Identification of adaptation in human postural control using GARCH models

Human postural dynamics was investigated in twelve normal subjects by means of a force platform recording body sway induced by bipolar transmastoid galvanic stimulation of the vestibular nerve and labyrinth. We modeled the stabilizing forces actuated by the feet as resulting from complex muscular activity subject to feedback of body velocity and position. Time series analysis demonstrated that a transfer function from stimulus to sway-force response with specific parameters could be identified. In addition, adaptation to the vestibular stimulus was demonstrated to exist. Residual GARCH modeling (generalized autoregressive conditional heteroskedasticity) suggested a postural adaptation time constant in the range of 40-50 s. The results suggest means to evaluate adaptive behavior in postural control and in other physiological contexts.