Does the distribution and variation in cortical bone along lower limb diaphyses reflect selection for locomotor economy?

Abstract

It has been hypothesised that limb tapering reflects an energetic trade-off between bone strength and weight, and selection for tissue economy, resulting in lighter distal limb segments. If adaptive mechanisms constrain the response of osseous tissue to mechanical loading one might expect a higher level of constraint, and therefore less variability, in more distal aspects of the limb. High-resolution CT was used to quantify the distribution and variation in strength (Z p ), cortical area (CA) and shape (I max/I min) at 5 % intervals along the femoral and tibial diaphysis for a skeletal sample of mid- to late Holocene Native American agriculturalists and foragers (M = 21, F = 19). Z p and CA are highest in the proximal femur, decrease at a fairly consistent rate (Z p increases in the distal femur) and reach their lowest values at the distal tibia. By contrast, inherent morphological variability (coefficient of variation) for both Z p and CA are relatively constant along both the femur and tibia. The distribution and variation in I max/I min is greater than that of CA or Z p . These findings support earlier studies that have identified tapering in human limbs, yet, because morphological plasticity appears to be generally consistent across the diaphyses of the femur and tibia, morphological constraint (canalisation) does not seem to be the overriding mechanism dictating the tapering of limb bone structure.