trying to identify and quantify motor development in this population. For example, individuals with ASD may respond differently to a controlled experimental design due to different sensitivity to sensory stimuli or a lack of understanding of the described procedures. Furthermore, each individual responds differently to different interventions. All in all, typical developmental trajectories are all but impossible to determine. However, researchers have identified some general characteristics related to movement and motor development that suggest that individuals with ASD move differently from their typically developing counterparts (Pope et al., 2010). The importance of determining motor differences, particularly in younger children, cannot be underscored enough: If early developing motor differences exist, then assessment of these deficits may aid in identifying children at risk for ASD at a younger age than is currently possible.

Primarily through retrospective video analysis, researchers have identified early motor differences in children with ASD. Kanner (1943), who first identified ASD, noted that the infants exhibited hypotonia, or low muscle tone (similar to infants with DS). Other researchers have also observed hypotonia (Adrien et al., 1993; Ming, Brimacombe, & Wagner, 2007). For example, Ming et al. (2007) reviewed the clinical records of 154 children diagnosed with ASD in an attempt to determine the types and prevalence of motor impairments in this population, particularly hypotonia, delayed motor milestones, coordination issues, and toe walking. Children ranged in age from 2 to 18 years, with a mean age of 6 years. Reflecting the gender differences within the ASD population, there were 126 males and 28 females within the study. All of the children were on the autism spectrum: 74 were diagnosed with autism, 70 with PDD-NOS, and 10 with AS. The researchers determined that 51% of the group demonstrated hypotonia as infants; the prevalence of hypotonia decreased over time to 38% in the age range of 7 to 18 years. Not all retrospective studies have indicated that infants with autism have hypotonia. Saint-Georges et al. (2010) performed an exhaustive research review using retrospective video analysis and determined that across the 41 studies reviewed, hypotonia was not a consistent finding.

Given that at least a percentage of children with ASD exhibit hypotonia, subsequent issues with the acquisition of motor milestones may exist (as in children with DS). Low muscle tone affects infants' ability to raise the head while in the prone position, which affects the ability to sit supported, and so on. In fact, another consistent finding among individuals with ASD is a delay in achieving motor milestones and fundamental motor skills as young children. Acknowledging the limitations of looking back at records after diagnosis, Landa and Garrett-Mayer (2006) performed a 2-year longitudinal prospective study in which they followed a group of 84 infants who were either high (n = 60) or low (n = 27) risk for ASD.

Infants were tested at 6, 14, and 24 months using the Mullen Scales of Early Learning (MSEL), a standardized developmental test for children aged 0 to 69 months. The MSEL consists of five subscales: gross motor, fine motor, visual reception, receptive language, and expressive language. At 2 years, all children were given the Autism Diagnostic Observation Schedule (ADOS), which is a semistructured, play-based interview that provides systematic probes for symptoms of autism in social interaction, communication, play, and repetitive behaviors. Of the 60 children deemed at risk, 21 met the ADOS algorithm criteria for ASD as well as received a clinical diagnosis of ASD; 2 children from the low-risk group met the criteria and were included in the high-risk group. Early on, no statistical differences existed between groups. However, by 24 months, the high-risk group showed delays in both gross and fine motor skills, and group members were significantly different from the typically developing group in all domains.

In related research, a group of investigators assessed motor development in children younger than 3 years of age (Provost, Lopez, & Heimerl, 2007). In this study, 19 children with ASD, ranging in age from 21 to 41 months, were examined with the Bayley Scales of Infant Development II (BSID II; Bayley, 1993) and the Peabody Developmental Motor Scales, Second Edition (PDMS-2; Folio & Fewell, 2000). The researchers determined that 84% of the children were significantly delayed on the BSID II and PDMS-2. In a similar study, Ozonoff et al. (2008) examined the early movement behavior of children later identified as ASD and reported a delay in the appearance of motor milestones, including lying in prone and supine position, rolling, sitting, crawling, and walking, with the most significant delay occurring in walking onset. Therefore, it appears that delays in motor milestones exist in many of the children diagnosed with
ASD.

Acknowledging that delays appear to exist in the ASD population, Ozonoff et al. followed strong inference (see chapter 4) and asked the next logical question: Can motor delays differentiate children with autism from children who have more general developmental delays? If motor delays could be used for differentiation, then practitioners could use motor delays to identify infants at risk for ASD. In the resulting study, Ozonoff et al. (2008) placed 103 children between 24 and 60 months of age in 1 of 3 groups: children with ASD, children with developmental delays (DD), or children with typical development (TD). Home videotapes of the participants were analyzed and coded with the Infant Motor Maturity and Atypicality Coding Scales (IMMACS), which rate six motor milestones (lying in prone and supine position, roll, sit, crawl, walk) as well as protective skills (e.g., righting the body after losing balance). What Ozonoff et al. (2008) found, in contrast to earlier reports and speculation, is that the only group that differed in terms of number of movement abnormalities and protective skills was the DD group; the ASD and TD groups were not significantly different. The ASD and DD groups both showed motor delays, but they were not significantly different from each other. Therefore, the notion that motor delays can be used to identify children at risk for ASD was not supported in this research. Obviously, a prospective study examining both qualitative and quantitative variables is necessary as a next step to confirm this finding.

Are individuals with ASD merely delayed in motor skill acquisition, or do they differ in the ways in which they move? Several research studies (see table 9.5) have found at least one consistent difference between children with ASD and children without ASD, and that is their walking patterns. Over three decades ago, researchers found that children with ASD demonstrate abnormal limb movements, shortened steps, and persistent toe walking (Damasio & Maurer, 1978; Vilensky, Damasio, & Maurer, 1981). More recently, Ming et al. (2007), in their retrospective study, determined that nearly 20% of children with ASD walked on their toes rather than on their entire foot. Esposito and Venturi (2008) used retrospective video analysis as well as an observational scale to examine 42 children with ASD who had been walking at least 6 months and also found differences in early walking patterns. Vernazza-Martin et al. (2005) found significant differences in gait when comparing 15 children aged 4 to 6 years with and without ASD, as did Woodward (2001) in a study of children with ASD aged 3 and 10 years. One consistent finding is that the gait cycle is slower and less consistent in children with ASD.

Dewey, Cantell, and Crawford (2007) compared the performance of children with different disabilities on the Bruininks-Oseretsky Test of Motor Proficiency-Short Form. Participants included 49 children with ASD, 46 children with developmental coordination disorder (DCD) and attention-deficit/hyperactivity disorder (ADHD), 38 children with DCD, 27 children with ADHD, and 78 children with typical development. The results indicated that although all the atypical groups displayed significant impairment of motor skills, children with ASD were significantly more impaired compared with their cohorts with specific motor skill deficits. They were also the only group to show impairment on gestural skills. Several researchers have found deficiencies in fine motor skills in children with ASD. These range from delays in manual dexterity (Miyahara et al., 1997) and graphomotor skills (Mayes & Calhoun, 2003) during early and middle childhood to motor control issues in prehension (Mari, Castiello, Marks, Marraffa, & Prior, 2003). Such fine motor skill deficits influence handwriting as well as many functional activities involving the hands and arms. Across these studies, it appears that individuals with autism do move differently when compared with their typically developing counterparts and that deficits range from fine to gross motor skills.

In sum, although motor deficits are not listed as part of the DSM-V diagnostic criteria for ASD, individuals with these disorders tend to exhibit motor skill deficiencies as well as differences in motor development. Infants with ASD may exhibit hypotonia (although this finding is not as robust as it is in infants with DS). In addition, delays may exist in the acquisition of motor milestones and fundamental motor skills. Differences in skills such as gait and manual dexterity persist into childhood.

Developmental Coordination Disorder

Another disorder with an unknown etiology that has garnered considerable empirical attention as of late is DCD (Wilson & Larkin, 2008). DCD, also known as developmental dyspraxia, is characterized by extreme lack of motor coordination and by other movement deficits in the absence of neurological defects. DCD has been calculated to affect as much as 6% of the elementary school population (American Psychiatric Association, 1994; Barnhart, Davenport, Epps, & Nordquist, 2003; Barnett, Kooistra, & Henderson, 1998). Although the disorder was described as early as 1937, it took until 1994 for a group of 43 experts in various movement disorder fields to come to a consensus on a name and description for DCD in order to facilitate both research and diagnosis (see the sidebar for the DSM-V description of DCD). Individuals characterized as having DCD fall into the 0 to 10th percentile of performance on the Movement Assessment Battery for Children (M-ABC), a standardized motor skills test used to assess motor proficiency in children with disabilities (Geuze, Jongmans, Schoemaker, & Smits-Engelsman, 2001; Henderson & Sugden, 1992; Miyahara & Möbs, 1995). Because DCD is categorized as a learning disability, it also may influence a child's ability to perform academically. Motor difficulties can range from gross to fine motor to balance skills and can include motor planning deficits and visual or spatial difficulties (Cermak & Larkin, 2002; de Castelnau, Albaret, Chaix, & Zanone, 2007; Kaplan, Wilson, Dewey, & Crawford, 1998; Przysucha & Taylor, 2004; Whitall et al., 2006). Table 9.6 provides examples of functional motor issues that children with DCD may have.