Figure 3.

ATPase histochemistry (pH 9.6) demonstrating differences in muscle fiber types and size in 3-week (A) versus 12-week (B) old rats. Dark staining fibers are fast, while light staining fibers are slow. The average fiber diameters in this figure were 17 µm for fast fibers and 22 µm for slow fibers at 3 weeks of age. At 12 weeks of age, these diameters increased to 24 and 41 µm, respectively. In the sample shown, the 3-week-old soleus comprised approximately 50% fast fibers. At 12 weeks, the proportion of fast fibers was fewer than 10%. Scale bar = 100 µm.