The functional role of sensory inputs from the foot: stabilizing human standing posture during voluntary and vibration-induced body sway.

The functional role of sensory inputs from the foot in the stabilization of upright standing in humans was studied using an ischemic nerve block which was applied bilaterally at the level of the ankle. Subjects were asked: (1) to lean forward or backward by pivoting around their ankle joints; and (2) to hold a standing posture during the bilateral application of vibration (140 Hz) to the Achilles tendons. After 30-40 min of ischemia, the magnitude of maximum body leaning was equally reduced to about 70% of controls for both the "eyes-open" and "eyes-closed" conditions with bare feet. This decrease of body leaning caused by ischemia was not observed when a foot was fixed firmly on a "fixation" board. During vibration, body sway was augmented, and characteristic oscillations of this body sway around 3 Hz were observed under ischemic conditions with eyes closed and with bare feet. We concluded that foot sensation may be an important source of information for controlling the magnitude of body leaning and for stabilizing higher frequency components of body sway.