The dynamics of quasi-static posture control

The output of the human postural control system is highly irregular, even u nder quiet-standing conditions. To describe the observed dynamics of human posture control, we propose a simple, analytical stochastic model, consisti ng of a pinned polymer. For such a system, there exists a fluctuation-dissi pation theorem (FDT), which provides a linear relationship between the corr elations 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 per turbation can be predicted from the fluctuations exhibited by the system un der 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 postu ral-sway data, We also show that the estimated correlation and response fun ctions can be described by our model. These results demonstrate that the FD T exists for human balance control and that postural sway can be modeled by an equilibrium stochastic process. From a physiological standpoint, these findings suggest that the postural control system utilizes the same neuromu scular 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. (C) 1999 Elsevier Science B.V. All rights reserved. PsycINFO classification: 2330; 2240.