Fig. 1. In an inverted pendulum (IP) gait, the potential (PE; red line) and kinetic energy (KE; blue line) fluctuate out-of-phase, and energy exchange between PE and KE is possible. If the amplitudes of the PE (F_PE) and KE fluctuations (F_KE) are similar, there will be an optimal energy recovery via the IP mechanism. In a spring-mass gait, PE and KE fluctuate in-phase and there is no energy transfer between PE and KE. Instead, energy can be stored in elastic tissues during initial stance and released at push-off. This mechanism is active during human running and effectuates an energy recovery of ca. 35%.