Biomechanics of Step Initiation After Balance Recovery With Implications for Humanoid Robot Locomotion

Abstract

Balancerecovery stepping is often necessary for both a human and humanoid robot to avoid a fall by taking a single step or multiple steps after an external perturbation. 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 by moving the back foot forward from a static, doublesupport position, comparing parameters from normal step length (SL) to those from increasing SLs to the point of step failure, to provide inspiration for a humanoid control strategy. Healthy young adults instrumented with joint reflective markers executed a prescribedlength step from rest while marker positions and ground reaction forces (GRFs) were measured. The participants were scaled to the Gait2354 model in opensim software to calculate body kinematic and joint kinetic parameters, with further postprocessing in matlab. With increasing SL, participants reduced both static and pushoff backfoot GRF. Body center of mass (CoM) lowered and moved forward, with additional lowering at the longer steps, and followed a path centered within the initial base of support (BoS). Step execution was successful if participants gained enough forward momentum at toeoff to move the instantaneous capture point (ICP) to within the BoS defined by the final position of both feet on the front force plate. All lower extremity joint torques increased with SL except ankle joint. Front knee work increased dramatically with SL, accompanied by decrease in backankle work. As SL increased, the human strategy changed, with participants shifting their CoM forward and downward before toeoff, thus gaining forward momentum, 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 the SL that can be completed with backankle pushoff before additional knee flexion and torque is needed. For biped control, the results support stability based on capturepoint dynamics and suggest strategy for centerofmass trajectory and distribution of ground force reactions that can be compared with robot controllers for initiation of gait after recovery steps.