Passive quadrupedal bounding with a segmented flexible torso

This paper examines the effect of torso flexibility on the dynamics of quadrupedal running with a bounding gait. A reduced-order passive and conservative model with a segmented flexible torso and compliant legs is introduced to study torso-leg coordination. Numerical return map studies reveal that a large variety of cyclic bounding motions can be realized passively, as a natural mode of the system. Despite the simplicity of the model, the resulting motions correspond to torso bending movements that resemble those in galloping mammals without explicit reliance on the fine structural and morphological details. This way, the proposed model offers a unifying description of the task-level locomotion behavior, and can be used to inform feedback control synthesis by serving as a behavioral target for the control system.