Body Freezing

Complex  systems  in  nature  are  defined  as  having many individual components that are free to vary and interact with each other, exemplified by a sand pile, a weather system, and social collectives such as  animal  colonies  and  sports  teams.  An  athlete can  also  be  studied  in  this  way.  In  the  complexity sciences, the term degrees of freedom typically refers to the independent components of a system that can be reorganized in many different ways as surrounding constraints change. When considering the  human  body  as  a  complex  system,  an  important challenge is to understand how coordination emerges  among  the  large  number  of  motor  system  degrees  of  freedom  (e.g.,  the  muscles,  joints, limb  segments).  In  motor  learning,  this  challenge is  known  as  Nikolai  Bernstein’s  degrees  of  freedom  problem:  How  can  humans  organize  the large number of motor system degrees of freedom to  consistently  produce  functional  actions  such as  catching  a  ball  with  one  hand?  Even  a  simple movement of reaching and grasping an object with the hand and arm could require a catcher to regulate 7 degrees of freedom (dfs) of the arm, involving  flexion–extension,  medial–lateral  movement, and rotation of joints (3 at the shoulder, 1 at the elbow, and 3 at the wrist). Of course, more degrees of  freedom  need  to  be  regulated  in  coordinating more complex actions such as performing a triple somersault in gymnastics.

Bernstein  proposed  that  performers  initially cope  with  the  large  number  of  motor  system degrees  of  freedom  by  rigidly  fixing  or  freezing a  small  number  into  a  basic  motor  pattern  to achieve a task goal. This strategy leads to the characteristic  stiffness  that  many  individuals  portray early  in  learning.  The  freezing  of  motor  system degrees of freedom is a completely understandable coping  mechanism  when  anyone  is  placed  in  an unfamiliar  performance  context  and  shows  how an  individual’s  intentions,  perception,  and  action interact  to  constrain  the  movement  pattern  that emerges. For example, when novices learn to swim their main intention is to remain afloat and maintain stability in the water in order to breathe and not sink. This intention contrasts with those of an Olympic-level  swimmer  seeking  to  move  rapidly and  efficiently  through  the  water  to  reach  a  race endpoint  in  the  shortest  time  possible.  An  initial coordination mode in the breaststroke corresponds to an iso-contraction of the nonhomologous limbs: the  in-phase  muscle  contraction  of  arms  and  legs together. System stability is enhanced by synchronizing the flexion and extension of both arms and legs together, rather like the directional movements  of  an  accordion.  The  accordion  mode  of  coordination  corresponds  to  a  juxtapositioning  of  two contradictory  actions:  leg  propulsion  during  arm recovery  and  arm  propulsion  during  leg  recovery.  It  is  not  mechanically  effective  and  does  not provide high swim speed because each propulsive action is thwarted by a recovery action. However, this freezing coordination strategy is functional for novice swimmers because it is the most stable and easiest to perform early in learning.

As  learners  become  more  familiar  with  a  task, their intentions change quickly and they can abandon  the  coping  strategy  of  freezing  degrees  of freedom  by  reorganizing  them  into  specific  functional muscle–joint linkages or synergies. Bernstein advocated  that  these  more  functional  groupings help  learners  compress  the  numerous  physical components of the movement system to make the relevant dfs for an action become mutually dependent. Synergies between motor system components help make the body more manageable for learners when they discover and assemble strongly coupled limb  relations  to  cope  with  the  huge  number  of movement system degrees of freedom.

Synergies  are  functional,  being  designed  for  a specific purpose or activity, such as when groups of muscles  are  temporarily  assembled  into  coherent units to achieve specific task goals, like throwing a  ball  or  performing  a  triple  salchow  in  ice  skating. Good quality perceptual information is necessary in assembling coordinative structures because the  details  of  their  specific  form  or  organization are  not  completely  predetermined  and  emerge under  the  constraints  of  each  performance  situation. Assembling a synergy is a dynamical process dependent on relevant sources of perceptual information related to key properties of the performer (e.g., haptic information from muscles and joints) and  the  environment  (e.g.,  vision  of  a  target  or surface).  Synergies  emerge  from  the  rigidly  fixed configurations  that  learners  use  early  on  to  manage the multitude of motor system dfs and become dynamic  and  flexible  as  learners  use  information to tune their functional organization.

Bernstein’s  ideas  were  a  precursor  to  recognition of the human body as a complex system and were  instrumental  for  movement  scientists  seeking  to  understand  how  coordination  can  emerge in human movement systems with their huge number of degrees of freedom, such as muscles, joints, and limb segments. It has been suggested that the degrees-of-freedom  problem  can  be  resolved  in a  human  movement  system  if  the  human  movement  system  is  conceptualized  as  a  complex, dynamical  system  in  which  cooperation  between subsystem components can lead to a reduction in system  dimensionality  through  the  emergence  of synergies  or  more  compact  movement  patterns. Some research on how skilled and unskilled individuals kick a football has supported these ideas. D. I. Anderson and Ben Sidaway’s detailed analysis of kicking confirmed the different ways that motor system degrees of freedom are reorganized during learning.  They  demonstrated  that  novice  kickers did not display the same coordination patterns as skilled  individuals.  The  rigidity  of  novice  movement  patterns  and  the  flexible  nature  of  skilled kicking  patterns  were  clearly  depicted  in  their work.  Before  practice,  the  joint  range  of  motion (ROM)  for  knee  flexion  and  extension  during kicking  by  unskilled  participants  was  smaller  in magnitude  than  the  values  observed  in  skilled kickers. Smaller ranges of joint ROM tend to signify  greater  rigidity  of  movement  patterns.  After practicing  for  10  weeks  at  15  minutes  per  week, the  novice  group’s  coordination  pattern  began to  lose  its  rigidly  fixed  characteristic  and  tended to  resemble  the  more  flexible  pattern  of  skilled kickers.

References:

  1. Anderson, D. I., & Sidaway, B. (1994). Coordination changes associated with practice of a soccer kick. Research Quarterly for Exercise and Sport, 65, 93–99.
  2. Bernstein, N. A. (1967). The coordination and regulation of movement. London: Pergamon Press.
  3. Seifert, L., & Davids, K. (2012). Intentions, perceptions and actions constrain functional intra and interindividual variability in the acquisition of expertise in individual sports. The Open Sports Sciences Journal, 5, 68–75.

 

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