Understand Human Walking Through a 2D Inverted Pendulum Model

This paper gives some macroscopic understandings on human walking about the limitations on walking speed and step length, the reachable region, capture region, and disturbance recovery through a 2D inverted pendulum model. Our concern is the most basic problems in human walking, such as what are the limitations on walking speed and step length, how people change speed during step-to-step transition, and how people prevent a fall. The concept of walking orbit is proposed as a tool to study these problems. It describes the walking motion in the state space under walking constraints, giving us an intuitive way to study human walking during a step and switch between steps. The model has a point mass on the hip and two massless legs. The two dominant control inputs, hip and ankle actuation are idealized into a free determined foot placement and an impulsive push off. Based on this model, some quantitative and qualitative analysis are given, leading to some macroscopic understandings on human walking. Although this paper does not talk about any details on how to realize the control for a real biped robot, it may serve as a helpful guide for biped robot design and control in the future.