Human balance out of equilibrium :

During quiet standing, the human body sways in a stochastic manner. Here we apply the uctuation-dissipation theorem (FDT) to the human postural control system and show that the dynamic response of the postural control system to a weak mechanical perturbation can be predicted from the uctuations exhibited by the system under quasi-static 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 results demonstrate that the FDT exists for human balance control and that postural sway can be modeled by an equilibrium stochastic process. These ndings also suggest that the postural control system utilizes the same control mechanisms under quiet-standing and dynamic conditions. 4] provides a theoretical framework for studying stochastic systems , a classic example being Brownian motion 5, 6]. For many of these systems, there exists the uctuation-dissipation theorem (FDT) 2, 3, 4, 7, 8, 9], which provides a relationship between the correlations of the uctuations of a system and its relaxation to equilibrium. Besides many applications in physics and chemistry, the FDT has been used to study protein dynamics 10], biochemical ki-netics 11, 12], and population risk mortality 13]. Here 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. Our speciic aims are to show that: (1) human postural sway is an equilibrium stochastic process for which the FDT holds, and (2) the response function and the derivative of the correlation function can be mod-eled by the analytical solution of the recently considered pinned-polymer model of posture control 14]. The human postural control system is highly complex { it involves multiple sensory systems and motor components. Numerous studies have investigated human balance control under quasi-static (unperturbed) conditions or dynamic (perturbed) conditions 15]. Despite these eeorts, it remains unclear as to how the various sensorimotor components are integrated into the postural control system and whether the system utilizes similar mechanisms and strategies under quiet-standing and perturbed conditions 16]. Given the intrinsic complexity of the postural control system, it is not surprising that its output is highly irregular. For example, during quiet standing, the center of pressure (COP) under an individual's feet continually uctuates in a stochastic manner (see Fig. 1b). Recently, Collins and …