Motor unit discharge pattern in soleus muscles associated with rapid body sway

Motor unit discharge (MUD) patterns of the soleus muscles associated with rapid forward body sway were studied in relation to displacement of the center of foot pressure (CFP) and to angular changes in the position of the ankle joint. Subjects were instructed to shift the CFP from a relaxed bending posture (N) to a leaning posture half of maximally forward (1/2F) by swaying their bodies, pivoting at the ankles as rapidly and accurately as possible following an auditory signal. Ten motor units recorded from 5 subjects were studied. During body sway accompanied by elongation of the soleus muscle, a stereotyped serial motor unit discharge pattern was observed: cessation of unit discharge (acceleration phase), and then resumption of unit discharge (deceleration and stop phase). The rate of MUD during the deceleration and stop phase was higher than that when subjects held their bodies during holding his body at N or 1/2F. A strong positive correlation was observed between the timing of the resumption of MUD and both the CFP changes and the angular changes in the position of the ankle joint. These results strongly suggest that peripheral inputs are important for control of both the timing of the resumption and the frequency of MUD in the performance of a rapid body sway.