SPRING-MASS MODEL CHARACTERISTICS AS A FUNCTION OF RUNNING SPEED:

1678 A simple spring-mass model can explain many aspects of animal and human locomotion. Three properties characterize gait in this model. Leg stiffness(Kleg) is the ratio of the peak force to the displacement of the leg spring. Effective vertical stiffness (Keffect), which describes the motion of the system, is the ratio of the peak vertical force to displacement of the center of mass. Theta is the half angle swept by the leg-spring during stance. It has been reported (using animal and human data) that Kleg was relatively speed independent, with only small changes as speed increased, while Keffect and theta increase systematically with speed (Farley, et al. 1993, He, et al. 1991). The purpose of this study was to assess spring-mass model characteristics for human runners across a range of typical distance running speeds. Ten seconds of vertical ground reaction force (VGRF) measurements at speeds of 3.35, 3.83, and 4.47 m/s for a group of four well trained runners were made using an instrumented treadmill. Peak VGRF, center of mass displacement, change in leg length, and angle theta were calculated for each of 24-28 stance periods at each speed and used to determine Kleg and Keffect. As predicted with the model, Keffect and theta increased significantly with speed. However, Kleg was not speed independent. The data below show the consistent increase of leg stiffness for each subject as running speed increased. TableTable