This paper proposes a relatively simple function of assistive force for a belt-type hip assist suit developed by the authors’ group. The function, which is inspired by the muscle force of the rectus femoris, contains only two parameters, the magnitude and a phase shift factor. Thus, it can reduce the amount of calculation in generating the desired assistive force during walking. Tests were performed on three healthy subjects to confirm its effect and to investigate its influence on the motions of hip, knee and ankle joints. It was demonstrated that the effect of the assist depended greatly on the phase shift factor, i.e., the location of the peak of the assistive force in a swing period. A large effect was observed when the peak of the assistive force came at mid-swing phase. The results of the tests showed that the proposed force function could help to increase walk ratio (the ratio of step length to the number of steps per minute) by an average value of 11.2% at a force magnitude of 35 N, which could produce an assistive torque of the same order as the magnitude of the muscle force of the rectus femoris around the hip joint.
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