Biomedical subjects

Rebecca M Walter

Publications and source records attributed to Rebecca M Walter.

4 recordsLinked to original sources

PubMed2007

Ground forces applied by galloping dogs.

The gallop differs from most other quadrupedal gaits in that each limb plays a unique role. This study compares the ground forces applied by the four limbs and uses force differences between limbs to address the question of why the gallop is the fastest quadrupedal gait. Individual ground forces were recorded from each of the four limbs as six dogs galloped down a runway at constant speed. Trials were videotaped at high speed using a camera positioned perpendicular to the runway, and velocity was measured using photosensors. The trailing forelimb applied greater peak vertical forces than the lead forelimb, however the vertical impulses from the two forelimbs were similar because the lead forelimb had a longer contact interval. The trailing forelimb and lead hindlimb applied greater peak accelerating forces and accelerating force impulses than their contralateral limbs despite their tendency to have shorter contact intervals. The accelerating impulse of both forelimbs combined did not differ significantly from that of both hindlimbs. The forelimbs applied a greater decelerating impulse than the hindlimbs, such that their net fore-aft impulse was decelerating whereas that of the hindlimbs was accelerating. The greater accelerating impulse applied by the trailing forelimb and greater decelerating impulse applied by the lead forelimb are consistent with the forelimbs acting as elastic struts rather than being actively retracted. In contrast, greater accelerating forces were produced by the lead hindlimb while the center of mass was lifted, suggesting that the hindlimbs are more actively extended or retracted during stance. The differences in ground forces measured between paired limbs suggest that the lead forelimb and trailing hindlimb are limited in their ability to apply forces by their positions in the stride cycle rather than by their muscular capacity. Although a bound or half-bound would allow more limbs to produce their maximal forces, a gallop may generate higher speeds because it is more efficient. Galloping could be more efficient than other gaits involving sagittal bending if the increased number of ground contact intervals decreased either the decelerating forces applied at the onset of ground contact or the vertical motion of the center of mass.

Animals↗

PubMed2004

Effects of mass distribution on the mechanics of level trotting in dogs.

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. Steady speed locomotion requires that braking and propulsion of the stance limbs be equal in magnitude. We predicted that changes in body mass distribution would alter individual limb braking-propulsive force patterns and we tested this hypothesis 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. Vertical and fore-aft impulses were calculated by integrating individual force-time curves and Fourier analysis was used to quantify the braking-propulsive (b-p) bias of the fore-aft force curve. We predicted that experimental manipulation of antero-posterior mass distribution would (1) change the fore-hind distribution of vertical impulse when the limb girdles are loaded, (2) decrease the b-p bias of the experimentally loaded limb and (3) increase relative contact time of the experimentally loaded limb, while (4) the individual limb mean fore-aft forces (normalized to body weight + added weight) would be unaffected. All four of these results were observed when mass was added at the pelvic girdle, but only 1, 3 and 4 were observed when mass was added at the pectoral girdle. We propose that the observed relationship between antero-posterior mass distribution and individual limb function may be broadly applicable to quadrupeds with different body types. In addition to the predicted results, our data show that the mechanical effects of adding mass to the trunk are much more complex than would be predicted from mass distribution alone. Effects of trunk moments due to loading were evident when mass was added at the center of mass or at the pelvic girdle. These results suggest a functional link between appendicular and axial mechanics via action of the limbs as levers.

Analysis of Variance↗

PubMed2003

Kinematics of 90 degrees running turns in wild mice.

Turning is a requirement for locomotion on the variable terrain that most terrestrial animals inhabit and is a deciding factor in many predator-prey interactions. Despite this, the kinematics and mechanics of quadrupedal turns are not well understood. To gain insight to the turning kinematics of small quadrupedal mammals, six adult wild mice were videotaped at 250 Hz from below as they performed 90 degrees running turns. Four markers placed along the sagittal axis were digitized to allow observation of lateral bending and body rotation throughout the turn. Ground contact periods of the fore- and hindlimbs were also noted for each frame. During turning, mice increased their ground contact time, but did not change their stride frequency relative to straight running at maximum speed. Postcranial body rotation preceded deflection in heading, and did not occur in one continuous motion, but rather in bouts of 15-53 degrees. These bouts were synchronized with the stride cycle, such that the majority of rotation occurred during the second half of forelimb support and the first half of hindlimb support. In this phase of the stride cycle, the trunk was sagittally flexed and rotational inertia was 65% of that during maximal extension. By synchronizing body rotation with this portion of the stride cycle, mice can achieve a given angular acceleration with much lower applied torque. Compared with humans running along curved trajectories, mice maintained relatively higher speeds at proportionately smaller radii. A possible explanation for this difference lies in the more crouched limb posture of mice, which increases the mechanical advantage for horizontal ground force production. The occurrence of body rotation prior to deflection in heading may facilitate acceleration in the new direction by making use of the relatively greater force production inherent in the parasagittal limb posture of mice.

Animals↗

PubMed2002

Scaling of rotational inertia in murine rodents and two species of lizard.

Because the force required to rotate a body about an axis is directly proportional to its rotational inertia about the axis, it is likely that animals with high rotational inertia would be constrained in their turning abilities. Given that rotational inertia scales with mass(1.67) in geometrically similar animals, whereas the ability to apply torque scales with mass(1.00), larger animals would be expected to have more difficulty turning than smaller animals of similar shape. To determine how rotational inertia scales with body mass, we used the fact that the period of a physical pendulum is proportional to its rotational inertia(0.50), and measured rotational inertia in two groups of vertebrates with greatly different body shapes: murine rodents (Mus domesticus and Rattus norvegicus) and lizards (Iguana iguana and Varanus exanthematicus). Rotational inertia did not deviate significantly from isometric scaling in the murine rodents as a group or in the varanid lizards, scaling with mass(1.63) and mass(1.59), respectively. Although rotational inertia did scale with negative allometry in iguanas and rats alone, with mass(1.56) and mass(1.42), respectively, it still increased much more quickly with increasing mass than the predicted ability to apply torque. This suggests either that these animals are not constrained by rotational inertia because of their relatively small size or that larger rodents and lizards are poorer turners than smaller ones. The murine rodents had a 3.0- to 4.9-fold lower rotational inertia than similarly sized lizards of either species. Given that the basal synapsids had body proportions and limb configurations similar to those of modern lizards, we suggest that the loss of the large muscular tail and elongated body form during the evolution of cynodonts and mammals reduced rotational inertia and probably improved turning ability.

Animals↗