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 hav e identified two discrete strategies commonly employed by humans, i.e. hip and ankle strategies. It has hence been implied that the knee joint prays a relatively minor role in balance recovery. The purpose of this study was t o determine whether the size of the feasible stability region (FSR) would b e affected by allowing knee motion in sagittal plane movement termination. The FSR was defined as the feasible range of anterior Velocities of the cen ter 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 co mputed using a four-segment biomechanical model and optimization routine ba sed on Simulated Annealing algorithm for three scenarios: unrestricted knee motion (UK), restricted knee motion (RK), and unrestricted knee motion wit h an initial posture that matches RK (UKM). We found that movement terminat ion could benefit little from UK condition when the COM (x(COM)) was initia lly located in the forefoot region [0.00 (toe) > x(COM) greater than or equ al to -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 an d the rest to movement termination per se. These findings illustrated a the oretical role of knee motion in standing humans' repertoire of effective po sture responses, which include hip and ankle strategies and their variants for balance recovery with stationary BOS. (C) 2000 Elsevier Science Ltd. Al l rights reserved.