Asymmetry in linear measurements and cross-sectional geometry in the humerus during ontogeny

Adult upper limb asymmetry is used to reconstruct behavior. However, the developmental trajectory of asymmetry in bone length, cross-sectional geometry (CSG), and joint dimensions is poorly understood. This study examines the development trajectory of humeral asymmetry and if asymmetry in bone length, joint size, and CSG develop in concert.

Linear measurements of bone length and metaphyseal/epiphyseal breadth, bending rigidity (I_max and I_min), and cross-sectional shape (I_max/I_min) at 30%, 50%, and 70% of bone length were acquired from 3D models of humeri from four skeletal samples of prehistoric hunter-gatherer populations (n = 82). Dental age cohorts were used to assess ontogenetic trends. Percent absolute (%AA) and directional (%DA) asymmetry were calculated for paired measures. Percentage of matching direction of asymmetry across variables and correlation analysis tested relationships between variables.

Within the total pooled sample, I_max shows the highest %AA and %DA, followed by shape and linear dimensions. Asymmetry is lowest in neonates and increases with age, particularly %DA of I_max in mid-proximal sections. Correlations among variables are low to moderate and strongest between I_max measures. Matching direction of asymmetry between variables is low and generally increases with age.

Higher correlations with age in CSG likely indicate greater responsiveness to mechanical loading. Low correlations in magnitude of asymmetry and side dominance suggest independence in the development of asymmetry between maximum rigidity, shape, and linear measures. Differences in how asymmetric loading affects the ontogeny of linear and CSG variables may account for the heterogeneous development of asymmetry.