Fig. 1. Optimization model for cortical bone responses to loading. The periosteal modeling rate (PM RATE) is predicted to decrease and the Haversian remodeling rate (HRR) to increase with higher costs of adding mass [roughly proportional to R, the functional distance from each midshaft to the axis of rotation of the hind limb at the hip]. If Haversian remodeling functions to repair or prevent fatigue damage, then HRR is expected to increase as PM RATE activity declines with age for both models.