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pccontrol - 86 Infant Motor Development ne of the basic...

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Unformatted text preview: 86 Infant Motor Development ne of the basic functional components of motor development is I . Opostural control (Reed, 1989). In order for an infant to acquire the many motor skills accomplished in the first year of life, a prerequisite is appropriate postural control. Despite its importance, this aspect of motor development has been largely neglected in the literature until quite recently. Part of the prob- lem was that individuals are often unaware of many of the postural responses that are made. As a result, posture has been regarded as having lower “status” or less importance than other voluntary move- ments. Regardless of the status accorded to this aspect of motor devel- i3[ opment, there is also a fundamental difficulty in researching posture. i That is, it is very difficult to examine postural responses in isolation. In this chapter, the primary postural achievements made in infancy are reviewed in the context of recent research evidence. The most com- monly cited postural achievements include the development of head control, sitting, and the ability to stand erect prior to walking. The main variables affecting early postural development are examined in j ‘ relation to the various theoretical approaches outlined in chapter 2. As much of the postural research has also been carried out in relation n f to particular motor skills such as reaching and locomotion, chapters 5 j l and 6, which outline these developments in detail, also contain relevant HT literature on postural control. DefinitionnndDescriptlon.offiosiumLConiroL The term posture simply means the positions adopted by the body or parts of the body (Jouen 8; Lepecq, 1990). Appropriate postural control is necessary for stability, balance, and orientation. Stability is achieved through maintaining the center of body mass low and within the object’s or individuals base of support (Shumway—Cook & Wool- [ "’ lacott, 1993). The center of body mass, or center of gravity, refers to the point of concentration of the earth’s gravitational pull on an object ,5; or individual. It is a balance point where “all the particles of the object i“ are evenly distributed” (Enoka, 1994, p. 43). When one considers the changes in body dimensions with age depicted in figure 2.6, it is clear that there are considerable changes in the center of gravity throughout infancy that would influence stability. The lower the center of gravity, the more stable the object or individual. As the head takes up a larger proportion of the body mass in young infants as compared to older children and adults, the center of gravity is higher. It becomes lower with increasing age. When an individual maintains equilibrium, this is termed balance. This term is often used interchangeably with “stability” (e..g., Shum- way-Cook & Woollacott, 1993). However, balance can be achieved even in an unstable situation (Haywood & Getchell, 2001). For example, a ballerina canibalance on her toes on one leg even though this is a very unstable posture. 1: ll l ‘. Postural Control 87 Postural control also provides orientation. Appropriate orientation ensures that the alignment or configuration of the body segments is maintained in the appropriate order with respect to one another for the movement or task required (Shumway-Cook & Woollacott, 1993). In the case of walking, for example, a vertical posture or alignment is required. Develnpm e.nta|......Imn s,_iiion.,s, . Bald nc.e-.,.CQ,ntroL- Newborns appear to have little postural control, although they can balance their head very briefly (a few seconds) when held or placed ’ in a sitting position (Prechtl, 1977). Within a few months they have developed head control, followed by trunk control, which allows the infant to sit, initially with support but eventually without any support. By around 12 months of age, infants achieve independent vertical pos- ture involving support on two legs. This eventually leads to locomotor activities such as walking, running, and skipping. It appears from this description that postural control develops in a cephalocaudal direction as originally described by Gesell and Amatruda ( 1945) in their principle of developmental direction. The early matu- rationists such as Gesell argued that these “stages” of development are determined through internal or genetic mechanisms. However, it is now evident that many factors, internal and external, guide the developmental process. Hence, the term developmental transition is used rather than stage. The Development of Head Cantrell The ability to raise the head in the prone position is seen as one of the first major motor achievements of the newborn. Given the dimen— sions of the head in relation to the body at birth, it is no mean feat to raise the head, as the head has a large proportion of weight and body length at birth. Even at seven weeks of age the young infant in figure 4.1 is struggling to raise his head. Indeed, although 90% of neonates at two weeks of age can lift their head from a horizontal surface, it is not until they are three months of age that they? can fully extend their neck while lying in a prone position (Frankenburg et a1., 1992). Fol- lowing this achievement, the infant soon develops the ability to raise the head and chest, eventually pushing up on extended arms. This has been achieved by the infant pictured in figure 4.2. At three months of age, the infant can usually ,maintain the head erect and upright while being held in a sitting ors'standingyposition. A more difficult task is raising the head while supine, a feat that is generally achieved by five months of age. As the head contains two sensory systems essential for balance con— trol, namely the vestibular system and the Visual system, head control 88 Infant Motor Development Figure 4.1 A 7-week-old infant struggles to overcome gravity in order to raise his head. e? 652;: Figure 4.2 At l2 weeks, this intant has developed good head control. V is “especially important” in the development of postural control (Ber— tenthal, 2001). One issue that has received considerable attention is li 1» the ability of young infants to track targets with their eyes and the role F, that head movements play in this. The infant in figure 4.1 would have til considerable difficulty tracking objects by turning the head. Although g 5; neonates can track objects at birth, they tend to use eye movements if initially and turn their head only after their eye movements have t reached the? periphery (Bloch 8: Carchon, 1992). By one month of age, head movements play a significant role in object tracking (Bertenthal 81 von Hofsten, 1998). Postural Control 89 The Development of Sitting In order to sit unsupported, the infant must achieve appropriate trunk and head control. By five months of age, most infants can sit unsup- ported but cannot sit upright. The trunk is supported over, or aligned with, the pelvis and legs, which form a relatively stable base of support. However, an infant at this stage (figure 4.3; the infant in this figure is 22 weeks of age) shows an exaggerated forward lean necessary to support the head. Around six to eight months of age, the upper half of the body has gained adequate control to enable the infant to sit upright unsupported. This is an important achievement as it allows the infant to free her arms and hands for exploration, as can be seen in figure 4.4. The complex interrelationship between postural control and reaching is discussed more fully in the next chapter. Developing the Upright Posture for Walking One of the prerequisites for independent walking is the ability to main- tain postural stability on two legs. Maintaining balance in the upright position requires that the center of gravity be kept over the supporting surface. Latash (1998a) described the ability of humans to maintain the vertical posture as “a miracle,” as the area of support in humans is relatively small. Figure 4.3 Although infants can sit ’iinsupported at Figure 4.4 Once an infant has achieved adequate five months of age, they show an exaggerated forward postural stability while sitting, she is able to more easily lean required to support the head. manipulate objects. 90 Infant Motor Development Infants are usually able to pull themselves into the upright position by around eight months of age. However, when infants first achieve this upright posture, they make use of the surrounding environment for support. The infant shown in figure 4.5 makes use of a toy box to support herself in the upright posture. It is not generally until around 1 1 months of age that infants can stand unsupported (Capute et al., 1985). It is even more difficult for an infant to acquire the ability to main- tain balance when producing forward propulsion such as in walking or running. Again, this is first achieved by the use of supporting objects, a behavior called cruising, as shown in figure 4.6. Cruising involves the use of all four limbs. In the initial stages of cruising, the infant usually moves only one limb at a time as he struggles to maintain the upright position. However, with practice, the infant is soon able to involve mul- tiple limbs as balance becomes more stable (Haehl et al., 2000). Eventually, the infant makes his or her first unsupported steps, which require that the weight of the whole body be supported on one leg at a time as the other leg swings forward. However, once infants achieve these first steps they rapidly become competent walkers. The following sections explore how infants develop the appropriate postural control for these difficult and complex motor tasks. l Figure4.5 The environment is utilized tor support Figure 4.6 An intant cruising, using both arms when the infant first achieves the vertical posture. for support Postural Control hfihienceionlbefleyelopmentoflbflumlfionkolm Assiante (1998) suggests two main functional principles that describe the types of strategies adopted in maintaining appropriate postural control. The first principle, relating to appropriate balance, involves choosing a frame of reference, which can be either the gravitational vector or the type of surface that the individual is standing on. In the first case, balance control is considered to occur temporally from the head down to the feet (i.e., in a cephalocaudal direction), whereas in the latter it is from the feet to the head. Assiante considers that in the first six months infants are guided primarily by gravitational forces and thus develop balance in a cephalocaudal direction. Once infants acquire the upright stance, their frame of reference shifts to the type of surface they are standing on, and balance control is then initiated in the feet and moves up to the head. The second principle, based on Bernstein’s notion of degrees of freedom (see chapter 2), involves mastering the degrees of freedom that need to be simultaneously controlled during postural control. This is relevant to maintaining the appropriate orientation, as “pos- tural orientation requires an understanding of how the many degrees of freedom represented by the body’s muscles and joints are to work together in a coordinated fashion” (Metcalfe & Clark, 2000, p. 392). Assiante and Amblard (1995) note that the head-trunk unit and the degrees of freedom of the neck are of particular importance. As the head contains both the Visual and vestibular sensors essential for appropriate postural control, it is important to minimize head move- ments. This reduces degrees of freedom, thus producing a reference point for further movements. Developmentalists have recently become interested in understand- ing the processes that lead to developmental transitions. In particular, they have investigated the rate-limiting variables that may be respon- sible for behavioral shifts. Appropriate postural control is considered a rate-limiting variable as it is an essential requirement for the devel- opment of motor skills (Shumway-Cook & Woollacott, 1993). However, the development of appropriate postural control is also dependent on a number of variables, and recent research has identified several variables that influence postural development (Woollacott, 1993). One such variable is the development of the appropriate sensory systems, in particular, the somatosensory, vestibular, and Visual systems. Other variables that have been investigated include neuromuscular develop- ment, muscle strength, body mass, and the changing center of gravity through changes in body morphology. ‘ l The Importance of Sensory Input in Postural Control Although individuals are generally unaware of it, the body is continu— ally making changes to posture in response to the information that is received from our sensory systems. Such a relationship is termed a 92 Infant Motor Development closed-loop system by information-processing theorists. On the other hand, from an ecological or dynamic systems perspective, this is a perfect example of perception—action coupling as outlined by Gibson (197 9). Regardless of the theoretical distinction, the systems that are considered of key importance for postural control are visual, vestibu- lar, and somatosensory. One of the key questions asked in postural research is whether one sensory system is dominant over another and if so, whether this priority changes with development. 7/7e Visua/ Sysfem In the adult, exteroceptive information from the visual system (and to a lesser degree, the auditory system) plays an important role in pos- tural control. This is best illustrated by standing with the eyes closed, which usually results in increased body sway. It was also clearly dem- onstrated by David Lee and colleagues (Lee & Thomson, 1982), who designed a room where the walls moved but the floor did not. This is illustrated in figure 4.7. The individual placed within the room does not move. As a result there are no changes in the information provided by proprioceptors (vestibular, kinesthetic, or tactile). Despite this, the misleading visual information provided by the moving walls results in the individual’s overbalancing, thus demonstrating the dominance of the visual information over the proprioceptive information. Does vision play as significant a role in postural control for the young infant? The visual system is considered the most poorly developed of the senses in the newborn infant (see chapter 1), suggesting that it may Room moved toward the subject Room moved away from subject Subject sways Subject sways back to compensate forward to compensate p a , b , ['I/ Figure 4.7 David lee's "moving room” setup. This experimental paradigm has been used to demonstrate visual dominance in balance control. Adapted from tee and Thomson, 1982, « (3:525 Postural Control play a less significant role. Despite this, there is evidence to suggest that neonates utilize vision to achieve postural control. Jouen (1988) placed infants as young as three days of age in an infant seat that could measure the infant’s forward and backward head movements. When a light stimulus was moved toward and away from the infants, 83% of the newborns reacted, although they did not consistently respond in the appropriate direction. This implied that although the infants could perceive the moving stimulus and make postural adjustments, they did not have the correct reference value to respond appropriately. The “moving room” just described for adults has also been used in a doctoral study to test the dominance of vision in infants and children. Pope (1984, cited in Woollacott & Jensen, 1996) observed that two- month—old infants sitting on a stationary platform reacted similarly to adults, responding to the visual information and ignoring the vestibular and kinesthetic information. Barela and colleagues (2000) also used the “moving room” paradigm with sitting infants aged six, seven, eight, and nine months. These authors also found the relationship between the visual information and the body sway to be similar to that in adults. There was no evidence of any developmental differences between the four age groups, despite the greater proficiency of the older groups in sitting. This is consistent with the findings of Bertenthal and colleagues (2000), Who argued that although infants develop faster reactions to movement perturbations and their coordination increases with age, their “postural control system is not fundamentally different than the adult’s system” (p. 313). It has been suggested that infants may rely even more on visual information than other types of information when shifting from one level of postural control to another (Jouen & Lepecq, 1990]. From a dynamic systems perspective, thesgwould be periods in which there is a phase transition from one behavior to another and therefore a period of instability. However, once stability is achieved, there is a shift in dependence from exteroceptive input from the visual system to the proprioceptive and vestibular systems. Woollacott (1993) considers the adaptive mechanisms that implement this modality shift a further limiting variable. Ves/I'bU/ar and Somafosensory Sysfems In order to maintain a stable posture, the segments within the body must maintain appropriate muscle tone. This is thought to occur through the use of sensory information from kinesthetic and vestibu— lar receptors, which initiate the appropriate postural reactions. The vestibular system is highly developed at birth, and it is assumed that the kinesthetic system must also be highly develdped, although very little research has been undertaken in this regard. f. In one of the few studies that have addressed the contribution of somatosensoi'y information on postural control in infants, Barela and colleagues (1999) investigated the importance of sensory information from surface contact when infants were in the upright position. They 94 Infant Motor Development examined the relationship between body sway and the force applied by the infant’s hand to a supporting surface during four different stages of development, namely, pull to stand, standing alone, the onset of independent walking, and experienced walking (around 1 1/2 years). The authors identified a shift in the function of the somatosensory information provided by the supporting surface. In the first three stages, they argued that the infant’s hand provided mechanical support, as the forces applied appeared in phase with body sway. However, in the last stage, in which the infants had become accomplished walkers, the somatosensory information from the hand appeared to take on more of an informational rather than supporting role, evidenced by a shift in the time lag between the forces applied and body sway. The applied force, which also decreased to a light touch in the‘last stage, led the body sway by around 140 msec, which is similar to the value observed in adults. Barela and colleagues argued that this was evidence for a feed—forward mechanism that enabled the infant to control body sway and suggested that these feed-forward relationships are an important part of the development of appropriate postural control. Musculoskeletal Components The ability to support the body is dependent on appropriate muscular strength. This is an obvious variable, yet it has not been adequately researched, especially in relation to postural control. Muscular strength is also closely related to body mass, as an infant who is “chubbier” than another would be required to lift or m0ve a greater mass. Hence the muscles would need to be stronger, or development may be delayed. This issue is discussed by Thelen (1983) in relation to the ability to perform the stepping response and._by Kawai and colleagues (1999), who examined spontaneous arm movements in the first week of life (as described in chapter 1). Both studies manipulated body mass by immersing the infants in water. Thelen also added weights to the infants’ legs. Movements were found to be influenced by the changes in body mass produced by the different conditions that were imposed. As described earlier, the body proportions change dramatically in the first year of life. The young infant is grossly restricted at birth because a large proportion of the body mass is composed of the head. Conse- quently, this is one of the variables limiting early performance. The rapid growth of the infant redistributes these proportions, giving the infant increasing control of the head. Following this, neck and trunk development allows the infant to roll over, sit up, and eventually become mobile through creeping, for example. Finally, as the infant’s center of gravity is lowered, the infant can gain the appropriate postural control and balance for standing and walking. It can be seen that muscular strength, body mass, and the notion of gravity are closely interrelated. Breniere (1996) describes a natural body frequency (N BF) in adults, a parameter incorporating gravity, body parameters of mass, body center of mass, height, and body moment Postural Control of inertia. Despite variations in anthropometric data, no changes in the NBF were evident from one adult to another. The author suggested that this may be a kinetic invariant that is specific to posture and gait. The NBF “integrates gravity and the body’s material structure and establishes a correspondence between the programming of locomotor parameters and their postural consequences” (Breniere, 1999, p. 197). Natural body frequency does, however, change with growth. Breniére (1999) examined how the NBF altered on a sample of children from the onset of independent walking to seven years after the onset of inde- pendent walking. As gravity has a greater effect on younger children compared to those who are older, the NBF decreases with age. The concept of gravity as one of the constraining variables was also examined by Jensen and colleagues (1997). They investigated the impact of physical growth on the relative change in joint moments produced by gravity. Their results showed that as the infant became older, there was a decrease in the slope of the gravitational moment, suggesting that changes in the gravitational moments during infancy may be a variable influencing phase shifts in motor patterns during development. Neuromusculor Development A further variable is the development of the appropriate neuromuscular pathways necessary for maintaining balance. Several different theo— retical approaches view the development of the relationship between nervous system and muscle quite differently. One is based on a matu- rational approach proposing that postural responses are controlled by innate central pattern generators,_(CPGs; see chapter 3), which are neural networks that generate the rhythmic motor patterns found in locomotion (e.g., Hadders-Algra et al., 1996). Unlike the muscle syn- ergies or coordinative structures described from a dynamic systems perspective, these neuronal connections are thought to be predeter- mined through endogenous maturation. That is, their maturation is not dependent upon experience. Forssberg and Hirschfeld (1994) suggested two levels of control based on CPGs. At the first level is selection of the appropriate CPG. At the second level, the CPG is fine-tuned through sensory feedback from visual, vestibular, and kinesthetic systems. It is only this fine—tuning that is influenced by experience. ’ ' The notion of postural control developing only as a result of the maturation of specific CPGs is considered no longer appropriate given more recent research findings (Thelen & Spencer, 1998). For example, the high variability in postural development would not be expected if the appropriate responses are “prewired.” Evidence of suchrvariability was provided by Harbourne and colleagues (1987, cited in Woollacott, 1993), who examined the transition to sitting in infants at two to three months and «then at four to five months of age. The infants were held around the trunk for support and then were released. Electromyo- graphic (EMG) recordings from the back and hip muscles were taken 95 96 Infant Motor Development Figure 4.8 Research on cruising uses; markers to track the movement dynoma to determine the patterns of muscular activity present at each of the two stages. In the earlier stage, there was greater variability in the order in which each muscle was activated; but by four to five months, the infants developed their own particular pattern of muscular acti- vation. Although some patterns were more common than others, the emergence of several solutions to the problem of maintaining balance while sitting demonstrated that the solution did not exist as an innate pattern. Rather, it was proposed that these synergies emerged as a result of the infants’ past experiences. From a dynamic systems perspective, muscle synergies can be con— sidered in terms of Bernstein’s notion of degrees of freedom (Whiting, 1984). Rather than the appropriate postural responses being innate CPGs, there are many possible solutions, and synergies are assembled “online” when needed. Through trial and error, the infant discovers the most appropriate response. Several recent studies have utilized Bernstein’s notion of degrees of freedom to investigate muscle syner- gies in the development of postural control. For example, Harbourne and Stergiou (2003) investigated sitting posture, and Haehl and col- leagues (2000) have examined cruising. Cruising has been identified as an important stage of development and has been investigated using techniques such as motion analysis (see figure 4.8]; Cruising appears to be responsible for the development of the appropri- ate postural control for the transition to independent walking (Haehl et al.). Harbourne and Stergiou (2003) examined three dif- ferent stages of sitting, namely sitting with support when infants could hold up their head and upper trunk, independent sitting for only a brief period, and controlled independent sitting. These three stages of development demonstfated very different levels of postural control. The authors’ measure of postural stability or sway was used to determine the changes in the center of pressure while the infant sat on a force platform. In order to determine the number of degrees of freedom in each of these levels of control, they mea— sured dimensionality (Newell, 1997], described as “a way to characterize the geometry of the attractor organization and the number ofindependent degrees of freedom in system control” (p. 7 3). Using a method called correlation dimension, Harbourne and Stergiou found that the dimensionality diminished from stages 1 to 2. This supports the notion that degrees of freedom are reduced or “frozen” as a new skill is being learned (Vereijken et al., 1992). There was then an increase in dimensionality from the second stage to the third, in which independent sitting had been mastered. Again, ics. This infant is cruising using onty one this supports Bernstein’s notion of degrees of freedom, hand for support. suggesting that once a skill is accomplished there is a Postural Control 97 freeing of the degrees of freedom, “providing the infant with increased adaptability or flexibility in maintaining postural support over the base of support in sitting” (Harbourne & Stergiou, 2003, p. 375). Haehl and colleagues (2000] also considered the transition from cruising to independent walking in terms of Bernstein’s notion of reduc- ing the degrees of freedom (Bernstein, 1967). They provided a graphi— cal representation of this as shown in figure 4.9. Phase I represents Bernstein’s notion that initially the degrees of freedom are “frozen” through tight coupling in order to minimize the initial complexity of the task. Haehl and colleagues suggested that this stage was important for cruising to provide a stable base for an infant to experiment with moving more than one limb at a time. As development progresses, these tight couplings are loosened so that the individual can explore more complex patterns (phase IIa). With practice, preferred patterns are iden- tified, usually described as synergies or coordinative structures, and the degrees of freedom are again reduced as this preferred solution is implemented. This final solution emerges as a result of a combination of biological, environmental, and task variables, although Haehl and colleagues suggested that postural control is the primary component that is improving throughout the infant’s cruising to allow the shift to independent walking. This model was investigated by Haehl and colleagues (2000) through examination of changes in the coordination of the thorax and pelvis using motion analysis techniques. They identified an initial “wobble” Degrees of freedom Degrees of freedom V Progress in skill acquisition Progress in skill acquisition 0 b a Figure 4.9 Haehl and colleagues (2000) used (a) Bernstein's phases of skill acquisition, namely (lllreezing degrees at treedom, (Ila) release at degrees of treedom, and (Nb) selecting the most efficient or economical move- ment patterns to hypothesize (b) the phases'ot skill acquisition during cruising in infants, namely (i) poor control and wobbly, (Ill freezing the degrees at Ffieedom, (Illa) releasing the degrees at freedom, and (Illb) selecting the most efficient or economical movement patterns. Reprinted from Haehl et al, 2000. 98 Infant Motor Development stage that they attributed to the infants’ limited experience in the upright position. “Cruising presents a unique challenge to infants; they must coordinate multiple segments of the body while locomot- ing in a new posture” (p. 709). However, the findings failed to support Bernstein’s sequence of change in respect to the degrees of freedom. Rather, the authors found a gradual reduction in the degrees of free- dom leading to a plateau a few weeks before the onset of independent walking. They suggested that “the task of cruising itself may simply not require infants to tightly constrain the movements of the trunk” (p. 710), as there were many solutions that could be adopted. Prior Motor Experience Another rate-limiting variable that one needs to consider when inves- tigating the development of motor abilities is prior motor experience, which is often determined by the opportunity the infant has for action. Throughout, this volume refers to the importance of early movements to later development. As discussed in chapter 1, there is evidence to sug- gest that a reduction of early fetal movements may have implications for later development. Differences in child—rearing practices also indicate that movement opportunity may affect development. The importance of opportunity was highlighted by Bower (1977) when he commented that “it seems clear that the environment-initiated opportunities for practice in fact have a great deal to do with both the rate and direction of motor development” (p. 9 1). Early spontaneous activity has been linked to later motor develop- ment (Piek & Carman, 1994; Thelen, 1979) and appears to influence postural development. For example, Haas and Diener (1988) argued that the early bouncing, rocking, arithythmical swaying that young infants produce once they pull themselves to the upright stance is an important precursor to postural control as it provides valuable information in feedback mechanisms. That is, the rocking on the hands and knees shown in figure 1.10 is an important precursor to creeping. Haas and Diener (1988) found that less active infants with delayed motor development had ’difficulty in adjusting to postural perturbations. Prior activity is also clearly important for the develop- ment of muscle strength, again emphasizing how different variables or subsystems interact to produce the appropriate conditions for a developmental transition. The impact of prior motor experience on postural development was examined in a series of longitudinal studies by Sveistrup and Wool- lacott (1996, 1997). They examined the automatic postural response from pull to stand to late independent walking in infantsrThey were interested in how these postural synergies, defined as “multiple mus- cles that are constrained to act together as a functional unit, With fixed temporal and spatial parameters” (Sveistrup & Woollacott, 1996, p. 58), developed in infancy. These synergies are highly consistent y» Postural Control in adults during a perturbation while standing. On the basis of EMG recordings, Sveistrup and Woollacott found evidence to suggest that these postural synergies developed in the infant in a generative pro- cess as a result of experience. A rudimentary form of the automatic postural response was found in infants prior to independent walking. It appears that muscles are activated individually initially and then gradually integrated one at a time into the “motor map” producing the functional synergies. The investigators pointed out that these changes could also have resulted from an “unfolding of a preprogrammed genetic code” (p. 68), that is, through maturation rather than experience. However, a further study (Sveistrup & Woollacott, 1997) addressed the effect of different early experience on the onset of the postural response by varying the number of perturbations experienced on the movable platform. Extensive training was found to produce a larger propor- tion of trials with functionally appropriate muscle activity for postural control compared with no perturbation training. Although experience clearly had an effect on postural muscle development, Sveistrup and Woollacott pointed out that maturational effects are also important to consider, as muscle onset latency was not affected by practice. Factors such as myelination or the maturation of appropriate neural connec- tions may be an important rate limiter for this aspect of postural control. Early experience has not received as much attention as other variables in the search for rate—limiting variables, perhaps because it requires quite intensive longitudinal research. Also, for ethical reasons, there are situations in which we cannot manipulate experi— ence in research on human infants—another reason why this factor has not received a great deal of attention (von Hofsten, 1993). Lon- gitudinal studies are essential in the investigation of developmental transitions, as cross-sectional analyses will give only the present state of the group investigated, saying nothing about how this level was achieved. 99 lOO Intont Motor Development WWWWW Infants have several major postural accomplishments in their first year, namely the development of head control, trunk control, and the abil— ity to maintain upright posture that is necessary for walking. In this chapter, several variables that influence the development of postural control were discussed. The essential relationship between sensory systems and the motor mechanisms required for postural control was highlighted as a major component of appropriate postural control. In particular, visual, vestibular, and somatosensory information are of primary importance. Neuromuscular and musculoskeletal variables were also discussed, both from a maturational and from a dynamic systems perspective. The developing infant undergoes major neuro- logical and morphological changes, and one needs to consider these when examining the processes that influence postural development. Of particular importance to balance is the change in the infant’s center of gravity during growth. The final variable described was the influence of experience, a factor that has been largely neglected in the literature on postural control. It is important to note that these factors are all interrelated. Balance, like any other behavior, develops as a result of the changes that occur in the organism, the task required, and the surrounding environment (Newell, 1986). An integral part of all motor tasks is appropriate postural control. This is essential for the maintenance of balance during movement and also ensures the appropriate body orientation for the specific task required. Postural control is considered a rate-limiting variable in the achievement of motor tasks and is discussed more fully in relation to manual and locomotor skills in‘ the following two chapters. \n 7’ ‘s §\- ...
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