Fig. 2.

Bone performance during loading is primarily governed by bone structure (from microstructure to macrostructure). Genetics and history of loading (exercise) are the two main determinants of bone structure (Eisman 1999; Ferrari et al. 1999). Genetic background can mediate the effect of exercise in two ways. First, the propensity to exercise has a genetic basis, and this desire for activity is the target of selection (gray line). Second, the physiological response to exercise can be modulated by genetic background (dashed line). For example, Kodama et al. (2000) showed differential sensitivity to exercise in C57BL/6J mice compared with C3H/HeJ mice, with a greater response to loading in the former.