Phenotypic plasticity and constraint along the upper and lower limb diaphyses of Homo sapiens

Department of Archaeology and Anthropology, University of Cambridge, Cambridge, UKKEY WORDS peripheral quantitative computed tomography; limb morphology; cross-sectionalgeometry; mechanical loading; long boneABSTRACTObjectives: Morphological variation along the human limb reflects complex structural trade-offs between bonestrength and mass. Here we assess how varying levels of plasticity and constraint affect this structure and influencethe response to habitual loading along the diaphysis.Materials and Methods: Cross-sectional geometric properties including total area, cortical area, and rigiditywere compared from the upper (humerus: 50% of length, radius: 66%, 50%, 4%) and lower (tibia: 50%, 38%, 4%)limbs of male varsity-level athletes and matched controls with distinct habitual loading histories.Results: Geometric properties among cricketers and swimmers were significantly greater at the humeral mid-shaft, mid-proximal radius, and radial midshaft compared to controls. By contrast, no significant differences werefound among athletes or controls at the distal radius. The tibial midshafts of hockey players and runners also dis-played greater area and rigidity compared to controls. Differences in geometry among the three groups became lesspronounced distally, where structure was comparable among athletes and controls at 4% of tibial length. Addition-ally, coefficients of variation revealed that variation among athletes of the same sport was highest distally in boththe upper and lower limb and lowest at midshaft, where structure most closely reflected the activity pattern of eachloading group.Discussion: These results support previous research suggesting that distal limb sections are more tightly con-strained by safety factors compared to midshafts and proximal sections. Overall, it appears that plasticity and con-straint vary not only between limb segments in correspondence to known activity patterns, but also along specificsections of the diaphysis. Am J Phys Anthropol 000:000–000, 2015.