Abstract
This study explores how curvature in the quokka femur may help to reduce bending strain during locomotion. The quokka is a small wallaby, but the curvature of the femur and the muscles active during stance phase are similar to most quadrupedal mammals. Our hypothesis is that the action of hip extensor and ankle plantarflexor muscles during stance phase place cranial bending strains that act to reduce the caudal curvature of the femur. Knee extensors and biarticular muscles that span the femur longitudinally create caudal bending strains in the caudally curved (concave caudal side) bone. These opposing strains can balance each other and result in less strain on the bone. We test this idea by comparing the performance of a normally curved finite element model of the quokka femur to a digitally straightened version of the same bone. The normally curved model is indeed less strained than the straightened version. To further examine the relationship between curvature and the strains in the femoral models, we also tested an extra-curved and a reverse-curved version with the same loads. There appears to be a linear relationship between the curvature and the strains experienced by the models. These results demonstrate that longitudinal curvature in bones may be a manipulable mechanism whereby bone can induce a strain gradient to oppose strains induced by habitual loading.
Highlights
Many long bones in animal limbs are curved, but such curvature is thought to reduce the bones strength under longitudinal loading
The forces we applied were 10% of the maximum possible forces based on physiological cross sectional area (PCSA), but there is no reason to assume that all the muscles operate at the same proportion of their potential, and it seems likely that in reality the vasti and longitudinal forces are relatively larger than the adductor and ankle plantarflexor forces
This study further developed the theoretical model of bone curvature as a strain-reducing adaptation to habitual loading (Milne, 2016)
Summary
Many long bones in animal limbs are curved, but such curvature is thought to reduce the bones strength under longitudinal loading. Several authors have constructed hypotheses to explain this apparent paradox of bone curvature. It has been suggested that curvature may serve to induce strain (Lanyon, 1980), accommodate musculature (Lanyon, 1980), or warn of impending strain limits (Currey, 1984). The most widely accepted hypothesis suggests that bone curvature is the result of a trade-off between strength and predictable bending (Bertram & Biewener, 1988). A curved bone, while not as strong as a straight bone, will only ever experience unidirectional bending strain. Straight bones are liable to experience bending in any direction. While the ‘‘predictability hypothesis’’ is potentially powerful, it lacks an operational mechanism
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