Body oscillations in balancing due to segmental stretch reflex activity

Summary While subjects balanced on a seesaw consisting of a platform with a curved base, the antero-posterior sway of head and body as well as changes in the angle of the ankle joint were recorded and analysed for their frequency power spectrum. The EMG of leg muscles and the position of the resultant force exerted by the seesaw on a force-measuring platform were simultaneously registered and analysed. Balancing oscillations of 4–5 Hz were observed under this condition. They were accompanied by short, reciprocally organized bursts of EMG activity in the leg muscles. When stimulating the tibialis nerves to produce a displacement, the delay until the counterbalancing EMG activity started (about 40 ms) was in the time range of a fast-conducting segmental reflex. After partial ischaemic blocking of group I afferents from the leg muscles or fixation of the ankle joints, the predominant sway frequency was lacking, bursts of EMG activity became longer and stronger, and body balance was more unstable. Altering the height of the seesaw showed that a threshold change in the ankle angle was the determining factor in the production of spinal stretch reflex activity for fast regulation of balance.This work was supported by the Deutsche Forschungsgemeinschaft (SFB 70 — Hirnforschung und Sinnesphysiologie)