Meeting Abstract
We use geometric mechanics (e.g. Hatton et al, PRL, 2013) to understand the coordination patterns between limb movements and back bending during the locomotion the fire salamander (S. salamandra). To characterize the footfall patterns as the salamanders walked on a bed of level sand (0.3 mm diameter glass particles), we used Hildebrand’s gait diagram, which classifies gaits based on two parameters: duty factor and leg phase shift (Science,1965). Our geometric mechanics model assumes that each limb has two degrees of freedom and one degree of freedom for body bending in the lateral plane. To model the interaction of the feet with the ground (assuming little belly drag) we used granular resistive force theory (Zhang & Goldman, 2014) with new drag and intrusion force relations. Natural variation in gait (among 3 individuals tested) enabled comparison of experiment and theory at different points in the Hildebrand space. One animal used a lateral sequence tripod gait (duty factor 75%, leg phase shift 25%) with a phase offset (defined as the phase between the peaks of front right leg angle and body angle) of 89 deg (with ~20% variation) in good accord with theoretical prediction (88 deg). The second animal used a diagonal gait (duty factor 75%, leg phase shift 50%) with a phase offset of 43 deg (theoretical prediction 45 deg). The third animal tested used a lateral sequence diagonal couplet gait (duty factor 75%, leg phase shift 37.5%) displaying a phase offset of 66 deg (theoretical prediction 67 deg). The theory (and a morphologically similar robophysical salamander model) also allows study of gaits and phase shifts that are not observed in the animal, revealing disadvantages of improper phasing (or no body bending at all).
