Femoral diaphyseal shape and mobility: An ontogenetic perspective

Abstract

The ratio of midshaft femoral anteroposterior (I x ) to mediolateral (I y ) second moments of area has been suggested as a possible indicator of mobility, and has been applied as an analytic tool to paleoanthropological and archaeological samples with variable success. Under this model, biomechanical loads associated with increased mobility modify the shape of the femoral midshaft from a roughly circular cross section to an anteroposteriorly reinforced one. While previous research indicates that immature femora respond to changes in the manner of locomotor loading (Ruff 2003; Cowgill et al. 2010), relatively fewer studies have examined population-level differences in immature femoral shape as a product of overall group mobility. This study uses seven immature Holocene human samples (n = 522) to explore two questions: (1) When do population differences in midshaft femoral shape emerge during ontogeny? (2) Does the midshaft femoral mobility index correlate with other cross-sectional properties of the femur? The results of this analysis indicate that while population-level shape differences appear relatively early in human ontogeny (>6 years of age), these differences may not correspond to expected levels of group mobility, and may be a product of differences in body proportions. In addition, while many factors are implicated, it remains unclear what intrinsic dynamics directly impact midshaft femoral shape during ontogeny, and, while mobility levels may be a factor, additional influences need to be evaluated.