Relation between muscle response onset and body segmental movements during postural perturbations in humans.

This study has examined the individual movements of the body segments of a group of 10 standing adults during anterior and posterior platform displacements (3 and 6 cm amplitudes), and compared body movements to neck and ankle muscle response onset times. Differences in the kinematics of movement were observed for anterior vs. posterior platform displacements: hip, shoulder, and head began to move much earlier for posterior compared to anterior platform movements. This could explain differences in postural muscle temporal response organization for the two directions of body movement. Though anterior/posterior neck and head displacements were late in comparison to neck flexor muscle response onset, small vertical movements of the shoulder and head occurred early (40 and 67 ms after platform movement onset). These movements were consistently directed upward for anterior platform displacements and downward for posterior platform displacements. In order to determine whether neck proprioceptors were responsible for response activation in the neck we repeated the experiment using a neck stabilization device, on one of our subjects. In this condition, we found normal neck muscle response latencies. This suggests that neck proprioceptors are not the primary contributors to the early neck muscle responses seen during horizontal support surface displacements. In studying the effect of repeated exposure to horizontal platform displacements we found a diminution in the amplitude and an increase in onset latencies in neck and antagonist ankle muscle responses over the sequence of 16 trials, in many of the subjects tested. This corresponded to smaller head accelerations, and smaller displacements of the head and shoulder in later trials in the experimental sequence. The result implies that these subjects changed their postural "set" during the course of the experiment, possibly by relaxing the muscles of the body to allow the viscoelastic properties of the lower body segments to absorb more of the impact of platform displacement.