Schematic representation of changes at the femoral midshaft with variable exercise treatment of mice aged 7 and 11 weeks. Shaded regions indicate areas of bone growth. Cortical areas of 7-week-old mice treated with voluntary running exercise were ~4% greater than age-matched controls, although this difference was not significant (P > 0.05). Cortical areas of exercise-treated mice aged 11 weeks was ~10% greater compared with controls (P < 0.05). Mice in both age groups exhibited about 3.8 times more endocortical growth than periosteal growth in a pattern indicative of anterior diaphyseal drift (anterior is to the left in this image).