The femoral shaft waist, an alternative robusticity measure: its distribution, relation to midshaft, and applicability to behavioral reconstructions
Midshaft is the most commonly used site for behavioral and robusticity inferences in cross-sectional studies of the femur. This work tries to revive an alternative location because the midshaft, as much as it is easy to locate, does not necessarily reflect the same adaptive remodeling in every individual. Femoral waist which is defined as the shaft’s mechanically weakest point is reintroduced as an alternative. The aim of this work is to describe waist’s general patterns of distribution along the shaft, to test which morphological characteristics influence its position, to compare its inferential potential with the midshaft, and to evaluate applicability of the concept under the mechanical predictions about stress and strain distribution along the femur. A total of 251 individuals representing four temporal samples spanning the Eneolithics to 19th century were analyzed using CT-derived cross-sectional properties. The results showed that the femoral waist tends to center around the midshaft but with a rather large amount of variation and that the samples do not seem to differ in this respect. General levels of mechanical loading and robusticity may influence its distribution as evidenced from the Early Middle Ages males who were the most robust group (in body size adjusted parameters) and had their waists positioned more proximally. Variables that influence waist’s location are primarily related to strain distribution but not its magnitude. Thus, neck-shaft angle, anteversion angle, femoral inclination, crural index, and femoral curvature proved to be significant predictors (their importance varies between sexes), while body size measures were insignificant. Behavioral and robusticity inferences made from the femoral waist and midshaft are rather incompatible. In closely related populations, results from the two locations would probably lead to different interpretations while in rather distant units (species or genera), this would not be as problematic. Whenever possible, true cross-sectional properties should be used to locate the femoral waist. External methods using subperiosteal contour are more acceptable than methods estimating cross-sectional properties only from external dimensions. Lastly, the femoral waist position can potentially be used for taxonomic purposes in earlier hominins as well as for the reconstruction of other femoral characteristics (e.g., femoral length).