FULL, R.J.*; SEIPEL, J.; HOLMES, P; Univ. of California, Berkeley; Princeton University; Princeton University: Dynamic Stability Model Predicts Constraints In Sprawled Posture Running

A simple, passive dynamic model of sprawled posture animals represented by a single, lateral leg spring has shown remarkable dynamic stability using mechanical feedback (Schmitt et al. 2002). We replaced the single virtual leg spring with six legs to examine the effects of the large lateral and opposing leg forces measured in sprawled posture runners. Each leg was modeled as a linear spring endowed with two inputs, force-free length and �hip� position, representing the effects of neurally-activated muscles. Inputs were determined from measured foot force and kinematic body data from the cockroach, Blaberus discoidalis. We solved 3 degree of freedom Newtonian equations for coupled translation-yawing motions in response to these inputs, and determined branches of periodic gaits over the animal’s typical speed range. The model predicted stable and unstable regions of stride frequencies, stride lengths and leg touchdown positions. The animal�s actual locomotor kinematics fell within the space of stable gaits. Despite the lateral and opposing leg forces, legs generate yaw moments around the center of mass that add synergistically in a common direction to allow rapid stabilization using only feedforward activation while keeping individual joints moments low (< 0.1 N mm). Supported by DARPA/ONR N00014-98-1-0747.