Fiber-type composition of nine rat muscles. I. Changes during the first year of life.

There are few data available that describe the changes in fiber-type profiles with age in a range of muscles; thus fiber-type profiles and areas were determined in nine muscles of female rats from weaning to 1 yr. The results not only demonstrated the difference in fiber-type composition, size, and hence functional area in the various muscles examined but also illustrated the changes in these parameters with age. In particular, it was clear that two ends of a spectrum of fiber compositions are represented in the muscles studied: tensor fasciae latae, mainly fast-twitch glycolytic at one end and soleus (and adductor longus), mainly slow-twitch oxidative at the other; the remaining muscles were mostly dominated by fast-twitch fibers of a variety of metabolic types. Aging resulted in an increase in fiber area that was most dramatic in fast-twitch glycolytic fibers. Changes in fiber type with age were noted in all muscles, but the exact nature of the developmental changes appeared muscle specific.