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Abstract
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BACKGROUND: Balance-recovery or push-recovery stepping is often necessary for both a human and humanoid robot after an external perturbation, taking a single step or multiple steps to avoid a fall. The determination of where to step to come to a complete stop has been studied, but little is known about the strategy for initiation of forward motion from the static position following such a step. The goal of this study was to examine the human strategy for stepping from a static, double-support position as might occur after a balance-recovery step, comparing parameters from normal step length to those from increasing step lengths to the point of step failure.
METHOD OF APPROACH: Healthy young adults instrumented with joint reflective markers executed a prescribed-length step from rest while marker positions and ground reaction forces were measured. The subjects were scaled to the Gait2354 model in OpenSim software to calculate body kinematic and joint torque parameters, with further post-processing in MATLAB.
RESULTS: With increasing step length, subjects reduced both static and push-off back-foot ground reaction force. Body center of mass lowered and moved forward, with additional lowering at the longer steps, and followed a path centered within the initial base of support. Step execution was successful if subjects gained enough forward momentum at toe-off to move the capture point to within the base of support defined by the final position of both feet on the front force plate. All peak joint torques increased with step length except ankle joint. Front knee work increased dramatically with step length, accompanied by decrease in back ankle work.
CONCLUSIONS: As step length increased, the human strategy changed, with subjects shifting their center of mass forward and downward before toe-off, thus generating kinetic energy, while using less propulsive work from the back ankle and engaging the front knee to straighten the body. The results have significance for human motion, suggesting the upper limit of step length that can be completed with back-ankle push-off before additional knee torque is needed. For biped control, the results support stability based on capture-point dynamics and suggest strategy for center-of-mass trajectory and distribution of ground force reactions that can be compared with current balance controllers.
Language: en |