Effects of grade and mass distribution on the mechanics of trotting in dogs.

Quadrupedal running on grades requires balancing of pitch moments about the center of mass (COM) while supplying sufficient impulse to maintain a steady uphill or downhill velocity. Here, trotting mechanics on a 15 deg grade were characterized by the distribution of impulse between the limbs and the angle of resultant impulse at each limb. Anterior-posterior manipulation of COM position has previously been shown to influence limb mechanics during level trotting of dogs; hence, the combined effects of grade and COM manipulations were explored by adding 10% body mass at the COM, shoulder or pelvis. Whole body and individual limb ground reaction forces, as well as spatiotemporal step parameters, were measured during downhill and uphill trotting. Deviations from steady-speed locomotion were determined by the net impulse angle and accounted for in the statistical model. The limbs exerted only propulsive force during uphill trotting and, with the exception of slight hindlimb propulsion in late stance, only braking force during downhill trotting. Ratios of forelimb impulse to total impulse were computed for normal and shear components. Normal impulse ratios were more different from level values during uphill than downhill trotting, indicating that the limbs act more as levers on the incline. Differential limb function was evident in the extreme divergence of forelimb and hindlimb impulse angles, amplifying forelimb braking and hindlimb propulsive biases observed during level trotting. In both downhill and uphill trotting, added mass at the up-slope limb resulted in fore-hind distributions of normal impulse more similar to those of level trotting and more equal fore-hind distributions of shear impulse. The latter result suggests a functional trade-off in quadruped design: a COM closer to the hindlimbs would distribute downhill braking more equally, whereas a COM closer to the forelimbs would distribute uphill propulsion more equally. Because muscles exert less force when actively shortening than when lengthening, it would be advantageous for the forelimb and hindlimb muscles to share the propulsive burden more equally during uphill trotting. This functional advantage is consistent with the anterior COM position of most terrestrial quadrupeds.