Meeting Abstract

65.3  Thursday, Jan. 6  Importance of wrist rotation for high performance terrestrial locomotion of a sea turtle inspired physical model MAZOUCHOVA, N.*; UMBANHOWER, P.; GOLDMAN, D.I.; Georgia Institute of Technology; Northwestern University; Georgia Institute of Technology nmazouch@gatech.edu

To reach the ocean after hatching, juvenile sea turtles use their flippers to traverse a beach environment typically composed of granular media (sand), which can exhibit both solid and fluid-like response to stress. Previously we found that during rapid (3 BL/sec) locomotion on loosely packed sand, the fore-limbs did not slip during ground contact, resulting in speeds on sand comparable to that on hard ground. We hypothesized that judicious control of limb penetration and wrist bending allows the hatchlings to maintain propulsive forces below the sand’s yield force during stance phase. To explore this hypothesis, we use a turtle-inspired physical model to test the influence of wrist bending on locomotor performance on granular media. The device propels itself with either a symmetric or diagonal gait on sand using two servo-motor driven limbs consisting of flat-plate flippers and passively flexible or rigid wrists. We use high speed video to record kinematics in a trackway filled with a granular medium (poppy seeds). With flexible wrists and a symmetric gait there was no visible limb slip, and average forward speed increased with limb frequency (t-test, p<0.0001) such that the slope was equal to the linear retraction distance β (t-test, p<0.0001). With rigid wrists, β was significantly lower (t-test, p>0.05) and observations revealed that the flipper slipped relative to the substrate during stance and body lift was smaller. A flexible wrist prevents slipping and enables the model to advance geometrically making use of granular induced constraint forces (yield force) instead of through slipping-induced granular drag forces.