Activity, body shape, and cross-sectional geometry of the femur and tibia

Both bi-iliac breadth and stature are considered key aspects of body shape, vary ecogeographically, and have been proposed to influence femoral midshaft shape, complicating interpretations of activity. This chapter explores patterns of variation in cross-sectional geometry [especially shape, as measured by I max/I min or midshaft anteroposterior (AP) and mediolateral (ML) diameters] in the femur and tibia using three data sets that comprise a large amount of external measurements and some data from cross-sectional geometry. These data show that the midshaft shapes of the femur and tibia are only weakly correlated: r = -0.12 for AP/ML diameters; r = 0.33 for I max/I min ratios. Femoral midshaft shape is weakly, but significantly, associated with bi-iliac breadth and the ratio of bi-iliac breadth to femoral length in some, but not all, data sets. The results indicate that variation in body shape does not drive the low correlations observed between femoral and tibial midshaft shapes. We should look to other factors to explain the mismatch.