Movement regimens drive region-specific humeral adaptation: whole-bone insights from an experimental animal model

Repetitive movements impose distinct loading regimens on long bones, potentially eliciting predictable osteogenic responses and morphological change. Cortical responses to such movements are seldom investigated along whole diaphyses as opposed to selected locations. Morphometric mapping is a technique that is well-suited for comprehensively examining whole diaphyses. To assess responses to different movement regimens, we evaluated humeri of thirty-five female C57BL/6J mice raised from weeks 4 to 20 post-birth in one of three scenarios: linear-dominated (n = 12), turning-dominated (n = 12), and open cage (n = 11) movements. We measured aspects of humeral structure such as diaphyseal cortical thicknesses (Ct.Th), second moments of area (SMA), and deltoid crest angles (DCA) on 3D models and 2D images of humeri generated from high-resolution (microCT) image data. Turning and open-cage individuals exhibited slightly greater Ct.Th than linear individuals in proximal shaft regions. Turning individuals were distinctive in mediolateral reinforcement (SMAs) in distal shaft regions. Individuals in the open cage group tended to exhibit overall greater SMAs than individuals in the other groups. Bayesian probabilities support mean DCA differences between the linear (25.0°) versus the open cage (20.9°) and turning (22.0°) groups. Ultimately, three different movement regimens elicited unique combinations of site- and direction-specific shaft reinforcements and muscle attachment orientations without any substantial differences in overall humeral bone mass. The generally overall greater SMAs of the open cage group suggest benefits to skeletal health (e.g., diversified shaft reinforcement) may be accrued in a non-pharmacological intervention strategy that would emphasize multidirectional rather than repetitive activities.