Motor Commands in Sport

There are two causes of body movement. First, an external force can act on the person or animal; a strong  gust  of  wind,  for  example,  may  cause  the movement  of  living  organisms.  Second,  through biological machinery such as muscles, force is produced  internally,  leading  to  movement.  The  truly interesting aspect of our movement within a physical  environment  is  that  without  exception,  our movement is a consequence of both muscles generating force internally and the environment producing forces on our body externally.

How  then  does  the  nervous  system  deal  with the  generation  of  instructions  to  the  muscles  to control  movement?  In  other  words,  what  is  the “language” for motor commands? Let’s begin with a straw person model, which we know cannot be correct  but  is  nonetheless  useful  to  enhance  our understanding.  Perhaps  the  motor  commands specify an exact sequence of muscular forces along with  the  actual  temporal  pattern  of  these  forces. This type of idea is seen in the impulse-variability theory  for  the  speed–accuracy  trade-off.  This model  is  based  upon  the  assumption  that  motor commands  specify  the  intensity  and  duration  of muscular contractions.

Nikolai  Bernstein  argued  that  the  central  nervous  could  not  directly  specify  muscular  forces. His  argument  is  quite  simple.  Imagine  flexing and  extending  your  elbow  joint  in  rhythm  to  a metronome.  In  one  case  you  are  moving  in  the horizontal  (transverse)  plane  so  that  gravity  is neutral. In the second case, you move in the vertical  (sagittal)  plane.  In  this  latter  case,  the  effects of  gravity  are  dependent  upon  whether  you  are extending  the  elbow  (moving  down)  or  flexing the elbow (moving up). In these two conditions of planar  motion,  the  movement  trajectories  are  the same, but the actual internally generated muscular forces  are  different.  In  other  words,  motor  commands cannot be specific because we cannot know (predict) the exact nature of external forces acting upon us. Motor commands must be more general.

Delos  Wickens,  in  a  classic  experiment  published in 1938, showed that motor commands are not  linked  directly  to  muscle  commands.  Human research  subjects  placed  their  index  finger  on  a button, with the palm of the hand facing down. A stimulus was presented, and a short time later, the index finger received a mild electrical shock. In a short time, the subjects were conditioned to avoid the shock. What did these people learn? Did they learn  to  contract  a  muscle,  or  did  they  learn  to move away? Wickens then had these subjects put their index finger on the button but this time with the  palm  up.  If  in  fact  the  individuals  had  developed  motor  commands  to  contract  their  flexor muscle  of  the  index  finger,  they  would  not  avoid the  shock.  If  the  individual’s  motor  commands were written to “move away”—if, in other words, the  motor  commands  were  a  spatial  code—then the participant would easily avoid the shock. For9 out of 10 subjects, the movement was immediate and away. In other words, at the highest levels of the central nervous system (CNS), the motor commands  were  set  up  in  reference  to  what  is  called allocentric  space  (with  reference  to  the  external spatial world) compared to muscular space.

Are specific muscular commands unimportant? The answer is negative. One can design a very easy experiment to capture this point. Take your favorite  pen,  and  sign  your  name  on  a  piece  of  paper. Now, use your nondominant hand and sign your name. Research clearly has shown that these two signatures look very similar but at the same time are  different.  One’s  nondominant  hand  signature is not as clear as that of one’s dominant hand. In other  words,  motor  commands  have  generality (the spatial commands can be transferred to different effectors) and also have specificity. Specificity means  that  one’s  nervous  system  has  specialized commands  for  well-practiced  muscular  patterns. These  commands  might  be  much  more  motor specific. We do not know much about the nature of this type of specificity.

Another  way  to  examine  the  nature  of  motor commands is to make the assumption that human beings strive to minimize a particular characteristic of movement. For example, to minimize movement time (MT), muscular contractions would produce force  as  quickly  as  possible  and  then  decelerate (“jam on the brakes”) at the last possible instant. This  is  the  driving  strategy  of  a  typical  16-yearold.  However,  unless  there  is  a  sense  of  urgency, it seems as though human beings attempt to move to minimize some aspect of the energy expended. The way to do this is to move smoothly. Smooth is defined  as  lacking  changes  in  acceleration.  Pointto-point  movement  trajectories  look  as  though the  minimization  principle  is  related  to  reducing changes  in  acceleration.  Movement  trajectories tend to show a bell-shaped velocity profile so that there is a period of positive acceleration and then a  negative  acceleration.  Scientists  are  not  in  full agreement concerning the actual muscular and/or neural  quantity  that  is  minimized,  but  it  is  clear that biological motion is geared to reduce energy expenditure.

Finally,  motor  commands  must  possess  a  high degree  of  generality.  Handwriting  is  the  most popular  example.  People  can  sign  their  name  on a whiteboard, a check, a letter, and using a mouse with  a  computer  screen.  Signatures  are  large  or small, written fast or slow, but a signature maintains  the  spatial–temporal  pattern  that  makes  it “yours.”  What  type  of  motor  commands  allows humans to do this? Perhaps it makes sense to think of a set of motor commands like a set of rules, or equations,  that  define  the  trajectory  in  space  and time. These rules, or equations, are scalable so that the essence of our signature or our tennis forehand exists, but the particular muscle fibers, or muscle units,  are  organized  independently  of  these  rules or commands. Thus, our motor commands are not in muscle language, which allows for flexibility as well as specificity.

References:

  1. Flash, T., & Hogan, N. (1985). The coordination of arm movements: An experimentally confirmed mathematical model. The Journal of Neuroscience, 5,1688–1703.
  2. Wickens, D. D. (1938). The transferance of conditioned excitation and conditioned inhibition from one muscle group to the antagonistic muscle group. Journal ofExperimental Psychology, 22, 101–123.

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