The importance of body-limb coordination in a walking tetrapod

Sprawled-posture legged-locomotors (like salamanders) generate movement via cyclic self-deformation using appropriate coordination of limb and body motions. We systematically study how such limb-use patterns (gaits) and body undulation affect locomotor performance in a robophysical and a mathematical model. Our servo-driven salamander-like robot (450 g, 40 cm long) moves on a level bed of poppy seeds using its four limbs, each with two degrees of freedom (up/down and fore/aft), and a joint in the middle of the body which controls horizontal bending. Our mathematical model, which has the same morphology as the robot, extends geometric mechanics [e.g., Hatton, 2013] to legged systems and use granular resistive force theory [Zhang & Goldman, 2014] to model the interaction of the limbs with the ground. The robot and model move using symmetric gaits (i.e., gaits with laterally alternating limb movement) [Hildebrand 1965]. Stride lengths of the robot and the model are in good agreement (± 0.05 body lengths/cycle) over a range of symmetric gaits. Different footfall patterns require different body bending coordination to maximize stride length; the optimal coordination generates stride lengths that are twice as large as that of the worst coordination.