The effects of muscle growth and training on the maximum tension (MT) per cross-sectional area (CSA) were studied in fast twitch (FT) and slow twitch (ST) skinned fibers of extensor digitorum longus and soleus in the male mice. The fiber classifications, fiber diameters and isometric tensions were investigated from 8 to 86 days after birth. The results were summarized as follows:
　1. Body weight, CSA and MT increased rapidly until about 50th days, and reached to steady level thereafter. After 50th days CSA were higher for FT fibers relative to ST fibers.
　2. Hypertrophy of fibers and MT increases were observed in the trained group.
　3. MT per CSA (kg/cm²) increased rapidly up to 50th days in both fiber types. Relative the rate at 80th days against 10th days were 1.98 and 1.93 in ST and FT fibers, respectively. However, there were no significant differences between trained and control groups and between ST and FT fibers in the rates.

DESCENTE SPORTS SCIENCE Vol.7/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE