The Dynamics of Quasistatic Posture Control

The output of the human postural control system is highly irregular, even under quiet-standing 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 uctuation-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 uctuations 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 show that the estimated correlation and response functions can be described by our model. These results demonstrate that the FDT exists for human balance control and that postural sway can be modeled by an equilibrium stochastic process. From a physiological standpoint, these ndings suggest that the postural control system utilizes the same neuro-muscular control mechanisms under quiet-standing and dynamic conditions. The parameters extracted from the model can be interpreted from a physiological standpoint and could thus be used as an assessment tool for the evaluation of patients subsequent to pharmacological and surgical treatment.