The Spring-mass Model for Walking

INTRODUCTION In general, two different models are employed when addressing animal and human locomotion on a simple mechanical level: the inverted pendulum model for walking, where the body is reduced to a point mass m at the center of mass (COM) vaulting over a rigid stance leg of length l0, and the spring-mass model for running or hopping, where the rigid stance leg is substituted by a compressing spring of rest length l0 and stiffness k. The spring-mass model reproduces salient features of the characteristic ground reaction force (GRF) pattern observed in running, which renders it ideal to explain experimental observations, predict functional dependencies, and formulate biological control hypotheses. By contrast, the inverted pendulum model suffers from GRF patterns inconsistent with experimental observations. Consequently, experiments also demonstrate that instead of vaulting over rigid legs (characterized as ‘compass gait’), the COM experiences much less vertical excursion necessitating significant stance limb compressions, which at high speeds are even comparable to those observed in running [e.g. 1]. Motivated by these experimental findings, we here ask in how far the characteristic GRF patterns of walking can be explained by purely elastic leg behavior.