Fig. 1. (A) Spring-mass model (or `spring-loaded inverted pendulum', SLIP) for the dynamics of legged terrestrial locomotion. The body is represented by a point mass m, located at the body center of mass (CoM; black circle), and the leg by a linear compression spring with leg stiffness k<sub>leg</sub> and contact angle <sub>o</sub>. (B) Despite its simplicity, the spring-mass model accurately describes the fluctuations in mechanical energy of the body during running (PE<sub>g</sub>, gravitational potential energy; KE<sub>v</sub> and KE<sub>h</sub>, vertical and horizontal kinetic energy, respectively; E<sub>com</sub>, center of mass energy) (Daley and Biewener, 2006; Daley et al., 2006). (C) Furthermore, all terrestrial animals appear to exhibit spring-mass dynamics, whether they run on two, four, six or eight legs. Multiple legs act in concert to produce the effective `leg-spring' dynamics (Holmes et al., 2006). (D) Similarly, a lateral spring-mass model describes well the medio-lateral dynamics of cockroach locomotion, in which three legs operate as a single effective `leg-spring' in the medio-lateral plane (Full et al., 2002).