Organization of rapid responses to postural and locomotor-like perturbations of standing man

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Abstract
This study has described the organization of EMG activities among the muscles of a standing subject's legs during rapid postural adjustments (95–120 ms latencies). Adjustments were elicited by the horizontal translation of both feet (causing antero-posterior sway), by the synchronous vertical displacement of both feet (causing changes in height) and by the reciprocal vertical displacement of the feet (causing a locomotor-like motion of the legs and lateral sway of the body). The resulting patterns of EMG activity were highly specific for each kind of displacement, and all subjects completely reorganized the pattern of activity from one form to another within the first trials, even immediately following unexpected stimulus changes. The organization of EMG activities during reciprocal vertical displacements was qualitatively quite similar to those observed during the comparable swing and stance phases of the locomotor step cycle; flexor muscles of the ankle and knee (those being shortened by the displacement) contracted in the upwardly displaced leg while extensor muscles were active in the downwardly displaced leg. This pattern was in marked contrast to the activation of lengthening muscles during synchronous vertical and antero-posterior sway displacements. Finally, electrical cutaneous stimulation of the dorsum of one foot during reciprocal vertical displacements always enhanced the EMG activity of the agonist leg muscles, in-phase with the vertical movement.