The Dynamics of Quasistatic Posture Control

The output of the human postural control system is highly irregular, even under quietstanding conditions. To describe the observed dynamics of human posture control, we propose a simple, analytical stochastic model, consisting of a pinned polymer. For such a system, there exists a fluctuation-dissipation theorem (FDT), which provides a linear relationship between the correlations of the system and its dynamical response to an external perturbing force. We apply the FDT to the human postural control system and use it to test the hypothesis that the system's dynamic response to a mechanical perturbation can be predicted from the fluctuations exhibited by the system under quasi-static conditions. We measured the postural sway of healthy young subjects under quiet-standing and dynamic (perturbed) conditions. We found that the response of the postural control system to weak perturbations is similar to the decay of the correlations obtained from quiet-standing postural-sway data. We also sho..