Inadequacy of an Inverted Pendulum Model of Human Gait

Introduction There remains some disagreement on the source of anterior ground reaction forces (Fx) in normal gait. For some, a passive "roll-off" results from the position of the body’s center-ofmass (COM) forward of its base of support [1], and to others, an active "push-off" results from plantarflexion power generation at the ankle [2]. The former theory essentially models the body as an inverted pendulum and has support in walking toys developed as far back as the late 1800s, and in recent passive dynamic robots [3]. A limitation in these devices is the need for a downward slope upon which to "walk." That we can walk on level and upwardly sloped surfaces suggests that we do indeed input mechanical energy. Based upon these observations, and our experience with pathological gait, we hypothesized that COM position alone would be insufficient to fully explain Fx.