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First published online March 28, 2008
Journal of Experimental Biology 211, 1187-1202 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.012989

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Mechanics of limb bone loading during terrestrial locomotion in river cooter turtles (Pseudemys concinna)

Michael T. Butcher and Richard W. Blob^*

Department of Biological Sciences, 132 Long Hall, Clemson University, Clemson, SC 29634, USA

View larger version (4K): [in this window] [in a new window]   Fig. 1. (A) Outline sketch (right lateral view) of the hindlimb skeleton of Pseudemys concinna illustrating the lines of action of the major muscle groups contributing to stresses in the femur during the stance phase of terrestrial locomotion. Pelvic girdle bones and tail vertebrae are colored black and femur is shaded grey. Some proximal hip muscles that do not span the femoral midshaft and do not contribute directly to femoral stress (e.g. puboischiofemoralis externus, ischiotrochantericus) have been omitted for clarity. Rotational forces exerted by caudi-iliofemoralis (dashed arrow) were not calculated (see text). (B) Outline sketch of the right femur and tibia (same as in A) of P. concinna illustrating the planes defining the anatomical frame of reference for force platform analyses. Both surfaces of the plane are labeled, with solid arrows and filled circles indicating surfaces in view and dashed arrows and open circles indicating surfaces hidden from view (i.e. surfaces that can only be seen if the planes are transparent). A, anterior; P, posterior; D, dorsal; V, ventral; L, lateral; M, medial.

View larger version (4K): [in this window] [in a new window]   Fig. 2. Representative kinematic profiles of right hindlimb joints for river cooter turtles (P. concinna) during a walking step over a force platform. Top to bottom: femoral (hip) protraction (Pro.)/retraction (Ret.) angle, femoral (hip) abduction (Ab.)/adduction (Add.) angle, knee angle and ankle angle (Ext., extension; Flex., flexion). Kinematic profiles represent mean (±s.e.m.) angles averaged across all four turtles (N=18–21 trials per individual, 78 total steps per data point). Note that axis scales differ for these plots to provide increased resolution for smaller angles.

View larger version (7K): [in this window] [in a new window]   Fig. 3. Mean ground reaction force (GRF) dynamics of the right hindlimb from an individual cooter. All plots show means (±s.e.m.) over N=21 trials. (A) Vertical, anteroposterior (AP) and mediolateral (ML) GRF components in body weight (BW), with positive values indicating upward, anterior and lateral forces, respectively (top to bottom). Axis scales differ for these plots to provide increased resolution for the small AP and ML forces. All trials were normalized to the same duration, allowing values to be graphed against the fraction of time through the contact interval. (B) Limb segment positions at the mean time of peak net GRF (41% contact) during a representative step by P. concinna, with the direction and magnitude of the GRF vector illustrated. The femur is highlighted by bolder lines; note that it is foreshortened in lateral view. H, hip; K, knee; A, ankle. (C) AP and ML orientations of the net GRF vector. AP angles were determined relative to vertical at 0° (90° indicates GRF horizontal, pointing forwards; <0° indicates posteriorly directed GRF). ML angles were determined relative to vertical at 0° (positive values indicate laterally directed GRF; negative values indicate medially directed GRF).

View larger version (4K): [in this window] [in a new window]   Fig. 4. Moments exerted by the GRF about the hindlimb joints and the long axis of the femur from an individual cooter. All plots show means (±s.e.m.) over N=20 trials. Note that axis scales differ for these plots to provide increased resolution for smaller moments. Directions of moments are labeled to the right of the figure plots. Hip AP, the GRF moment about the hip in the anatomical anterior and posterior directions; Hip DV, the GRF moment about the hip in the anatomical dorsal and ventral directions; Right prox. clock., torsional GRF moment, clockwise when viewing the right femur from the proximal end; Right prox. counter., torsional GRF moment, counterclockwise when viewing the right femur from its proximal end.

View larger version (4K): [in this window] [in a new window]   Fig. 5. Components of bending stress in the femur induced by muscles and GRF components from an individual cooter. All data are mean (±s.e.m.) stresses over N=21 trials. Stresses plotted are those occurring on the dorsal surface for forces acting to cause dorsoventral (DV) bending, and those occurring on the anterior surface for forces acting to cause anteroposterior (AP) bending. Tensile stress is positive and compressive stress is negative. `Muscles' indicates stresses induced by major muscle groups in the direction indicated; `external' indicates stresses induced by the GRF acting in the direction indicated; `axial' indicates stresses induced by the axial component of the GRF due to bone curvature in the direction indicated. Bending stresses induced by axial forces are very small and overlap along the zero line for the DV and AP directions. Note that the retractor moment generated by the GRF for most of stance (Fig. 4) precludes calculation of muscular contributions to AP bending stress in the femur, but that resulting stress underestimation is minimized to the extent possible (see Appendix).

View larger version (9K): [in this window] [in a new window]   Fig. 6. (A) Maximum tensile (t, open circles) and compressive (c, filled circles) stresses acting in the right femur and neutral axis angle from the anatomical AP axis of the femur from an individual cooter. Plots show means (±s.e.m.) over N=20 trials. Frame stills show limb position at the time of maximum tensile stress (left image) and at the time of peak net GRF magnitude (right image). Solid vertical lines mark the relative timing of these loading events. (B) Schematic cross-sections of a right femur illustrating neutral axis orientations for bending (red line and values) at peak tensile stress (upper) and peak net GRF (lower). Neutral axis is illustrated offset from the centroid (dark circle) due to axial compression superimposed on bending loads. Mean rotation of the neutral axis >45° over the course of a walking step indicates that the `posterior' cortex of the femur experiences compression (shaded) and the `anterior' cortex experiences tension (unshaded), placing the plane of bending nearly parallel with the anatomical dorsoventral (DV) axis of the bone. The curved black arrow indicates the inward rotation of the femur during a step, which shifts the anatomical plane of bending to align more closely with the anatomical DV axis.