Muscle fiber length adaptation to static stretch or shortening depends on age, with sarcomere addition in young muscle being dependent on mobility. Series sarcomere number can also increase in young animals in response to increased muscle excursion, but it is not clear whether adult muscles respond similarly. The ankle flexor retinaculum was transected in neonatal and adult rats to increase tibialis anterior muscle excursion. Sarcomere number in tibialis anterior was determined after 8 wk of adaptation. Muscle moment arm and excursion were increased 30% (P < 0.01) in both age groups. Muscle cross-sectional area was reduced by 12% (P < 0.01) in response to the increased mechanical advantage, and this reduction was unaffected by age. Fiber length change was also unaffected by age, with both groups showing a trend (P < 0.10) for slightly (6%) increased fiber length. Retinaculum transection results in shorter muscle length in all joint configurations, so this trend opposes the fiber length decrease predicted by an adaptation to muscle length and indicates that fiber length is influenced by dynamic mechanical signals in addition to static length.