Maximum Likelihood Identification of a Posture Control System

To identify characteristics of a sensory feedback path of a posture control system without external disturbances, the maximum likelihood identification method was applied to data of an antero-posterior sway during quiet stance. A subject stood on a force plate and the sway angle of a body (input) was measured by a position sensor camera and the ankle joint moment (output) was measured by the force plate. Using data of 500 input-output pairs sampled every 0.1 s, parameters of the sensory feedback path were estimated and frequency response functions were calculated. These showed derivative characteristics that are necessary for stabilization of the posture control system. Gain characteristics under the closed eyes condition tended to be greater than those under the open eyes condition.