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Fig. 9. Average fore–aft ground reaction forces (F_y) and kinetic energy fluctuations (E_ky) for all dogs walking at 0.8 m s^–1. (A) The limbs generated propulsive and braking forces simultaneously throughout most of the stride. Consequently, the summed limb fore–aft force was smaller than the individual limb forces. Shaded areas indicate the net propulsive and braking impulses, which determine the velocity fluctuations of the center of mass. Limb phase was 15% of stride time, as observed in dogs. (B) Kinetic energy fluctuations were smaller for the center of mass than for the fore and hind quarters because the nearly out-of-phase fluctuations of the fore and hind quarters partly offset each other. Data assume that (1) the fore and hind quarters were, respectively, 63% and 37% of the total body mass (37.8 kg), (2) the fore and hind quarters each had a mean velocity of 0.8 m s^–1 and (3) the velocity fluctuations of the fore and hind quarters were determined completely by their respective fore–aft ground forces. The first two assumptions are reasonable, but the third assumption is likely to be false because forces transmitted via the trunk probably play a role. The fore and hind limbs generate net braking and propulsive forces, respectively, so trunk forces would presumably counteract these net forces. Otherwise, the net propulsive ground reaction force on the hind quarters would cause them to overtake the fore quarters. The trunk is most likely loaded in compression during steady-speed walking because the hind quarters must, on average, push the fore quarters forward, and the fore quarters must, on average, push backwards on the hind quarters over a complete stride. If these trunk interaction forces were accounted for, we would expect the kinetic energy values of the fore and hind quarters to return to their respective initial values at the end of the stride instead of having net changes as shown in B. (C) The dogs' average footfall pattern; LH, left hind limb; LF, left fore limb; RH, right hind limb; RF, right fore limb.

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