Showing papers by "Andrew A. Biewener published in 1988"

Kangaroo rat locomotion: design for elastic energy storage or acceleration?

Abstract: Mechanical stresses (force/cross-sectional area) acting in muscles, tendons and bones of the hindlimbs of kangaroo rats (Dipodomys spectabilis) were calculated during steady-speed hops and vertical jumps. Stresses were determined from both high-speed cine films (light and X-ray) and force plate recordings, as well as from in vivo tendon force recordings. Stresses in each hindlimb support element during hopping (1.6-3.1 m s-1) were generally only 33% of those acting during jumping (greater than or equal to 40 cm height): ankle extensor muscles, 80 +/− 12 (S.D.) versus 297 +/− 42 kPa; ankle extensor tendons, 7.9 +/− 1.5 versus 32.7 +/− 4.8 MPa; tibia, −29 +/− 5 versus −110 +/− 25 MPa (all values are for hopping versus jumping). The magnitude of stress in each structure during these locomotor activities was similarly matched to the strength of each element, so that a consistent safety factor to failure is achieved for the hindlimb as a whole (1.5-2.0). The large stresses during jumping were correlated with a three-fold increase in ground reaction forces exerted on the ground compared with the fastest steady hopping speeds. We conclude that, for its size, the kangaroo rat has disproportionately large hindlimb muscles, tendons and bones to withstand the large forces associated with rapid acceleration to avoid predation, which limits their ability to store and recover elastic strain energy. Middle ear morphology and behavioural observations of kangaroo rats jumping vertically to avoid predation by owls and rattlesnakes support this view.

Preferred speeds in terrestrial vertebrates: are they equivalent?

Abstract: Terrestrial animals have ‘preferred speeds’ within each gait, that are used much more frequently than others for moving along the ground. Energy costs reach minimal values at these speeds within each gait. In this study we asked whether these speeds are mechanically equivalent among different animals (i.e. speeds where the same levels of peak muscle stress occur). If so, this would help in establishing a link between the energetics and the mechanics of the active muscles at these speeds, providing a first step in understanding why energy costs are minimal. We also asked whether peak muscle stress reaches a similar fraction of the maximal isometric stress at these speeds. If so, this would suggest that muscles are structured so that a similar reserve capacity remains, with a similar safety factor for avoidance of injury in response to prolonged repetitive loading. We compared two species that use quite different locomotory methods at their preferred speeds: white rats that gallop and kangaroo rats that hop. We measured peak stress in the ankle extensor muscles of these two species, as they moved at their preferred speeds, using a force platform/cine analysis technique. We also measured the maximum isometric force that this muscle group could develop in situ in the same individuals. We found the ankle extensors of white rats and kangaroo rats developed virtually identical levels of peak stress at their preferred speeds (70 +/− 6 kPa and 69 +/− 6 kPa, respectively, mean +/− S.E.), despite a fourfold difference in peak ground reaction force per unit body mass exerted on each limb. The values of peak isometric stress in situ were also virtually identical (206 +/− 17 kPa and 200 +/− 9 kPa, respectively). Our finding that the peak muscle stress is about one-third of maximum isometric stress at the preferred speeds is consistent with the idea that these are mechanically equivalent speeds, where the same fraction of available muscle fibres is recruited. Finding nearly identical values in two species that move in such different ways (galloping vs hopping), and have such large differences in ground reaction force exerted by each limb, suggests this may be true more generally for terrestrial vertebrates.

Muscle forces during locomotion in kangaroo rats: force platform and tendon buckle measurements compared.

Abstract: The muscle forces and stresses occurring during normal locomotor activity in kangaroo rats are compared with the peak isometric force developed by the same muscles in situ. Two methods were used simultaneously to determine the stresses (force/cross-sectional area) acting in the ankle extensors during steady-speed hopping and during jumps when animals were startled: a direct measurement using a force buckle surgically implanted around a tendon; and an indirect measurement using a force platform/cine analysis technique. We obtained essentially the same values with the two techniques. We found that at slow speeds (0.7 m s-1) the ankle extensor muscles of kangaroo rats exerted 20% of the maximum isometric force developed when the muscles were stimulated via the tibial nerve. This increased to 53% at higher speeds (1.9 m s-1). At the animals's preferred hopping speed (1.5 m s-1), peak force was approximately 40% of maximum isometric force. In jumps when animals were startled, peak forces as high as 175% of the maximal elicited isometric force were recorded. These high forces always occurred when the muscles were being stretched. It appears that kangaroo rats utilize nearly the entire range of muscle force possible during normal locomotor events (i.e. up to 175% of maximum isometric force when muscles are stretched).

Mechanics of locomotion and jumping in the horse (Equus): in vivo stress in the tibia and metatarsus

Abstract: Principal stresses acting in the midshaft cortices of the tibia and metatarsus of the horse were determined from in vivo rosette strain gauge recordings for overground locomotion at different gaits, as well as for jumping and acceleration. Bone stresses were correlated with limb kinematics and ground reaction forces. The results for these two hind limb bones were compared to earlier determinations of locomotor stress in the forelimb radius and metacarpus (Biewener, Thomason & Lanyon, 1983b). Peak stresses generally increased with increasing speed; however, because of greater bending, stresses in the tibia were substantially higher (45%) than in the metatarsus over the range of steady state speeds. Bending of the tibia resulted from significant off-axis loading by the ground reaction force. In contrast, the metatarsus was loaded in compression due to its close alignment with the ground reaction force. Peak stresses as high as - 53 MPa (caudal cortex) in the tibia and -38 MPa (plantar cortex) in the metatarsus acted at a canter. Increased skeletal stress was matched by a corresponding increase in ground reaction force and a decrease in hind limb duty factor. In both bones, peak stresses were significantly greater and differed in their distribution during jumping and acceleration, compared to peak stresses during steady speed locomotion. Maximal values of - 126 MPa (cranial cortex) in the tibia and - 117 MPa (dorsal cortex) in the metatarsus were developed during jumping. These stresses are similar in magnitude to those reported for a range of different sized mammals during strenuous activity and correspond to a safety factor to yield failure of 1.5 to 3. Though generally consistent within an individual bone, the distribution and magnitude of stresses varied about 20% among individuals. This variation was greater for the metatarsus because of its lesser curvature, which diminishes the bone's ability to control for bending in a fixed direction.