Measurement of postural sway is a valuable research and clinical tool that can provide information related to various central and peripheral elements of the nervous system. The present study involved modeling single-limb standing as an inverted pendulum tethered to a supporting surface by two sets of springs that simulated the stiffness of muscles spanning the joint and the inherent stiffness of the joint itself. There are four key elements of this model: (a) joint stiffness is greater in the frontal plane compared to the sagittal plane (neither being affected by fatigue), (b) muscle stiffness is exponentially related to its extension from a resting position, (c) muscle stiffness is reduced by fatigue, and (d) an "ankle strategy" is used to maintain upright single-limb posture. It is concluded that an inverted pendulum model can be used to adequately predict sway frequencies and amplitudes in the mediolateral (ML) and anteroposterior (AP) directions for single-limb stance pre- and postfatigue. In particular, it is possible for acute muscle fatigue to increase sway in the ML direction but not necessarily in the AP direction.
Brian L. Davis and Mark D. Grabiner are with the Department of Biomedical Engineering, Cleveland Clinic Foundation, 9500 Euclid Ave., Cleveland, OH 44195.