Compensatory Strategies of a Sprawled Bipedal Runner Over a Sudden Drop

Abstract

Natural terrain constantly challenges locomotor stability. Bipedal parasagittal runners rely on proximo-distal control mechanisms and passive mechanical mechanisms to rapidly adjust to changing environments. However, it is not known how sprawled bipedal runners, like the basilisk lizard, adjust to unexpected perturbations. This study examines how basilisks navigate visible drop perturbations to elucidate the control strategies used to maintain stability. We ran four basilisk lizards along a 2.7 m long trackway with an embedded 6-d.o.f. force plate. Control trials were recorded with the force plate mounted flush to the track surface. We lowered the plate to 40% of the lizards’ limb length, relative to the track surface, for perturbation trials. We hypothesized that much like parasagittal runners, basilisks would rely on three distinct compensatory mechanisms to convert the potential energy (PE) change from the drop into fore-aft and vertical kinetic energy (KE) or to increase the total energy of the system (Ecom), as well as a fourth potential mechanism converting PE into medio-lateral KE, as a result of their sprawled limb posture. On average, lizards ran slower (T-ratio30=2.548, p = 0.0162) and with a more vertical limb posture (T-ratio28=-6.119, p < 0.0001) during the drop perturbation. As expected, vertical KE increased in drop surface trials. However, contrary to our hypothesis, the drop perturbation appeared to have little detectable effect on fore-aft and medio-lateral KE. Preliminarily, these results suggest that the sprawled limb posture may afford increased robustness to perturbations such as a sudden drop in surface height, facilitating kinematic compensations independent of significant kinetic changes.