Meeting Abstract

93.6  Monday, Jan. 6 14:45  Ground reaction forces during rapid escape turns in lizards. LIBBY, T.*; HWANG, M.; KOH, M.; XIE, B.; FULL, R.J.; Univ. of California, Berkeley tlibby@berkeley.edu

Body bending and appendage swinging can enhance maneuverability by exchanging angular momentum between body segments, allowing an animal to change direction even in the absence of external impulses. Multi-segment, rigid-body dynamics models of the lizard, Agama agama, revealed that inertial forces from C-start-like body curling and tail swinging significantly increase rotation in the first stride of an escape turn. Here, we combine our earlier momentum model of turning with measurements of whole body and single leg ground reaction forces measured using a force platform. Turns from 90° to 180° were executed in under 300 ms. Whole body forces showed that lizards did not simply rotate in place, but developed significant net center of mass (COM) linear acceleration in the escape direction during the first step. Some faster turns exhibited an aerial phase, where animals jumped and spun about the vertical axis. Peak single leg forces ranged from 5-10 times body weight. Ground reaction forces were oriented such that all limbs contributed to both angular and linear impulse during the first step, with the outer front limb being the dominant source of impulse. Foot forces in the first step appear to be a compromise between turning and accelerating the animal. No limb generated a net negative angular impulse to turning, although hind limbs often resisted net COM acceleration. Results are inspiring the design of a horizontal back bending robot with rapid turn capabilities.