In the figure here above, the upper curve (E_kf) represent the kinetic energy changes due to the forward velocity of the center of mass of the body (CG). The continuous line in the middle trace (E_p+E_kv) represents the sum of the kinetic (E_kv) et potential energy (E_p, dotted line) due to the vertical movements of the CG. The lower curve (E_cg) represents  the energy changes of the CG. Ecg is the sum of the other curves (E_cg= E_kf+E_p+E_kv).

In walking, E_kf and E_p+E_kv are in opposition of phase i.e. kinetic energy is transformed into potential energy and vice-versa. Due to this pendular mechanism, the variations of E_cg are reduced and the muscular work done to move the center of mass of the body is decreased.

In running, E_kf and E_p+E_kv are in phase, suggesting a bouncing mechanism. The movements of the center of mass of the body can be compared to those of a spring-mass system. During the aerial phase, E_cg is constant (the air friction is negligible). At the beginning of the contact phase, the center of mass of the body looses speed and height. The energy lost during this negative work phase is stored in the muscles and tendons and is recovered during the phase of positive work production to elevate and accelerate the center of mass of the body.

More information:  Patrick Willems

Main publication of lab on that topic  : WILLEMS P.A., CAVAGNA , G.A. & HEGLUND N.C. (1995) External, internal and total work in human locomotion. J. Exp. Biol., 198: 379-393

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