Figure 5.

MODEL OF THE MUSCLES AND REFLEXES involved in running consists of a rack-and-pinion element connected in series with a damped spring. The dashpot damping effect is created by a piston moving in a cylinder containing a viscous fluid such as oil (color). In the model the rack and pinion represents the role of movement commands from higher motor centers, the spring represents the muscle stiffness (assumed to be constant) and the dashpot represents both the force-velocity relation of muscle and the rate sensitivity of the reflexes. In other words, the setting up of leg positions is modeled by the motion of the rack and pinion, but the reaction to external force disturbances, such as the impact of the track on the contact leg, is modeled by the deflection of the damped spring. Hence the leg and hip positions at the beginning and end of the ground-contact phase, which determine step length, depend only on the movement of the rack and pinion, whereas the body motion during that phase, which determines ground-contact time, depends only on the deflection of the damped spring. With these simplifications, once runner’s inherent spring stiffness Ref. [3] and damping constants have been determined experimentally, effect of track stiffness on ground-contact time and step length can be calculated.