Predicted region of stability for balance recovery: motion at the knee joint can improve termination of forward movement.

Earlier experimental studies on balance recovery following perturbation have identified two discrete strategies commonly employed by humans, i.e. hip and ankle strategies. It has hence been implied that the knee joint plays a relatively minor role in balance recovery. The purpose of this study was to determine whether the size of the feasible stability region (FSR) would be affected by allowing knee motion in sagittal plane movement termination. The FSR was defined as the feasible range of anterior velocities of the center of mass (COM) of a human subject that could be reduced to zero with the final COM position within the base of support (BOS) limits. The FSR was computed using a four-segment biomechanical model and optimization routine based on Simulated Annealing algorithm for three scenarios: unrestricted knee motion (UK), restricted knee motion (RK), and unrestricted knee motion with an initial posture that matches RK (UKM). We found that movement termination could benefit little from UK condition when the COM (x(COM)) was initially located in the forefoot region [0.00 (toe) >x(COM)>/=-0.50 (mid-foot)] with no more than a 17% increase in FSR compared to RK. The effect of knee motion increased in the rear foot region with a 25% increase in FSR at x(COM)=-1 (heel). Close to half of this difference (12%) was attributable to the knee-related restriction on initial posture and the rest to movement termination per se. These findings illustrated a theoretical role of knee motion in standing humans' repertoire of effective posture responses, which include hip and ankle strategies and their variants for balance recovery with stationary BOS.