LEE, David V; University of Utah: Mechanics of Downhill and Uphill Running in a Quadruped

Two force platforms were used to measure individual limb ground reaction forces during downhill and uphill trotting on a 15 degree grade. Dogs trotted with no load and with 10% body mass positioned near the center of mass, fore- or hindlimbs. During uphill trotting, the limbs acted more as levers, exerting retracting moments to supply propulsive force. This result is consistent with the reduced force production of actively shortening muscles and the subsequent need to recruit large proximal retractor muscles. The forelimb supplied a greater fraction of braking impulse during downhill trotting and the hindlimb supplied a greater fraction of propulsive impulse during uphill trotting than would be predicted from the fore-hind distribution of normal impulse. These braking-propulsive affinities reflect fore- and hindlimb function during level trotting. The limbs exerted primarily braking force during downhill and propulsive force during uphill trotting. During downhill trotting, hindlimb duty factor decreased and the hindlimb was set down later in order to reduce nose-down pitching due to braking force associated with steady-speed downhill trotting. During uphill trotting, step period and contact times increased with respect to level, allowing more time for force production. Loading of the uphill limb produced fore-hind normal impulse distributions more similar to those of level trotting and shear impulse distributions that were more symmetrical. This result suggests a functional trade-off in quadruped design: a center of mass closer to the hindlimbs would distribute downhill braking force more equally, while a center of mass closer to the forelimbs would distribute uphill propulsive force more equally. The forward center of mass positions of most terrestrial quadrupeds seem to represent a compromise consistent with the force-velocity characteristics of skeletal muscle.