The step length able-bodied human tend to choose during stable straight forward level walking

Where the foot placement human tend to choose during stable level walking and what the intrinsic relationship between step length, step time, walking velocity and ankle moment are still unclear. The aim of this study is to investigate these issues. In this paper, human body is modeled as a linear inverted pendulum model (LIPM) with finite-sized foot, according to which the stable step length (SSL) and the stable velocity point (SVP) is derived. Biped gait simulations suggest that to keep the stability of step-to-step transition velocity with the lowest ankle energy consumption, human need to choose SSL to guarantee the foot placements locate at the SVP. The stable walking kinematic data collected from fifteen healthy young adults are analyzed in this paper. Analysis results show that real foot placements are very close to SVP $(R^{2}=0.99956)$ and the mean absolute errors between real foot placement and SVP are below 34 mm (about 6.1% of the average step length). Our main findings are that able-bodied human tend to choose SSL and place foot at SVP to maintain the stability of step-to-step transition velocity during stable level walking. Because of the imprecision of human motor control system, the error between real foot placement and SVP will always exist during stable walking. But due to the ankle effect, human can remain stable gait even though foot placement deviate from SVP within a range by applying ankle moment to reduce the step-to-step transition velocity fluctuation caused by the error between foot placement and SVP.