Analysis of the posture control system under fixed and sway-referenced support conditions.

To delineate the relative roles of each of the feedback sensors in the posture control system such as the visual, vestibular, and proprioceptive sensors, an identification technique was applied to measurements of antero-posterior sway angeles of the body and ankle moments under the following conditions: standing on a fixed support with eyes open (ox), standing on a fixed support with eyes closed (cx), standing on a sway-referenced support with eyes open (os), and standing on a sway-referenced support with eyes closed (cs). Frequency response functions from the sway angle to the ankle moment were calculated. Gain and phase characteristics for conditions (os) and (cs) were similar to those of Nashner's vestibular model in the high-frequency range, which shows that the vestibular system may be dominant. The gain was higher under condition (cx) than under (ox). Judging from the phase characteristics, this was probably due to increased weighting of the proprioceptive sensor over the vestibular sensor. There was a tendency for gain to increase as balance tasks became more demanding.