Hip retraction enhances walking stability on a ramp: an equilibrium point hypothesis-based study

Abstract

Hip retraction is a phenomenon observed in human walking. The swing leg rotates 8 backward at the end of the motion. Its positive effect on motion stability was reported in the 9 literature based on some simple models for running or walking. In this study, it is shown that 10 hip retraction angle increases in humans during their ascending and descending walk on a stair. 11In previous studies, hip retraction was modeled by defining a proper motion for the swing leg. 12 According to the equilibrium point hypothesis, the central nervous system (CNS) defines only 13 the equilibrium point(s) and stiffness(es) for body joint(s) to control the human motion. Human 14 body motion emerges as its natural response as a result of the external forces and the defined 15 equilibrium points of joints. Considering the hip torque as a spring-like model with an 16 equilibrium point and stiffness, this study revealed that the hip retraction can be generated by 17 the natural response of the swing leg. Besides, the stabilizing effect of hip retraction was 18 demonstrated by a model for human`s ascending and descending walking on a ramp with a 19 range of positive and negative angles, respectively. The findings suggest that the CNS needs 20 to define equilibrium point just ahead of the stance leg to take advantage of the hip retraction 21 effect on ascending and descending walks on a ramp.22 2