Fig. 2. Gravitational potential energy (E_p), kinetic energy (E_k) and total mechanical energy (E_com) of the center of mass versus time for a dog walking at four different speeds. E_p and E_k generally fluctuated out of phase so the fluctuations in E_com were smaller than either one. Bars indicate foot–ground contact times. Data are for typical trials for one stride beginning with the left hind limb ground contact for a 30 kg dog. LH, left hind limb; LF, left fore limb; RH, right hind limb; RF, right fore limb.