| The antero-posterior mass distribution of quadrupeds varies substantially amongst species, yet the functional implications of this design characteristic remain poorly understood. During trotting, the forelimb exerts a net braking force while the hindlimb exerts a net propulsive force. It was predicted that changes in body mass distribution would alter individual limb braking-propulsive force patterns and this hypothesis was tested by adding 10% body mass near the center of mass, at the pectoral girdle, or at the pelvic girdle of trotting dogs. Two force platforms in series recorded fore- and hindlimb ground reaction forces independently. During level trotting, the fore-hind distribution of vertical impulse changed in accordance with limb girdle loading and the propulsive bias of the hindlimb decreased when mass was added at the pelvic girdle. The observed relationship between antero-posterior mass distribution and individual limb function may be broadly applicable to quadrupeds with different body types. These data also suggest a functional link between appendicular and axial mechanics via action of the limbs as levers.; During trotting on a 15° grade, 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. Normal impulse ratios were closer to level values during downhill than uphill trotting, indicating that the limbs acted more like levers during uphill than downhill trotting.; Limb design is well conserved among quadrupeds, notably, the knees point forward and the elbows point back. A modeling study was undertaken to examine the effects of joint orientation on individual leg forces and center of mass (CoM) dynamics. Steady-speed trotting simulations revealed minimal pitch oscillations in a model with natural knee and elbow orientation, but substantially greater and more irregular pitch oscillations in a model with the opposite orientation. A passive mechanism for reducing pitch oscillations was an emergent property of directionally compliant legs with the knee-elbow orientation of living quadrupeds. Such intrinsic stability arising from mechanical design can simplify control tasks and lead to more robust running machines. |
