Relation of automatic postural responses and reaction-time voluntary movements of human leg muscles.

This study contrasts the properties of compensatory postural adjustments in response to movements of the support surface with those of reaction-time voluntary movements in human subjects. Subjects stood upon a six degrees-of-freedom movable platform and performed tone and movement-triggered voluntary sways about the ankle joints both under conditions of postural stability and instability. These triggered movements could be executed as rapidly as postural adjustments to support surface perturbations (80-120 ms), but only when the former were well practiced, single-choice (direction) and were performed under conditions of postural stability. Evaluation of the properties of postural adjustments and reaction-time voluntary movements revealed a number of clear organizational differences between the two categories of movement, but most interesting was the finding that, when reaction-time movements were triggered by or at the onset of platform movement, the postural adjustments always occurred first. Only when subjects were given a tone trigger 50 ms in advance of platform movement were they able to execute the reaction-time movement first. We found that the dichotomous voluntary/reflexive classification of movements was not consistent with all of the identified properties of postural adjustments and reaction-time movements. Instead, we find a system which classifies movements by function, as either stabilizating or orientational adjustments, to be more useful. In the context of whole-body movement then, intentional focal components would be closely associated with others directed towards postural stabilization.