Methods

In the present study, we compared skeletal muscle fiber characteristics between wheelchair-dependent young males with SCI (n= 8, 32±4 yr), healthy elderly males (n= 8, 75±2 yr), and young controls (n= 8, 31±3 yr). Muscle biopsies were collected to determine skeletal muscle fiber type composition, fiber size, and satellite cell content.

Results

Severe atrophy and a shift toward approximately 90% Type II muscle fibers were observed in muscle obtained from males with SCI. Muscle fiber size was substantially smaller in both the SCI (Types I and II fibers) and elderly subjects (Type II fibers) when compared with the controls. Satellite cell content was substantially lower in the wheelchair-dependent SCI subjects in both the Types I and II muscle fibers (0.049±0.019 and 0.050±0.005 satellite cells per fiber, respectively) when compared with the young controls (0.104±0.011 and 0.117±0.009 satellite cells per fiber, respectively). In the elderly, the number of satellite cells was lower in the Type II muscle fibers only (0.042±0.005 vs 0.117±0.009 satellite cells per fiber in the elderly vs young controls, respectively).

Conclusion

This is the first study to show that muscle fiber atrophy as observed with SCI (Types I and II fibers) and aging (Type II fibers) is accompanied by a muscle fiber type-specific reduction in satellite cell content in humans.