The Relationship between Body Mass Index and Long Bone Morphology: A Multidirectional Analysis

AGOSTINI, GINA MARIE. The Relationship between Body Mass Index and Long Bone Morphology: A Multidirectional Analysis. (Under the direction of Ann Helen Ross, D. Troy Case and Scott M. Fitzpatrick). Obesity has increased significantly during the last three decades in all ages and both sexes among European and African Americans, and Hispanic individuals of Mexican origin. Biomechanical literature is replete with evidence of compensatory adaptations made by overweight individuals to cope with adiposity in daily life, yet aside from correlations between weight and arthritis frequencies, little attention has been paid to the effect that obesity has on the human skeleton. Because a key goal of physical anthropology is to create a thorough and accurate biological profile of the individuals being analyzed, more research is needed to investigate implications of obesity, a condition which clearly affected how an individual appeared in life. The goal of this project was twofold: [1] to assess differences in diaphyseal crosssectional geometry of both humeri, the left femur and the left tibia on the basis of BMI; and [2] to test whether the expression of musculoskeletal stress markers (MSMs) of each bone were affected by weight. Both properties have been shown to be influenced by load and mechanical action resulting from stress-induced remodeling responses at the cellular level. A sample of modern males of European ancestry was utilized for this research. After controlling for age, multivariate statistics show significant (p-value < 0.05) elongation of the mediolateral (ML) dimension of the proximal and midshaft femur in overweight individuals after controlling for age. T-tests of group means confirm that overweight individuals have significantly large ML dimensions in this region (p-value < 0.05), suggesting that femora of overweight individuals undergo abnormally high rates of sagittal stress. These findings correlate well with biomechanical gait analyses, which show that overweight individuals display significant increases in step width and hip abduction, disproportionately large mediolateral ground reaction forces, and longer periods of stance during the walking cycle when compared to normal weight controls. All of these activities could explain abnormal sagittal stress of the proximal femur, especially when coupled with movement of excess mass. A significant bilateral effect of BMI on the ML dimension of the proximal humerus was also found (p-value < 0.01) after controlling for age. T-tests confirmed that overweight individuals have significantly large dimensions in this region (p-value < 0.05), perhaps due to high loads transmitted through the shoulder when an individual uses his arms to rise from a seated position. Despite their success in archaeological assessments of activity, MSMs were not found to be a suitable method of differentiating overweight individuals from normal or underweight individuals. This could be due to the presence of biological defense mechanisms at sites of muscle attachment due to the routinely high stresses associated with muscle pull. It is also possible that genetic influences of MSM expression or lack of significant activity differences between overweight and normal or underweight individuals could explain this result.