OPN ablation leads to increased muscle mass and size. The quadriceps and EDL muscles were evaluated for muscle mass at the ages indicated and data were normalized per tibial length. All cross sectional area and fiber analyses were conducted on the EDL. Vertical lines represent standard error. *, P < 0.05; ^#, P ≤ 0.02, determined by Student’s t test. Bars, 500 µm. CSA, cross-sectional area. Average fiber cross-sectional area was calculated from n = >9,000 fibers per genotype per age.