“…In particular, on the mechanical level, the planar spring-mass model for bouncing gaits (Blickhan, 1989;McMahon and Cheng, 1990) has drawn attention since, while advocating a largely reductionist description, it retains key features discriminating legged from wheeled systems: phase switches between flight (swing) and stance phase, a leg orientation, and a repulsive leg behavior in stance. In consequence, not only biomechanical studies investigating hopping (Farley et al, 1991;Seyfarth et al, 2001) or running (He et al, 1991;Farley et al, 1993), but also fast legged robots driven by model-based control algorithms (Raibert, 1986;Saranli and Koditschek, 2003) rely on this plant.…”