Meeting Abstract
103.5 Thursday, Jan. 7 Effects of CoM Position on Forelimb and Hindlimb Mechanics during Incline and Decline Trotting LEE, D.V.; University of Nevada Las Vegas dvlee@mac.com
Quadrupedal running on inclines and declines presents the mechanical challenge of balancing pitch moments about the center of mass (CoM) while maintaining a steady velocity in the direction of travel. The mechanical strategies by which dogs accommodate these demands were explored by loading manipulations of 10% body mass positioned over the CoM, forelimbs, or hindlimbs. Whole body and individual limb ground reaction forces, as well as spatial and temporal step parameters, were measured during downhill and uphill trotting on a 15 degree grade. Ratios of forelimb impulse to total (forelimb + hindlimb) impulse were computed for normal and shear forces. 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. 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 normal impulse distributions. This reflects the braking-propulsive affinities of fore- and hindlimbs during level trotting. In both uphill and downhill trotting, adding mass near the ‘uphill’ limb resulted in fore-hind normal impulse distributions more similar to those of level trotting and more equal fore-hind shear impulse distributions. This result suggests a functional trade-off in quadruped design: a CoM closer to the hindlimbs distributes downhill braking more equally, while a CoM closer to the forelimbs distributes uphill propulsion more equally. Because muscles exert less force when actively shortening than lengthening, it would be advantageous for the muscles of both limbs to share the propulsive burden during uphill trotting – a prediction consistent with the anterior CoM positions of most terrestrial quadrupeds.
