The gallop differs from most other quadrupedal gaits in that each limb plays a unique role. This study compares the ground forces applied by the four limbs and uses force differences between limbs to address the question of why the gallop is the fastest quadrupedal gait. Individual ground forces were recorded from each of the four limbs as six dogs galloped down a runway at constant speed. Trials were videotaped at high speed using a camera positioned perpendicular to the runway, and velocity was measured using photosensors. The trailing forelimb applied greater peak vertical forces than the lead forelimb, however the vertical impulses from the two forelimbs were similar because the lead forelimb had a longer contact interval. The trailing forelimb and lead hindlimb applied greater peak accelerating forces and accelerating force impulses than their contralateral limbs despite their tendency to have shorter contact intervals. The accelerating impulse of both forelimbs combined did not differ significantly from that of both hindlimbs. The forelimbs applied a greater decelerating impulse than the hindlimbs, such that their net fore-aft impulse was decelerating whereas that of the hindlimbs was accelerating. The greater accelerating impulse applied by the trailing forelimb and greater decelerating impulse applied by the lead forelimb are consistent with the forelimbs acting as elastic struts rather than being actively retracted. In contrast, greater accelerating forces were produced by the lead hindlimb while the center of mass was lifted, suggesting that the hindlimbs are more actively extended or retracted during stance. The differences in ground forces measured between paired limbs suggest that the lead forelimb and trailing hindlimb are limited in their ability to apply forces by their positions in the stride cycle rather than by their muscular capacity. Although a bound or half-bound would allow more limbs to produce their maximal forces, a gallop may generate higher speeds because it is more efficient. Galloping could be more efficient than other gaits involving sagittal bending if the increased number of ground contact intervals decreased either the decelerating forces applied at the onset of ground contact or the vertical motion of the center of mass.