Determinants of the center of mass trajectory in human walking and running

Walking is often modeled as an inverted pendulum system in which the center of mass vaults over the rigid stance limb. Running is modeled as a simple spring-mass system in which the center of mass bounces along on the complia nt stance limb. In these models, differences in stance-limb behavior lead t o nearly opposite patterns of vertical movements of the center of mass in t he two gaits. Our goal was to quantify the importance of stance-limb behavi or and other factors in determining the trajectory of the center of mass du ring walking and running. We collected kinematic and force platform data du ring human walking and running. Virtual stance-limb compression (i.e. reduc tion in the distance between the point of foot-ground contact and the cente r of mass during the first half of the stance phase) was only 26 % lower fo r walking (0.091 m) than for running (0.123 m) at speeds near the gait tran sition speed. In spite of this relatively small difference, the center of m ass moved upwards by 0.031 m during the first half of the stance phase duri ng walking and moved downwards by 0.073 m during the first half of the stan ce phase during running. The most important reason for this difference was that the stance limb swept through a larger angle during walking (30.4 degr ees) than during running (19.2 degrees). We conclude that stance-limb touch down angle and virtual stance-limb compression both play important roles in determining the trajectory of the center of mass and whether a gait is a w alk or a run.