CONTROL OF UPRIGHT STANDING POSTURE DURING LOW-FREQUENCY LINEAR OSCILLATION

We examined the effects of anteroposterior movement of a sled on human upright standing. Each of six healthy men stood on the platform of a sled in the dark. The sinusoidal acceleration was provided, from 0.02 to 0.04 G, followed by 0.06 and 0.08 G, at a stroke length from 6 to 1 0 m and then to 14 m. Low acceleration (0.02;Ind 0.04 G) induced body sway, pivoting on the ankle joint. High acceleration (0.06 and 0.08 G) increased body sway, but the head-neck joint remained locked upright. The electromyographic recordings of the lower leg muscles revealed co ntinuous tonic EMG activities of the gastrocnemius and tibialis anteri or muscles at acceleration of 0.02 and 0.04 G, while reciprocal activa tion was observed at 0.06 and 0.08 G. During head movement, the neck m uscles were slightly activated tonically at acceleration of 0.02 and 0 .04 G, but they were markedly and tonically activated at 0.06 and 0.08 G. We speculate that the sled oscillation caused body sway in proport ion to the acceleration, with the ankle joint playing a principal role . Analysis of neck movement also revealed that the head was held in a fixed upright position, indicating that the vestibulocollic reflex mig ht tonically activate the neck muscles. (C) 1998 Elsevier Science Irel and Ltd. All rights reserved.