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First published online July 14, 2008
Journal of Experimental Biology 211, 2397-2407 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018986
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In vivo strains in the femur of river cooter turtles (Pseudemys concinna) during terrestrial locomotion: tests of force-platform models of loading mechanics

Michael T. Butcher1, Nora R. Espinoza1,2, Stephanie R. Cirilo2 and Richard W. Blob1,*

1 Department of Biological Sciences, Clemson University, Clemson, SC 29634, USA
2 Department of Biology, Erskine College, Due West, SC 29639, USA


Figure 1
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  		Fig. 1. Representative strain recordings (simultaneous) from three gauge locations on the cooter femur during three consecutive walking steps. Left: principal strains, angle of principal tensile strains from the femoral long axis ({phi}t) and shear strains from ROS gauge recordings. Right: longitudinal strains from `dorsal', `anterior' and `ventral' sites. Note that strain scales differ among panels to facilitate presentation. Dark gray shading highlights the stance phase (contact) for a single step at all gauge locations. Light gray shading highlights the swing phase of a stride. {epsilon}t and {epsilon}c denote tensile and compressive (red line) principal strain traces, respectively.

 

Figure 2
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  		Fig. 2. (A) Shifts in the orientation of the neutral axis (NA) of femoral bending at five time increments (% of contact) through the step for four individual cooters. Each data point represents the angle of the NA to the anatomical anteroposterior (AP) axis of the femur averaged over N=3–6 steps. (B) Schematic femur cross section illustrating NA orientation and shift. Strain gauge locations are indicated by the black bars around the cortex of the femoral cross-section. Solid red line is an NA with an orientation of 60°. Directions of bending are indicated with respect to the anatomical axes of the bone as described in the text, not in an absolute frame of reference. AP, bending about an NA running from the anatomical dorsal to ventral cortex; DV, bending about an NA running from the anatomical anterior to posterior cortex.

 

Figure 3
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  		Fig. 3. Graphical comparisons of cross-sectional planar analyses of femoral strain distributions calculated for five time increments (% of contact) during representative walking for (A) individual pc05, (B) individual pc03 and (C) individual pc07. Time increments (% of contact) correspond to those plotted in Fig. 2. The centroid of each section is indicated by the black dot. Thin lines indicate contours of strain magnitude (all spaced at 100 µ{epsilon}). Peak strains calculated for these steps are labeled on the sections at either 30% or 50% depending on the individual. Compressive strains are shaded gray. The neutral axis (NA) of bending (strain=0 µ{epsilon}) is indicated by the red line (strain contour) separating compressive and tensile strains. Strain gauge locations are indicated by the black bars around the cortex of the femoral cross-sections. Anatomical directions are labeled in A and reflect the anatomical AP and DV axes illustrated in Fig. 2B.

 

Figure 4
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  		Fig. 4. Bivariate plot of strain magnitudes (in µ{epsilon}) versus corresponding strain rates (in µ{epsilon} s–1) for N=60 individual steps from river cooter turtles. All data are plotted as absolute values. Solid line reflects a linear least-squares regression of the pooled data (y=0.0326x+479.86). There was a significant positive correlation between strain magnitude and strain rate (P<0.001, r2=0.636).

 

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© The Company of Biologists Ltd 2008