THE INTERACTION OF MECHANICAL LOADING AND METABOLIC STRESS ON HUMAN CORTICAL BONE: TESTING ANTHROPOLOGICAL ASSUMPTIONS USING CROSS-SECTIONAL GEOMETRY AND HISTOMORPHOLOGY

This research investigates how the interaction of mechanics (i.e., physical activity) and metabolism (i.e., health status) shapes human cortical bone morphology during skeletal development. Understanding this interaction is important for research investigating human behavior from adult and subadult archaeological skeletal samples, as previous studies have demonstrated that interaction effects may confound the interpretation of either mechanics or metabolism independently from skeletal remains.

This study approaches this issue holistically through the analysis of human cortical bone morphology at dual scales (microscopic and macroscopic scales) and across multiple skeletal elements (femora, humeri, and ribs) exposed to different levels of mechanical loading. Because bone responds to environmental influences most strongly during growth, a subadult cemetery sample of 57 individuals from the medieval archaeological site of Alytus, Lithuania (A.D. 14^th-18^th centuries) was employed. Bone properties were compared among individuals who had experienced varying amounts of metabolic stress, as inferred from skeletal stress lesions. Analyses tested the hypothesis that to maintain proper bone strength, metabolic stress effects (i.e., bone loss) are increasingly attenuated as mechanical loading demands increase across the three skeletal elements.

Results suggest that mechanical loading compensates for metabolic bone loss at both macroscopic and microscopic scales. Macroscopically, loading attenuates metabolic bone loss by redistributing cortical bone further from the cross-sectional centroid, thus increasing bone strength properties and maintaining loading relationships across the skeleton. Additionally, although metabolic stress is associated with microscopic bone loss, this loss is distributed within the skeleton in a way that may mitigate reductions in strength at the tissue-level (i.e., preferentially in elements with more macroscopic bone mass). While these results point to enhanced effects of loading relative to metabolic stress, both factors have a detectable influence on cortical morphology.

The interaction between mechanical and metabolic factors, therefore, must be considered when interpreting physical activity and health status from human skeletal remains. As the majority of adult cortical morphology is formed during skeletal development, when these factors are strongest, accounting for interaction effects is important for both subadult and adult studies. Thus, the implications of this study will assist in improving analyses of human biocultural adaptation in the past.