Stabilization of human posture during induced oscillations of the body.

Displacements of the center of foot pressure, the hip and the head were recorded in six subjects standing on a platform, sinusoidally tilting in pitch (anterior-posterior). Stimulus frequencies ranged between 0.01 and 1 Hz. Stimulus amplitudes were 2, 4 and 6 degrees. With eyes open the displacements were minimal at 0.3 Hz. With eyes closed, however, induced sway was maximal at this frequency. The apparent lack of visual stabilization at the lowest frequency (0.01 Hz) might be attributed to a subthreshold velocity of the retinal image motion induced by the swaying body. A similar absence of visual stabilization of 1 Hz is assumed to indicate the limit of the working range of visual stabilization of posture. Independent of stimulus amplitude a phase lead of about 90 degrees was found at 0.01 Hz. This decreased with increasing frequency up to a phase lag of 100 degrees at the highest frequency (1 Hz). Head stabilization was generally more effective than hip stabilization. EMG recordings from the leg muscles suggest that with eyes closed the center of force is mainly stabilized by leg muscle activity, while with eyes open this stabilization is best, when vision allows for stabilization of body posture by intersegmental movements between head, trunk and legs.