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First published online October 5, 2006
Journal of Experimental Biology 209, 4154-4166 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02493

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

Andrew R. Lammers^1,*, Kathleen D. Earls² and Audrone R. Biknevicius²

¹ Department of Health Sciences, 2121 Euclid Ave. HS 108, Cleveland State University, Cleveland, OH 44115, USA
² Department of Biomedical Sciences, Ohio University College of Osteopathic Medicine, Athens, OH 45701, USA

View larger version (5K): [in a new window]   Fig. 1. Measurement of the craniocaudal center of mass. The dead animal was placed on its side, with the limbs arranged in a manner that resembled a standing position. The tail was positioned at about a 45° angle in the sagittal plane relative to the long axis of the body; this is approximately the same tail posture that is adopted during normal movement. l, length of platform between knife points; Ycom, distance between knife point and center of mass; Ws, weight of opossum; Wp, weight of knife point platform. See text for formula.

View larger version (16K): [in a new window]   Fig. 2. (A) Data collection setup, illustrating how forelimb force data were collected (first contact with force platform). In this diagram, the opossum is moving up the incline, and a single forelimb has stepped onto the force plate. (B,C) Digitized landmarks and the calculation of overall limb excursion angles. Protraction angle was measured at touchdown, retraction angle was measured at lift-off, and mediolateral angles were measured at both events.

View larger version (9K): [in a new window]   Fig. 3. Gait plot of limb phase against hindlimb duty factor. Trot and lateral-sequence trot-like (i.e. diagonal couplet) gait boundaries are denoted by broken lines. Following the convention of Hildebrand (Hildebrand, 1976), the axes are reversed. 67% confidence ellipses are drawn around each slope group (decline, incline and level).

View larger version (28K): [in a new window]   Fig. 4. Schematic of sagittal plane parameters for the forelimbs (above) and hindlimbs (below) of M. domestica on level, incline and decline trackways (left to right). Fore- and hindlimb touchdown (solid gray bar) and lift-off (open bar) angles are exaggerated to make differences between limbs and substrates more visible. Vertical impulse is represented by broken arrows, and braking and propulsive impulses by solid arrows; the magnitudes of these impulse vectors are also not shown to scale with each other for illustrative effect (see Table 1 for exact values). B, braking impulse; P, propulsive impulse.

View larger version (27K): [in a new window]   Fig. 5. Representative force traces for each limb (A,C,E, forelimb; B,D,F, hindlimb) and substrate type (A and B, level; C and D, incline; E and F, decline). Craniocaudal (CC) force profiles are shown in gray for clarity (negative, braking; positive propulsive). Mediolateral (ML) reaction forces are primarily medially directed (negative) except for hindlimbs on declined trackways. All axes are shown to the same scale. V, vertical force; BW, body weight.

View larger version (19K): [in a new window]   Fig. 6. Box and whisker plots of (A) vertical, (B) braking, propulsive and (C) mediolateral impulses. Each box represents 50% of the data, and the line within the box represents the median. Each whisker corresponds to 25% of the data. Asterisks represent outliers, and circles denote extreme outliers. Note that the scale of the y axis in each plot is different. BW, body weight; FL, forelimb; HL, hindlimb.

View larger version (19K): [in a new window]   Fig. 7. Relative effort (%) of vertical, braking and propulsive impulses exerted by fore- and hindlimbs. Absolute values of total impulse (forelimb + hindlimb) are indicated to the right. Because the total propulsive impulse on the decline was extremely low, percent limb effort was not calculated. These percentages were calculated for illustrative purposes; because they were calculated from the mean vertical, braking and propulsive impulses for each substrate slope, testing for significant difference among groups was not possible. BW, body weight.

View larger version (15K): [in a new window]   Fig. 8. Frictional conditions in locomotion. (A) Typical plot of the required coefficient of friction (µreq) in M. domestica (1.78 m s–1) on the level trackway. Broken line represents the median value of µreq. (B) Box plots of the median required coefficient of friction for each substrate and limb pair. Asterisk, outlier; circle, extreme outlier.