The figure shows that voluntary average daily running wheel distance is increased for young (4-wk-old), but not old (6-month-old),mdx mice over a 4-wk training period (⁴). These data suggest that voluntary endurance exercise when initiated at an early age can yield increased endurance capacity as a functional adaptation in mdx mice. [Adapted from Carter, G.T., M.A. Wineinger, S.A. Walsh, S.J. Horasek, R.T. Abresch, and W.M. Fowler Jr. Effect of voluntary wheel-running exercise on muscles of the mdx mouse. Neuromuscl. Disord. 5:323-32, 1995. Copyright ® 1995 Elsevier. Used with permission.]