Meeting Abstract

13.4  Thursday, Jan. 3  Mechanics of limb bone loading during terrestrial locomotion in river cooter turtles Pseudemys concinna: validation of stress analyses by in vivo strain recordings BUTCHER, M.T.*; ESPINOZA, N.R.; BLOB, R.W.; Clemson Univ.; Erskine College mbutche@clemson.edu

Studies of limb bone loading during terrestrial locomotion have focused mainly on birds and mammals, but data from a broader functional and phylogenetic range of species are critical to understand the evolution of limb bone design. Turtles are an interesting lineage in this context: although their slow walking speeds and robust limb bones might lead to low locomotor forces and limb bone stresses, their highly sprawled posture could produce high bending, leading to limb bone stresses similar to those of birds and mammals, as well as high torsion. To test between these possibilities, we evaluated stresses experienced by the femur of river cooters (Pseudemys concinna) during terrestrial walking by synchronizing measurements of 3D kinematics and ground reaction forces (GRF) of a single hindlimb. Mean net GRF at peak stress was 0.35 BW and directed vertically about midstance, nearly orthogonal to the femur. Peak bending stresses were low (tensile: 24.9�9.0 MPa; compressive: -31.1�9.1 MPa) and comparable to those in other reptiles, but the femur experienced higher shear stresses than those in other reptiles. In vivo femoral strain recordings verify bending and torsional loading, with average peak shear strains approaching 2000 με. These loading patterns may relate to several factors including the short length of the femur and highly sprawled limb posture of turtles. Femoral safety factors of 13.9 in bending and 6.3 in torsion suggest that not only do turtle limb bones seem overbuilt in terms of resisting bone loading, but further suggest that across tetrapod lineages, elevated torsion and limb bone safety factors may be primitive features of limb bone design. Supported by NSF I0B-0517340.