The Effect of Variable Stiffness Exoskeleton on the Hip Muscle Groups during Walking

The reasonable assistance pattern of passive exoskeleton can improve human walking performance effectively. The purpose of this study is to explore the optimal assistance mode of hip joint assisted by passive hip exoskeleton. Based on the mechanism of hip joint movement, especially the internal muscle force and moment in one gait cycle, we propose a passive hip exoskeleton with variable stiffness (vsPHE) to replace the moment of hip joint in both joint flexion and extension movement, and finally improve the efficiency of mechanical work of hip muscle groups and reduce their metabolic cost. In order to analyze the assistance effect of vsPHE, we use a simple mechanical model and a musculotendon actuator model to obtain the characteristics of biological mechanics and energetics of hip muscle groups. From the simulation results we find that the muscle force and activation decrease with the increase of assistance intensity, and the mono-articular muscle decrease more than the bi-articular, moreover, the vsPHE also reduce the negative mechanical work of the bi-articular muscle on the knee joint. The simulation results provides deep insights into the muscle-level mechanisms of hip muscle groups assisted by vsPHE, which is helpful to recover the change of metabolic cost of human body and present a guide for the design of hip exoskeleton, powered and unpowered, to improve the assistance efficiency and user performance.