Kinematics of sea star legged locomotion

Sea stars have slower crawling and faster bouncing gaits. Both speed and oscillation amplitude increase during the transition from crawling to oscillating. In the bouncy gait, vertical velocities precede horizontal velocities by 98&[deg], as reflected by clockwise circular hodographs. Potential energy precedes horizontal energy by 16&[deg] and so are nearly in phase. These phase relationships resemble terrestrial running gaits, except that podia are always on the ground. Kinetic and potential energy scale as mass1.1, with the change in kinetic energy consistently two orders of magnitude less, indicating that efficient exchange is not feasible. Frequency of the bouncy gait scales with mass-0.14, which is similar to continuously running vertebrates and indicates that gravitational forces are important. This scaling differs from the Hill model, in which scaling of muscle forces determine frequency. We propose a simple torque stabilized inverted pendulum (TS-IP) model to conceptuali...