|
| |
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
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
In vivo strains in the femur of river cooter turtles (Pseudemys concinna) during terrestrial locomotion: tests of force-platform models of loading mechanics
1 Department of Biological Sciences, Clemson University, Clemson, SC 29634,
USA
2 Department of Biology, Erskine College, Due West, SC 29639, USA
* Author for correspondence (e-mail: rblob{at}clemson.edu)
Accepted 15 May 2008
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 µ
. 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.
Key words: locomotion, biomechanics, bone strain, safety factor, turtle
