Figure 6. Fiber type number and muscle fiber composition show different age-related changes in the soleus and tibialis anterior. The myosin heavy chain (MyHC) fiber composition of tibialis anterior (A, C) and soleus (B, D) muscles from young (3 mo) and old (28-29 mo) mice was assessed by immunostaining (see also Figure S8). Total type I, IIA, IIX and IIB MyHC-expressing fiber counts (A-B, left panels), counts for each fiber type (based on single, double or triple positivity for type I, IIA, IIX and IIB MyHC; (A-B, right panels), and proportions of each fiber type (C-D) are shown. All data are means plus standard deviations of 2 sections/mouse, 3 mice/group. Fiber types are arranged in approximate order of most oxidative (type I) to most glycolytic (type IIB). *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001, ****p ≤ 0.0001.