Locomotor kinetics and kinematics on inclines and declines in the gray short-tailed opossum Monodelphis domestica

Small terrestrial animals continually encounter sloped substrates when moving about their habitat; therefore, it is important to understand the mechanics and kinematics of locomotion on non-horizontal substrates as well as on level terrain. To this end, we trained gray short-tailed opossums (Monodelphis domestica) to move along level, 30 inclined, and 30 declined trackways instrumented with a force platform. Vertical, craniocaudal and mediolateral impulses, peak vertical forces, and required coefficient of friction (mu(req)) of individual limbs were calculated. Two high speed video cameras were used to simultaneously capture whole limb craniocaudal and mediolateral angles at limb touchdown, midstance and lift-off. Patterns on the level terrain were typical for non-primate quadrupeds: the forelimbs supported the majority of the body weight, forelimbs were net braking and hindlimbs net propulsive, and both limb pairs exerted small laterally directed impulses. M. domestica moved more slowly on sloped substrates in comparison to level locomotion, and exhibited a greater mu(req). On inclines, both limb pairs were more protracted at touchdown and more retracted at liftoff, fore- and hindlimbs had equal roles in body weight support, forelimbs exerted greater propulsive impulse than hindlimbs, and mu(req) was greater in the forelimbs than in hindlimbs. On declines, only the forelimbs were more protracted at touchdown; forelimbs supported the great majority of body weight while they generated nearly all of the braking impulse and, despite the disparity in fore- vs hindlimb function on the decline, mu(req) was not significantly different between limbs. These differences on the inclined and declined surfaces most likely result from (1) the location of the opossums' center of mass, which is closer to the forelimbs than to the hindlimbs, and (2) the greater functional range of the forelimbs versus the hindlimbs.