A simple model of bipedal walking predicts the preferred speed-step length relationship

We used a simple model of passive dynamic walking, with the addition of active powering humans. We tested several hypothetical metabolic costs, with one component proportional swing leg. For this second component, a cost based on the amount of force needed to oscillate the leg divided by the rime duration of that force predicts the preferred speed-step length relationship much better than other costs, such as the amount of mechanical work done in swinging the leg. The cost of force/time models the need to recruit fast muscle fibers for large forces at short durations. The actual mechanical work performed by muscles on the swing leg appears to be of relatively less importance, although it appears to be minimized by the use of short bursts of muscle activity in near-isometric conditions. The combined minimization of toe-off mechanical work and force divided by rime predicts the preferred speed-step length relationship.