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Theories of Motor Development
Johanna Darrah, Gayatri Kembhavi
An infant is born, and the journey of documenting de- velopmental changes begins. Families celebrate and re- cord a series of firsts—first smile, first roll, first tooth, first step. When these milestones appear at the expected age, it is confirmation that their infant is developing as anticipated. For parents^1 of infants with an increased risk of developmental challenges, the appearance of these firsts at the expected time is affirmation that their child is developing appropriately for their age. It is also important to identify if an infant is not reaching mile- stones within the expected age range so that families can be supported. Infants at risk for developmental concerns are fol- lowed closely during their first 2 years to identify delays and, if necessary, to initiate early intervention programs to support the infant and family. Members of neonatal follow-up clinics (neonatologists, pediatricians, nurses, physical therapists, and occupational therapists) moni- tor the motor abilities of infants attending follow-up pro- grams.^2 Gross motor milestones provide one of the earli- est windows into an infant’s developmental progress. To identify atypical motor development, therapists require an in-depth knowledge of typical motor development. Requiring more than a developmental checklist of mo- tor skills, therapists need to understand the theoretical frameworks used to explain the process of development, in addition to the output of developmental maturation. A theoretical framework provides an explanation of how motor skills appear, and change can assist in the design of intervention programs to encourage the development of more mature motor skills. Consisting of a series of statements describing the laws, theoretical principles, or beliefs, a theory summarizes and explains observations and provides a basis for making pre- dictions about the behavior studied. By definition, theoreti- cal principles are tentative and require further research to be deemed valid (Lefrancois, 2006). These statements or hypotheses require thorough examination through experi- mental observation and manipulations. Beliefs are state- ments that are often personal observations and are even less substantiated by experimental results than theoretical principles. Laws, on the other hand, represent statements with accuracy beyond reasonable doubt; theories in the natural sciences such as chemistry and physics are charac- terized by a number of laws. Applied sciences such as physical and occupational therapy have typically relied on theoretical principles and beliefs rather than laws. Theoretical frameworks adopted in applied professions are not merely esoteric descrip- tions of observations and predictions; they guide clinical decision making. Theories of infant motor development provide a platform to generate hypotheses or predictions about which factors influence the emergence of gross mo- tor skills. These hypotheses then inform assessment and intervention approaches for infants who are either at risk for motor delay or who are exhibiting motor dysfunction. A paradigm shift in theoretical frameworks used to explain infant motor development has occurred over the last 3 decades. An array of contemporary theoretical frameworks has replaced the established neuromatura- tional theory (NMT) that dominated clinical approaches to gross motor assessment and intervention for young in- fants for almost a century. The new frameworks have been described collectively as “developmental systems” theories (Ulrich, 2010) and include specific theories such as dy- namic systems theory (DST) (Thelen & Bates, 1994), neu- ronal group selection theory (NGST) (Sporns & Edelman, 1993 ), neuroconstructionism (Karmiloff-Smith, 2006 ), (^1) The term parents is used to represent families and caregivers of varying structures. (^2) Throughout this manual we refer to therapists; however, the Al- berta Infant Motor Scale (AIMS) is appropriate for use by other health professionals who have a knowledge of infant gross motor development.
probabilistic epigenesis (Gottlieb & Lickliter, 2007), and perception action theory (Gibson, 1988). All of these the- oretical frameworks reject the NMT tenet that motor skill maturation is primarily dependent on neurological matu- ration. Instead they present interactionist models of devel- opment that consider factors within the child, the environ- ment, and the parameters of the functional task that behave synergistically to modulate the appearance of infant gross motor skills. Child-related influences include neurological, behavioral, and physical factors. Neurological integrity has a major impact on an infant’s movement choices, but con- temporary theories also consider how other child factors such as muscle strength, range of motion, anthropometric ratios, temperament, cognition, and motivation modify motor behavior. Environmental influences represent ex- ternal factors not related to a specific task. For example, gravity is a significant environmental factor shaping an infant’s motor abilities in the first few months after birth. Other examples of environmental factors that may influ- ence motor maturation include noise level, ambient tem- perature, and even restrictive clothing. Caregivers’ influ- ence on motor development is also receiving increasing interest. A review by Spittle and Treyvaud (2016) suggest that parents and the parent-infant relationship have the strongest influence on infant development. Gross motor skills are universally represented by de- scriptions of motor milestones such as rolling, reach- ing, sitting independently, creeping, and pulling to stand. Functional motor tasks represent the choices that infants use to explore their environments. Specific characteristics of the task can influence the functional motor solution in- fants use to accomplish a motor skill. For example, when first pulling to stand, depending on the height of the sur- face, infants may change their strategy from pulling up with their arms to using their legs. Or when creeping on a rough surface, infants may choose to creep on hands and feet (bear-walking) rather than on hands and knees. Functional motor solutions are influenced by environmen- tal constraints and facilitators. In this chapter we review the constructs of three theo- retical frameworks. Traditional NMT is included because many of its assumptions are still evident in current thera- peutic clinical decision making (Rahlin et al., 2019). DST and NGST are discussed because contemporary assessment and intervention strategies are based on these two theories (Akhbari Ziegler et al., 2019). Commonalities and differ- ences across the three theoretical frameworks are high- lighted, and the implications for assessment and therapy intervention for infants at risk of or demonstrating atypical gross motor development are discussed.
NEUROMATURATIONAL THEORY
Until the 1980s NMT theory dominated the literature de- scribing how infant motor skills appear and mature in the neonate, infant, and toddler. Arnold Gesell (1945), Myrtle McGraw (1945), and Mary Shirley (1931) undertook meticu- lous longitudinal studies to document the evolution of infant motor skills over time and to link that evolution to neurologi- cal maturation. The primary tenet of their NMT framework was that the emergence of infant motor skills is dependent on the inhibition of the subcortical centers of the brain as a result of the maturation of the motor cortex. Their hypotheses were influenced by the work of neuroanatomical and em- bryological researchers such as Coghill (1929) and Hooker (1952) who were investigating the direct relationship between stimulation of brain structures and motor output. Their work gave rise to a neuromaturationist paradigm suggesting that neural maturation dictated infant motor behaviors. Gesell, McGraw, and Shirley independently followed cohorts of infants from the first few weeks following birth to the emergence of independent walking and beyond, documenting not only the chronological appearance of new motor skills but also providing explanations of how they emerged. From rigorous longitudinal observation of infants, they described the emergence of motor skills according to three patterns of maturation: reflexive to voluntary, cepha- locaudal, and proximal to distal. Reflex movements preced- ed controlled voluntary movement. Purposeful movements emerged in a predictable manner; an infant achieved head control before trunk and pelvic control, and refined hand function appeared only after proximal shoulder control was achieved. They postulated that the primary driver of these developmental sequences was the inhibition of subcortical centers of the brain as a result of the maturation of higher cortical centers. They suggested that the blueprint for the emergence and maturation of gross motor skills was prede- termined or hardwired and dependent on cortical inhibi- tion of lower brain centers. Accordingly, they hypothesized that the environment played a secondary role, if any. Gesell (1945) and McGraw (1935) conducted longitu- dinal twin studies to evaluate the influence of the environ- ment on motor skill acquisition. One twin received sched- uled controlled environmental stimulation to enhance motor development while the other twin acted as a control subject. Both researchers concluded that the environment had a negligible or minimal impact on the development of an infant’s gross motor skills. Shirley (1931) sent assessors to observe 25 infants in their homes weekly in their first year of life and biweekly during their second year to docu- ment the appearance and maturation of their gross motor skills. She described the emergence of motor milestones as being orderly, that the sequence of skill development was
to explain changes in all developmental domains. Her re- search has challenged occupational and physical therapists to revisit a long-held belief system grounded in NMT that has guided clinical identification and intervention prin- ciples used with neonates and infants at risk for motor dysfunction for almost a century. One of the challenges of the DST framework is that it does not identify specific neural mechanisms responsible for the initiation of motor maturation—that is, how does the spiraling, interactive process begin (Sporns & Edelman, 1993)? Neuronal group selection theory draws attention to the neurological origins of motor maturation, while incorporating the influence of the environment and task.
Neuronal Group Selection Theory
NGST has been described as a “bridge” between the NMT and DST frameworks (Hadders-Algra, 2000) because it acknowledges the influence of both nature (genetics) and nurture (environment and task) on the emergence and re- finement of infant gross motor skills. Edelman (1987) pro- posed NGST as an overarching theory to explain how the maturational processes that occur in the brain and nervous system affect all areas of development. Sporns and Edelman (1993) applied this theory specifically to sensory motor de- velopment. Working from Bernstein’s premise that move- ment occurs in synergies, not through individual muscle actions, they suggested that maturation of movement re- sults from coordination among the musculoskeletal and neurological systems and afferent stimulation from the en- vironment. Their specific interest was how changes in the brain were related to both muscle coordination and envi- ronmental influences. NGST proposes that movement variability is a critical component of infant motor development. At birth, move- ment is organized through diffuse, disorganized neuro- nal circuits in cortical and subcortical areas of the brain. Interconnected neurons that fire in a temporally coordi- nated manner form neuronal groups that produce more organized movement patterns. These primary movement repertoires are not hardwired or stereotyped across all in- fants; their variability is a product of genetic information that provides general movement constraints for movement choices. While primary movement repertoires do not pro- vide specific solutions to environmental situations or tasks, these movements are more organized than those at birth. As an infant matures and practices primary movement repertoires, the brain receives increased afferent informa- tion from the environment, resulting in the emergence of secondary movement patterns linked to specific tasks and environmental situations. Although both primary and sec- ondary movement patterns exhibit variability, the source of variability differs. Primary movement variability is a of primary stepping was due to a critical fat:muscle ratio in the thighs of growing infants that made the leg too heavy to continue stepping. This hypothesis contradicted the be- lief that neurological maturation and reflex inhibition were solely responsible for the disappearance of reflex walk- ing, thereby suggesting that other internal mechanisms or subsystems could affect motor output. Second, she dem- onstrated that an external change in environment could change the motor behavior of infants. These two ideas have become tenets of contemporary descriptions of infant motor development. Her subsequent research on infant kicking and reaching (Thelen et al., 1993 ) supported her beliefs that motor skills emerged as a result of self-organization of an array of parameters within the child, the task, and the environment. No single sub- system can be considered the prime causal factor for the emergence of a new motor skill. Thelen also suggested that subsystems within the child do not mature at the same rate or in a linear fashion; a small change in an important sub- system or “control parameter” can result in a new motor behavior. Conversely, any subsystem within the child, task, or environment can act as a “rate-limiting factor,” imped- ing the emergence of a new motor skill. Infant motor solu- tions at any one point in time are flexible and the prod- uct of the dynamic interaction among many contributing factors within the child, the environment, and the specific task (Thelen et al., 1987). Thelen proposed that the func- tional task is the integral factor that shapes motor behavior. Interestingly, McGraw (1985), when reflecting on her aca- demic research, wrote that in her twin study she had im- peded the toddler’s ability to ride a tricycle by strapping his feet to the pedals. She concluded that by manipulating the task instead of permitting the child to craft his own solu- tion, she had made it more difficult for the child to perform the task. As such, she adopted a dynamic systems perspec- tive, suggesting that the characteristics of the task shape an infant’s motor solution. Thelen continued to apply the tenets of DST to cogni- tion (Thelen & Smith, 1994) and language (Thelen & Bates, 1994 ), and generated universal principles of development that could be applied across developmental domains. The first principle is the concept of nonlinear interactions of multiple internal and external subsystems influencing be- haviors. The second principle is that the resultant behaviors are loosely assembled, allowing for variability of solutions over time and environments. The third principle is indi- viduality, that children may choose different solutions for the same motor task. The fourth principle is described as embodiment, the suggestion that perception, action, and cognition are interrelated and cannot be partitioned into different developmental processes. Thelen constructed “a grand theory of dynamic systems” (Spencer et al., 2006)
In the same manner, the appearance of infant motor skills can be episodic with many skills appearing in a short time coupled with periods of consolidation when no new mo- tor skills emerge. Longitudinal studies of typically devel- oping infants’ scores on gross motor, fine motor, and lan- guage developmental scales have revealed variability both in an individual infant’s score over time and among infants’ scores at one time (Darrah et al., 1998, 2003 ). Infants do not maintain the same percentile score at all sampling ages, and there is not one universal pattern of the emergence of infant motor skills. Previous studies of emerging mo- tor milestones may have failed to capture these patterns of intraindividual and interindividual variability because the time interval between assessments in most longitudinal studies of infant development is long and thus may fail to capture the discontinuous shape of development (Adolph et al., 2008). Factors that affect variability can be intrinsic to the infant such as anthropometric change (height, weight, strength), behavioral characteristics (temperament, curi- osity), and cognitive abilities. External factors associated with the environment or the demands of the task can also affect an infant’s movement solution. For example, an in- fant with a mature gait may revert to a high guard pattern when walking on uneven surfaces, or an infant may creep to move in her home but decides to bear-walk on hands and knees when outside on an unfamiliar surface. In summary, contemporary theories of infant motor development reject the premise that motor skills are hard- wired and dependent solely on cortical maturation. In ad- dition to neurological integrity, many other physiologic and behavioral characteristics within an infant can influence motor behavior. Characteristics of the task and the envi- ronment also contribute to an infant’s motor solutions. How have these new principles and beliefs changed the as- sessment and intervention approaches of therapists?
CHANGING THEORIES, CHANGING PRACTICE
NMT dominated the landscape of infant motor develop- ment for over 50 years; as such, the theory informed and defined infant motor assessments and intervention prin- ciples during this time. Approaches to assessment were prescriptive and followed a reflex to voluntary movement pattern. Accordingly, therapists evaluated the integration of primitive reflexes, the emergence of righting and equi- librium reactions, and changes in muscle tone and resting postures, with little or no consideration of environmental influences. This assessment approach required extensive handling of the infant by the therapist, who often placed the infant in unnatural positions such as vertical and ven- tral suspension. Infants were expected to follow a similar course of motor development both in terms of the sequence product of genetics and internal afferent information, while secondary movement patterns are a result of added exter- nal afferent information from environmental experience. These secondary movement choices are situational and task specific, appearing at different ages for different tasks. The most mature form of secondary movement pat- terns evolves from one motor solution for a specific task to the emergence of multiple motor solutions for a specific motor task. This mature form of multiple solutions for one task may begin to appear for certain motor tasks as early as 2 years of age but may not appear for other more com- plex tasks until mature adolescence (Hadders-Algra, 2000). NGST synthesizes the interaction between neurological maturity and environmental influences; it views both as malleable and responsive. Motor maturation is not prepro- grammed or hardwired; it is accomplished by initial diffuse interconnected neurons that eventually are directed to spe- cific motor tasks as a result of afferent feedback from the environment. The common intent of all three theories presented in this chapter is to understand and describe the process of emergence of typical infant motor skills. They differ in their explanations of the initiation of developmental changes. NMT describes inhibition of subcortical areas of the ner- vous system by the motor cortex as the primary instigator of change. DST views the demands of the task as the pro- cess that shapes motor behavior. NGST views diffuse epi- genetic coding as the precursor of purposeful infant motor behavior. DST and NGST acknowledge that the final mo- tor solution for any given motor task reflects an interaction of factors represented by the child, the task, and the envi- ronment. In both of these theoretical frameworks, motor development is fluid and variable, with no one preferred motor solution that may be applied across different circum- stances. While McGraw and Gesell also recognized some variability in the timing and pattern of emerging motor skills both within a child and across children, their unre- lenting focus on the documentation of the rate and order of the emergence of universal motor milestones rather than on individual variability resulted in their adherence to the NMT. Movement variability, both within an infant (intraindi- vidual) and across infants (interindividual), defines typical motor development. Historically, variability in both the pattern and age of appearance of motor milestones was considered to represent measurement error (Vereijken, 2010 ); it was assumed that motor skills should emerge in a smooth linear fashion. The tenets of both DST and NGST suggest that infant motor skills emerge in a nonlin- ear manner. Nonlinear, unique individual patterns in the development of infants’ growth rates of height and weight have been reported (Lampl et al., 2001; Mei et al., 2004).
evaluation of the effectiveness of interventions more chal- lenging. New research evaluation methods may also need to be developed, as the gold standard randomized controlled trial may not prove to be the best method to use when eval- uating the efficacy of new intervention approaches. Infant motor development theory has evolved from pre- scriptive brain-controlled beliefs to contemporary frame- works that consider the role of numerous interacting vari- ables within the infant, the task, and the environment that shape an infant’s motor solution. Contemporary theories of infant motor control have made the understanding of infant motor development more complex and hence more challenging to translate easily into practice. The challenge for physical and occupational therapists is to avoid repeat- ing the error of blindly accepting the new theories, as was done with NMT, as truth. Theories are not truth; rather they are a set of assumptions to describe observations and as such need to be systematically evaluated. Occupational and physical therapists are well positioned, using innova- tive research methods, to evaluate assessment and inter- vention protocols based on the new theoretical paradigms, thereby linking practice to the underlying theory.
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