In vivo strains in the femur of river cooter turtles (Pseudemys concinna) during terrestrial locomotion: tests of force-platform models of loading mechanics

Abstract

Previous analyses of ground reaction force (GRF) and kinematic data from river cooter turtles (Pseudemys concinna) during terrestrial walking led to three primary conclusions about the mechanics of limb bone loading in this lineage: (1) the femur was loaded in a combination of axial compression, bending and torsion, similar to previously studied non-avian reptiles, (2) femoral shear stresses were high despite the possession of a reduced tail in turtles that does not drag on the ground and (3) stress-based calculations of femoral safety factors indicated high values in bending and torsion, similar to other reptiles and suggesting that substantial 'overbuilding' of limb bones could be an ancestral feature of tetrapods. Because force-platform analyses produce indirect estimates of bone loading, we sought to validate these conclusions by surgically implanting strain gauges on turtle femora to directly measure in vivo strains during terrestrial walking. Strain analyses verified axial compression and bending as well as high torsion in turtle femora, with peak axial strains comparable to those of other non-avian reptiles at similar walking speeds but higher peak shear strains approaching 2000 microepsilon. Planar strain analyses showed patterns of neutral axis (NA) of femoral bending orientations and shifting generally consistent with our previous force-platform analyses of bone stresses, tending to place the anterior and dorsal aspects of the femur in tension and verifying an unexpected pattern from our force studies that differs from patterns in other non-avian reptiles. Calculated femoral safety factors were 3.8 in torsion and ranged from 4.4 to 6.9 in bending. Although these safety factors in bending were lower than values derived from our stress-based calculations, they are similar to strain-based safety factors calculated for other non-avian reptiles in terrestrial locomotion and are still high compared with safety factors calculated for limb bones of birds and mammals. These findings are consistent with conclusions drawn from our previous models of limb bone stresses in turtles and suggest that not only are turtle limb bones 'overbuilt' in terms of resisting the loads that they experience during locomotion but also, across tetrapod lineages, elevated torsion and high limb bone safety factors may be primitive features of limb bone design.