Fig. 2. Examples of displacements, powers, joint angles and vertical ground reaction force vs time for hopping at 2.2 Hz on the 81, 15 and 11 kN m^-1 surfaces. All traces begin at landing and end at takeoff. Stick figures show body posture and surface position at instants of landing, peak force and takeoff on each surface. On the stiffest surface (81 kN m^-1), the legs first compressed and then extended, leading to in-phase kinematics and power of the leg, center of mass (COM) and surface. Also, the joints flexed and then extended. On the 15 kN m^-1 surface, the legs barely changed length and the center of mass followed the surface motion and power. On the softest surface (11 kN m^-1), the legs first extended and then compressed, resulting in out-of-phase leg and surface power. The transient changes in leg length on the 15 and 11 kN m^-1 surfaces are likely due to the fact that small changes in muscle force result in large changes in surface compression and thus leg length, on the softest surfaces.