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 propriocep tive sensors, an identification technique was applied to measurements of antero-posterior sway angles of the body and ankle moments under th e following conditions: standing on a fixed support with eyes open (ox ), standing on a fixed support with eyes closed (ex), standing on a sw ay-referenced support with eyes open (os), and standing on a sway-refe renced support with eyes closed (cs). Frequency response functions fro m the sway angle to the ankle moment were calculated. Gain and phase c haracteristics for conditions (os) and (cs) were similar to those of N ashner's vestibular model in the high-frequency range, which shows tha t the vestibular system may be dominant. The gain was higher under con dition (ex) than under (ox). Judging from the phase characteristics, t his was probably due to increased weighting of the proprioceptive sens or over the vestibular sensor. There was a tendency for gain to increa se as balance tasks became more demanding.