Motor activity and visually induced postural reactions during two-g and zero-g phases of parabolic flight.

Maintenance of an upright posture was measured in human subjects during increased and decreased gravitoinertial force phases of parabolic flight. Upright posture was maintained by tonic activity of the ankle joint extensor muscle (soleus) during the one-g or two-g phases. During the zero-g phase the same posture was maintained primarily by activity of the flexor muscle (tibialis). Downward and upward optokinetic stimulation induced strong postural reactions directed forwards and backwards, respectively. These reactions were accompanied by changes of motor activity of the flexor muscle. This suggests a rapid redistribution of motor programs between extensor and flexor muscles with an increase in visual input influence, and confirms results obtained during space flight.