WALTER, RM*; CARRIER, DR; Univ. of Utah: Effects of fore-aft body mass distribution on acceleration

A quadruped�s ability to apply propulsive ground reaction forces (GRF) and accelerate rapidly may be limited by 1) muscle power, 2) foot traction, and 3) ability to counteract the nose-up pitching torque due to propulsion. If foot slippage or pitching moments play a major role in limiting maximum acceleration, then fore-aft body mass distribution should strongly influence a quadruped�s ability to accelerate. Despite this, quadrupedal mammals vary greatly in their fore-aft body mass distribution from the front heavy American bison to the hind-weighted jackrabbit. We investigated the effects of fore-aft mass distribution on acceleration by comparing the mechanics of accelerating dogs with and without 10% body mass added in saddlebags just in front of the shoulder girdle or directly over the pelvic girdle. Dogs were videotaped and their GRFs measured during the initial pushoff and first two strides on a sandpaper coated trackway. During initial pushoff, dogs� hindlimbs applied the greatest peak propulsive forces and force impulses with caudal weights, whereas, in step two, these forces were greatest with cranial weights. Unweighted dogs applied greater peak vertical and propulsive forces during step one than during initial pushoff. Propulsive forces during pushoff appeared to be limited by substrate friction. If dogs� leg muscles exerted their maximum power during pushoff dogs could not have applied greater peak propulsive forces in step 1. Caudal weights led to greater hindlimb normal force and decreased the potential for foot slippage. In the second step, where cranial weights increased dogs� propulsive forces, propulsive force production was likely limited by a nose-up pitching torque. Cranial weights increased the propulsive forces dogs could apply while the net GRF remained in line with the center of mass.