Human Balance out of Equilibrium: Nonequilibrium Statistical Mechanics in Posture Control

During quiet standing, the human body sways in a stochastic manner. Here we show that the fluctuation-dissipation theorem can be applied to the human postural control system. That is, the dynamic response of the postural system to a weak mechanical perturbation can be predicted from the fluctuations exhibited by the system under quasistatic conditions. We also show that the estimated correlation and response functions can be described by a simple stochastic model consisting of a pinned polymer. These findings suggest that the postural control system utilizes the same control mechanisms under quiet-standing and dynamic conditions.