We have examined rapid postural adjustments associated with a class of voluntary movements that disturb postural equilibrium. In the test that follows, these motor activities are termed associated postural adjustments and voluntary focal movements, respectively. Standing human subjects performed a variety of movement tasks on a hand-held manipulandum, resulting in disturbances to their postural equilibrium. The experimental use of movement that interact with the subject's environment in a relatively simple way permitted a more precise comparison of the postural adjustments with their associated focal movements. Subjects either pulled or pushed on a stiff interface (the handle) or they responded in a predetermined way to handle perturbations. These activities were carried out with various degrees of steady-state postural stability. Prior to and during these movements, support surface and handle forces, electromyographic (EMG) signals, and body sway were monitored. In addition to previously shown postural adjustments associated with reaction-time arm movements, we have demonstrated these postural activities occur in concert with segmental stretch reflexes and self-initiated (untriggered) movements. Postural adjustments were initiated shortly before all focal movements tested except the short-latency component of the biceps stretch reflex (25- to 30-ms latency). However, this reflex component was rarely elicited by handle perturbations in free-standing subjects; therefore, postural adjustments usually preceded any biceps activity under this condition. By varying the degree of steady-state postural equilibrium, a reciprocal gain/threshold relationship between postural and focal components was documented, i.e., when stability was high, postural activity was reduced or absent and focal activity enhanced. Conversely, the biceps stretch reflex was difficult to elicit under any condition where the subject was not fully supported in the direction of movement and reaction times of focal movements were prolonged. Postural activities associated with focal movements were found to share a number of organizational properties with automatic postural adjustments to support surface movements. Specifically, the postural muscle synergies were equivalent in muscle composition, relative activation magnitudes, and relative temporal sequencing. Furthermore, both types of postural adjustments were highly specific in locus and magnitude to the quality of steady-state postural equilibrium (e.g., postural 'set). A conceptual model is proposed that suggests one simple way in which the reciprocal influence of postural set on postural and focal movement components and their temporal sequencing might be accomplished. Furthermore, we propose in this model a common central organization of postural adjustments associated with focal movements and those elicited by support-surface movements.
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