Motor Development

Motor development is the movement science discipline concerned with age-related changes in movement  patterns  and  the  underlying  processes  that drive  those  progressions.  The  changes  in  movement pattern tend to be sequential, occurring in a series of steps. In the early portion of the life span, change  is  from  execution  of  simple  (even  though coordinated) movement units to highly organized and complex movements. In the later portions of the life span, change reflects adaptations to physical aging and disease onset.

Early  in  the  establishment  of  motor  development  as  a  field  of  study,  researchers  tended  to study  the  young,  especially  infants.  While  many came  to  associate  motor  development  only  with the study of young performers, current interest is in  change  throughout  the  life  span.  Study  of  the entire life span has facilitated understanding of the underlying  processes  that  drive  development,  as researchers can observe how a change with growth compares to a change with aging. For example, the stride length of a newly walking toddler is short, compared to that of a young adult, as is the stride length  of  some  senior  walkers.  Motor  developmentalists  are  interested  in  whether  this  reflects underlying processes that are similar or different, with  the  answers  enriching  the  understanding  of developmental processes.

The  study  of  motor  development  includes  the description of change and documentation of when changes occur in the life span. Yet, the field goes far beyond description to focus on underlying causes of change or the why and how of change. This has not  always  been  apparent  to  researchers  in  other movement  sciences.  A  number  of  early  studies describing progression in movement patterns demonstrated  a  consistent  pattern  of  emerging  new skills across human infants. At the same time, the famous “nature–nurture” debate was ongoing in a number of fields concerned with development. The consistent  pattern  of  skill  emergence  was  often taken  as  support  for  the  view  that  “nature”  was the  predominant  influence  on  development.  As  a result, the perception of scholars outside the field of motor development was of a field dominated by a  maturational  perspective  and  using  only  methods of description as a research tool.

Motor-Development-Sports-psychologyDespite  the  perspective  of  others  that  motor development  emphasized  influences  of  nature (genetics), many early researchers in motor development  had  adopted  an  interactionist  perspective—one  that  acknowledged  that  nature  and nurture  (environment)  interact  and  both  influence  development  with  neither  dominating.  They sought  understanding  of  all  the  processes  driving developmental  change.  Nevertheless,  it  was  not until the later portions of the 20th and early 21st century  that  perception  of  the  field  changed  to acknowledge this great interest in how genetically driven development and the environment interact in the developmental change in motor patterns.

While  motor  development  far  exceeds  just  the description  of  change  in  movement  patterns,  the developmental  descriptions  produced  by  early scholars  introduced  a  rich  body  of  information that  is  used  even  today.  Those  rich  descriptions identified  sequences  of  new  skill  acquisition  in infancy (interskill sequences), such as the sequence of  rolling  over,  holding  the  head  up,  pushing  the chest up off the surface, and pushing up to hands and  knees.  Also  identified  were  sequences  of change  within  a  skill  (intratask  sequences).  One brief example for the skill of throwing is for trunk movement pattern: no rotation, simultaneous rotation of the upper and lower trunk, and then lower trunk rotation followed by upper trunk (differentiated) rotation.

Motor development has its roots in biology and psychology.  The  roles  of  physical  growth,  physiological  maturation,  and  physical  aging  in  driving  changes  in  movement  are  obviously  rooted in biology. Yet, the discipline of psychology, with its  emphasis  on  human  behavioral  development, was a greater influence on motor development as a field of study, particularly by proposing models of development and means of studying the human organism.

Focus  on  longer  periods  of  change  contrasts motor development with the field of motor learning that focuses on the change occurring in shorter time periods, for example, as a result of weeks of practice.  The  fields  are  further  distinguished  by an  emphasis  on  the  movement  product  in  motor learning,  but  an  emphasis  on  the  movement  pattern in motor development.

The current field, then, is concerned with development  over  the  life  span,  processes  underlying age-related  change  in  skill  performance,  and  the relationship  of  genetic  and  environmental  factors as  they  drive  motor  development.  It  is  important to understand how contemporary perspectives are shaping  both  research  and  practice  in  the  field. Two  are  particularly  important:  modeling  movement as arising from the relationship of constraining  factors  and  modeling  developing  systems  as dynamic systems.

The Model of Constraints

The  current  emphasis  on  the  interaction  of  multiple influences on development can be seen in the common use of Karl Newell’s model of constraints by both researchers and practitioners in the field. The model uses a triangle to represent movement as arising from the interaction of constraints that fall  into  three  categories:  the  organism  (here,  the human performer), the environment, and the task to be executed. The constraining, or shaping, factors associated with the performer can be further divided into structural and functional constraints. Examples  of  structural  constraints  include  the body’s  physical  structure  and  its  systems,  such  as the  muscular  system,  the  nervous  system,  and  so on.  Examples  of  functional  constraints  are  motivation,  experience,  and  fear.  Environmental  constraints include characteristics of the environment, such as gravity or altitude as well as the surfaces and  dimensions  of  the  setting  for  skill  execution. Task constraints include the intended goal for the movement,  equipment,  and  any  rules  established for achieving the goal.

A particular movement is shaped by the interaction of all relevant constraints. As any even single constraint  changes,  the  relationship  among  the constraints  differs,  potentially  resulting  in  a  different movement pattern. Usefulness of the model to  motor  developmentalists  is  obvious  when  one considers  how  the  structural  constraints  change with physical growth and maturation or aging. As an individual grows, for example, the relationship of the structural constraints with the environmental  and  task  constraints  changes,  giving  rise  to differing  movements.  Hence,  the  model  provides a  vehicle  for  relating  changes  in  these  relationships with growth and development to qualitative changes in movement patterns.

This  framework  is  useful  both  to  researchers who  focus  on  specific  relationships  among  the constraints  and  to  practitioners.  While  growth or  aging  typically  cannot  be  manipulated  in  the short  term,  the  environment  and  task  constraints can  be  manipulated  in  conjunction  with  changes in structural constraints. Practitioners can manipulate  environment  and  task  constraints  to  design developmentally appropriate tasks for those at any point on a developmental continuum of change in the  structural  constraints.  As  a  simple  example, if  the  movement  desired  is  to  shoot  a  basketball through a hoop, the height of the hoop from the ground and/or the weight of the basketball can be incrementally  changed  as  the  height  and  strength of  the  performer  change  to  achieve  the  desired goal  over  the  period  of  growth  and  maturation.  Another  example  is  how  practitioners  have designed  a  series  of  adapted  bicycles  for  children with  disabilities—for  example,  replacing  the  rear wheel  with  two  rollers,  to  facilitate  development to riding conventional bicycles.

Modeling Body Systems as Dynamic Systems

How  qualitative  change  in  movement  patterns emerges  is  one  of  the  most  interesting  questions in motor development yet to be answered. Motor developmentalists  have  modeled  the  developing body  systems  involved  in  movement  as  dynamic systems  to  address  this  question.  The  study  of dynamic  systems  has  its  roots  in  applied  mathematics.  Dynamic  systems  are  time-evolving  systems; hence, their usefulness to the study of motor development   becomes   obvious.   Body   systems evolve,  or  grow  and  mature  and  age,  over  time. The characteristics of dynamic systems have much to offer when addressing a number of fundamental issues in motor development today.

Consider  that  new  movement  patterns  are  not “more” of a previously executed pattern but rather different  patterns.  There  are  qualitative  changes. The  study  of  dynamic  systems  has  demonstrated that  elements  of  a  system  can  self-organize,  to place  themselves  in  new  relationships  with  each other. This could explain how new movement patterns appear: the body limbs and joints that are elements  of  a  movement  pattern  could  self-organize into a different movement pattern.

Dynamic systems are nonlinear, and their development is nonlinear. Hence, dynamic systems offer another advantage for modeling skill performance. Dynamic systems alternate between periods of stability and instability, and what prompts the change from one state to another might suggest how new behaviors  emerge  in  development.  Dynamic  systems are pushed out of a stable state by a change in one or more components of a subsystem. Likely the change must reach a critical value to push the overall  system  into  a  period  of  instability  from which  a  new  stable  state  eventually  emerges.  In motor  development,  changes  such  as  an  increase in  bone  length,  an  increase  in  muscle  strength, or  formation  of  neural  connections  might  be  the types  of  changes  that,  once  reaching  a  critical value,  stimulate  appearance  of  a  new  movement behavior. Dynamic systems theory predicts that a performer would enter a period of variability in a movement pattern before a new pattern eventually emerges and becomes stable. Whether this actually occurs is a question for future research.

One  of  the  multiple  systems  involved  in  a movement  might  develop  more  slowly  than  the other systems. Hence, that system discourages the appearance  of  a  new  skill  until  a  critical  value  is reached, changing the relationship among systems and facilitating appearance of a new pattern. For example,  the  leg  strength  and  body  weight  ratio might have to reach a critical level before an infant can  stand.  In  such  a  case,  the  slowly  developing system is described as the rate limiting system for a particular  movement  skill.  Motor  developmentalists  are  interested  in  identifying  rate  limiting  systems for various movements.

Another issue in the movement sciences in general  concerns  the  mechanism  for  controlling  so many body parts into a coordinated and smoothly controlled movement, and obviously, this too is an issue  in  the  development  of  motor  control.  Since body  parts  have  a  number  of  ways  of  moving— that  is,  multiple  degrees  of  freedom  (df)—a  challenge  for  movement  scientists  is  to  explain  how those  df  are  harnessed  to  execute  goal-directed movements,  especially  when  the  starting  position of limbs and joints can vary from time to time the movement is executed. For motor developmentalists, the challenge is extended to explain how control is initially achieved. Dynamic systems can be constrained into functional units, and this suggests that a human body comprised of dynamic systems can  spontaneously  organize  into  functional  units, thus  minimizing  the  number  of  individual  elements  that  must  be  controlled  by  the  developing nervous system. These functional units have been called  muscle  linkages  or  coordinative  structures. An  example  would  be  how  the  muscles  of  the ankle, knee, and hip act as a unit in swinging the leg  forward  in  walking.  Motor  developmentalists are interested in exploring how these coordinative structures develop, how they might reorganize into new movement patterns, and whether they change over  time  or  come  to  be  incorporated  into  new movement patterns.

As mentioned earlier, a particular challenge for modeling  the  control  of  movement,  especially  in developing individuals, is the probability that the initial  conditions  for  beginning  a  movement  are almost  always  different,  yet  movement  patterns are  successfully  completed.  Sensitive  dependence on initial conditions has become widely known as the  butterfly  effect,  so  named  for  the  notion  that a  butterfly  stirring  the  air  might  influence  storm systems halfway around the world a month later. The  challenge  specifically  for  motor  developmentalists  is  explaining  how  movement  patterns  are acquired,  including  how  they  sometimes  can  be executed correctly on the first attempt, despite this context-conditioned variability. As a simple example, developmentalists must account for how even infants  can  reach  to  an  object  despite  an  almost infinite number of starting positions of the arm.

Dynamic  nonlinear  systems  also  are  modeled as oscillators, including pendula. This has permitted  an  interesting  analogy  for  walking,  since  we might think of each arm, swinging back and forth with each walking stride, as a pendulum. The arms maintain  a  relationship,  systematically  staying  in opposition, and they maintain a relationship with the legs, moving forward and backward, opposite in  direction  to  the  same-side  leg.  This  synchronization  is  called  entrainment,  and  oscillators have  been  shown  to  entrain.  Entrainment  might be a means for the nervous system to more easily control what would otherwise be a set of complicated movements. If each limb, with its joints and muscle units, moved as an independent pendulum, control  of  walking  would  be  a  daunting  task. Entrainment might be an example of how dynamic systems constrain or harness df to simplify control of  some  movements.  Very  early  walkers  tend  to hold  their  arms  up  and  not  swing  them  but  with walking  experience  begin  to  swing  the  arms  at their sides like pendula, moving with the opposite leg.  This  suggests  that  early  walkers  might  begin to use entrainment as a means of more efficiently controlling the movement of multiple limbs.

Motor  developmentalists,  then,  have  used  the features  of  dynamic  systems  to  address  some  of the fundamental issues surrounding the emergence and  control  of  movement  patterns  in  developing movers. Current research into the development of control and coordination in movement is examining many of these features as contributions to our understanding of the underlying processes driving developmental change.

The Sequential Nature of Development

As  mentioned  earlier,  motor  development  is  considered  to  be  sequential.  Longitudinal  study  of the development of basic skills typically reveals a series of qualitative and quantitative changes that eventually  result  in  the  execution  of  a  skill  at  an advanced level. For example, very young children are  typically  observed  throwing  without  taking  a step and therefore with little weight shift. The distance the thrown ball travels is consequently short. At a higher level, children might be observed stepping with a throw but with the foot on the same side of the body as the throwing arm, hence limiting the contribution of trunk rotation to the throw. Then  at  even  more  advanced  levels  throwers  can be seen stepping with the foot opposite the throwing arm. The length of the step might also increase with development so that a throw for distance can be maximized. Hence, development of leg action in overarm throwing is a series of qualitative changes and occurs by progressing from level to level.

Motor developmentalists seek to identify those processes that drive change to the next level. The goal of assisting performers to progress from level to  level  (with  developmentally  appropriate  tasks) is  a  key  feature  of  a  developmental  perspective. Rather  than  seeing  each  early  level  as  an  error because  it  is  not  the  most  advanced  level,  developmentalists  give  credit  for  advancement  to  the next qualitative level, or for a quantitative increase within a level, all as progress toward the goal, the most advanced level.

Consider that performers move through developmental levels in different ways. Take the development  of  the  leg  action  in  throwing  described previously. There also are developmental levels for the action of the trunk and of the arm. One performer might advance a level of leg action before a level of arm action but another vice versa. Even as researchers explore what underlying processes are involved  in  these  variations,  performers  advance sequentially.  Perhaps  this  is  related  to  sequential change in the component body systems (serving as constraints  for  a  task).  Practitioners  who  present developmentally  appropriate  tasks  take  developing movers from level to level over time. This too characterizes a developmental perspective on complex skill acquisition.

As  motor  development  concerns  age-related, sequential  change,  the  ideal  research  design  for those studying motor development is a longitudinal design. The same individuals are observed over time.  This  design  permits  researchers  to  actually observe the phenomenon of interest as it changes in individuals. An obvious challenge for researchers  is  the  length  of  time  individuals  must  be observed to see the change in some phenomenon, as well as repeated measurement. Considering this, other designs are often used in motor development research. Cross-sectional designs allow researchers to observe a phenomenon in those of different ages to imply developmental change. So too are designs that  combine  longitudinal  and  cross-sectional designs. These allow developmental research to be conducted in a shorter time period and with fewer repetitions  of  a  measurement.  Nevertheless,  the longitudinal design remains the preferred method for confirming true developmental change.

Other Areas of Interest

Many  motor  development  researchers  are  concerned  with  the  sequential  changes  in  movement patterns,  the  development  of  postural  control, and  the  development  of  coordination  and  control of movements. Yet others study simultaneous change  in  other  constraining,  interacting  factors that  can  influence  movement  patterns.  The  study of growth and maturation and also aging is important  to  an  understanding  of  motor  development. Physiologically, children are not miniature adults, and there are a number of unique aspects in how children respond to exercise loads. Some researchers,  then,  have  specialized  in  age-related  changes to exercise or in the interaction of exercise with the structural constraints, especially body composition to address aspects of general health and wellness in young and old populations.

Motor  developmentalists  are  also  interested  in the  functional  constraints  and  their  age-related change.  Changes  in  self-efficacy  regarding  skill performance  are  studied,  as  are  motivational factors  in  participation  in  physical  activities. Mastery  learning  climates  have  been  studied  as a  means  to  provide  developmentally  appropriate tasks.  Societal  expectations,  culture,  and  values also  have  been  examined,  particularly  as  they might influence gender differences in performance or selection of particular physical activities.

Conclusion

Motor development as a field of study has become  an  important  component  of  the  movement  sciences,  both  for  practitioners  and  for  researchers. Practitioners  deal  with  movers  who  change  over time and attempt to facilitate that change by assisting movers to attain complex advanced movement patterns.  Researchers  have  found  that  models  of the  control  and  coordination  of  movement  are only  robust  if  they  account  for  developmental change.  Hence,  the  motor  development  field  is integral to the important issues taken up by movement  scientists.  The  fundamental  questions  taken up by motor developmentalists remain on the forefront  of  efforts  to  achieve  a  better  understanding of the movement sciences.

References:

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