Date of Award

12-2010

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Biological Sciences

Advisor

Blob, Richard W

Committee Member

DesJardins , John D

Committee Member

Higham , Timothy E

Abstract

Studies of limb bone loading in terrestrial mammals have typically found anteroposterior bending to be the primary loading regime, with torsion contributing minimally. However, previous studies have focused on large, cursorial eutherian species in which the limbs are held essentially upright. Recent in vivo strain data from the Virginia opossum Didelphis virginiana, a marsupial that uses a crouched rather than upright limb posture, have indicated that its femur experiences moderate torsion during locomotion as well as strong mediolateral bending. The elevated femoral torsion and strong mediolateral bending observed in opossums (compared to other mammals) might result from external forces such as a medial inclination of the ground reaction force (GRF), internal forces deriving from a crouched limb posture, or a combination of these factors. To evaluate the mechanism underlying the loading regime of opossum femora, we filmed opossums running over a force platform, allowing us to measure the magnitude of the GRF and its three-dimensional orientation relative to the limb, facilitating estimates of limb bone stresses. This three-dimensional analysis also allows depiction of muscular forces (particularly those of hip adductors) in the appropriate anatomical plane to a greater degree than previous two-dimensional analyses. At peak GRF and stress magnitudes the GRF is oriented nearly vertically, inducing a strong abductor moment at the hip that is countered by femoral adductor muscles on the medial aspect of the bone that place this surface in compression and induce mediolateral bending, corroborating and explaining the patterns identified from strain analyses. The crouched orientation of the femur during stance in opossums also contributes to levels of femoral torsion as high as those seen in many reptilian taxa. Femoral safety factors for bending (8.1) and torsional (18.6) loads were as high as those of reptiles and greater than those of upright, cursorial mammals, primarily because the load magnitudes experienced by opossums are much lower than those of most mammals. Thus, the evolutionary transition from crouched to upright posture in mammalian ancestors may have been accompanied by an increase in limb bone load magnitudes.

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Biomechanics Commons

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