Figure 5.

Relative proportion of fiber types in the rat soleus at different developmental ages. At 3 weeks of age, the soleus is comprised of about 60% type I fibers, with the rest being hybrid I/IIA or type IIA. By 6 weeks, approximately 90% of the fibers are type I, with only about 10% being made of hybrids or IIA fibers. The portion of type I fibers was significantly lower at 3 weeks, but the proportions were no different between the other ages. This indicates that the major transformation of fibers is complete by about 6 weeks.