The effect of substrate properties on hind foot use during locomotion of the zebra-tailed lizard
Complex Rheology And Biomechanics Laboratory
School of Physics
Host: Boris I. Prilutsky
Time: noon-1pm Friday, October 8
Location: GA Tech Student Center, Room 320
Understanding the mechanisms that organisms use to maintain high locomotor performance over a diversity of terrain is challenging due in part to the difficulty of observing and modeling foot-ground interaction. The zebra-tailed lizard (/Callisaurus draconoides/), a rapid desert runner, has a hind foot with elongate toes and moves effectively on substrates ranging from hard ground to loose granular media. To examine how the hind foot mechanics contribute to locomotor performance, we use high speed video to study in the laboratory the lizard's kinematics during running on hard ground and sand. At similar speeds (1.5 m/s), the center-of-mass (COM) kinematics are similar on both substrates, with the lowest vertical position occurring at mid-stance; stride frequency and duty factor are also similar. Body posture and hind foot kinematics differ between treatments: on hard ground, the body assumes a more horizontal posture, and the hind foot engages the ground in a digitigrade posture with the toe tip as the pivot. During stance, the tendinous foot curls into a c-shape, possibly aiding energy storage and return. On sand, the hind foot penetrates into the substrate in a plantigrade posture, while the body inclines upward, compensating for a lowered pelvis to maintain the vertical COM position. Subsurface x-ray video shows that during stance, the foot rotates downwards about the palm with less foot curvature than on hard ground. On weaker sand the foot penetrates deeper to generate similar net impulse. We hypothesize that the observed changes in foot configuration result from distribution of ground reaction forces over segments of the tendinous foot, and can passively generate effective ground engagement on a diversity of substrates.