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Whole-body mechanics and gaits in the gray short-tailed opossum Monodelphis domestica: integrating patterns of locomotion in a semi-erect mammal

Andrew J. Parchman¹, Stephen M. Reilly^1,* and Audrone R. Biknevicius²

¹ Department of Biological Sciences, College of Arts and Sciences, Ohio University, Athens OH 45701, USA
² Department of Biomedical Sciences, College of Osteopathic Medicine, Ohio University, Athens OH 45701, USA

View larger version (22K): [in a new window]   Fig. 1. Hildebrand gait plots for the 90 steps analyzed from seven Monodelphis domestica. Duty factor is the % of stride that the reference hind foot is on the ground; limb phase is the % of stride that the fore-foot follows the hind-foot on the same side. Gaits with duty factor 50% are classified as walking gaits (open squares), while those with duty factor <50% are classified as running gaits (open circles, non-aerial runs; filled circles, aerial runs). The limb-phase variable describes the general and specific footfall patterns associated with symmetrical gaits. Note that Monodelphis used only trotting gaits. The black triangle marks the velocity vector superimposed onto the Hildebrand plot from the three-dimensional plot of velocity, limb phase and duty factor. For comparison, the black outline encompasses 1178 symmetrical gait plots from 156 genera of tetrapods observed by Hildebrand (1985).

View larger version (28K): [in a new window]   Fig. 2. Spring mechanics (vertical, craniocaudal and lateral) in Monodelphis domestica typical of all steps analyzed over a fivefold increase in speed (0.3-1.8 m s-1). (A) Ground reaction forces, velocities of the center of mass, and vertical displacement of the center of mass. BW is body weight (0.833 N). Negative values in the craniocaudal record reflect braking effort whereas positive values are propulsive. The broken line indicates mean velocity. (B) Three-dimensional kinetic energies (KE), gravitational potential energy (PE), and total mechanical energy (ME) of the center of mass. Data are for a single diagonal couplet step from a 85 g individual moving at 1.046 m s-1 with a duty factor of 43.75, limb phase of 52.08, % recovery of 1.36%, and total KE—PE phase shift of 8.18°. The diagonal couplet step began when the first limb of the couplet touched the ground and ended when the last limb of the couplet was lifted off.

View larger version (15K): [in a new window]   Fig. 3. (A) Phase shift between the fluctuations in total kinetic energy and gravitational potential energy versus velocity. Phase shifts around 0° indicate that the two energies are in phase with one another and thus all steps analyzed exhibit spring mechanics. (B) Percentage recovery (measuring the efficiency of pendular mechanics) versus velocity. Low values indicate little to no pendular exchange of mechanical energy characteristic of spring mechanics. Note that neither variable changes significantly with speed, indicating that the opossums consistently use spring mechanics over the full range of speeds that they will perform sustained locomotion. Kinematic gaits are indicated by symbols: walking trots, open squares; non-aerial running trots, open circles; aerial running trots, filled circles.