![]() ![]() Among terrestrial tetrapods, the behavior generally thought to exert the greatest influence on the loading environment of limb bones is locomotion, because it usually imposes the largest and most frequent loads on the skeleton( Biewener, 1990 Biewener, 1993). A major factor commonly thought to have contributed to the diversity of limb bone form and function that has evolved across these species is variation in the mechanical loads that their bones experience( Currey, 1984 Currey, 2002 Blob, 2001 Lieberman et al., 2004). Tetrapod limb bones show a diverse range of shapes and designs, from the elongate bones of fast runners to the short, robust limbs of many fossorial species. Thus, not only do turtle limb bones seem considerably`over-designed' for resisting the loads that they encounter, but comparisons of bone loading across tetrapod lineages are consistent with the hypothesis that low limb bone loads, elevated torsion and high safety factors may be primitive features of limb bone design. Comparison of femoral stresses to measurements of limb bone mechanical properties in cooters indicates safety factors to yield of 13.9 in bending and 6.3 in torsion, considerably higher than values typical for birds and mammals, and closer to the elevated values calculated for other reptile species. Such high torsion is present despite cooters lacking a large tail, a feature that has been hypothesized to contribute to torsion in other reptiles in which the tail is dragged along the ground. Peak bending stresses experienced by the femur were low (tensile: 24.9☙.0 MPa compressive: –31.1☙.1 MPa)and comparable to those in other reptiles, yet peak shear stresses were higher than those in other reptiles, averaging 13.7±4.2 MPa. The net GRF magnitude at peak tensile bone stress averaged 0.35 BW (body weight) and was directed nearly vertically for the middle 40–65% of the contact interval, essentially orthogonal to the femur. Further, we evaluated femoral safety factors for this species by comparing our locomotor stress calculations with the results of mechanical property tests. To test between these possibilities, we evaluated stresses experienced by the femur of river cooter turtles( Pseudemys concinna) during terrestrial walking by synchronizing measurements of three-dimensional joint kinematics and ground reaction forces(GRFs) during isolated hindlimb footfalls. Although their slow walking speeds and robust limb bones might lead to low locomotor forces and limb bone stresses similar to other non-avian reptiles, their highly sprawled posture could produce high bending loads,leading to high limb bone stresses similar to those of avian and mammalian species, as well as high torsion. Turtles are an interesting lineage in this context. However, data from a broader functional and phylogenetic range of species are critical for understanding the evolution of limb bone function and design. Studies of limb bone loading during terrestrial locomotion have focused primarily on birds and mammals. ![]()
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