Skeletal muscle myosin heavy chain composition and resistance training.

We recently reported that 19 wk of heavy resistance training caused a decrease in the percentage of type IIb and an increase in the percentage of type IIa fibers as determined by qualitative histochemical analyses of myofibrillar adenosinetriphosphatase activity of biopsies of musculus vastus lateralis (Hather et al. Acta Physiol. Scand. 143: 177-185, 1991). These data were interpreted to suggest that resistance training had caused transformation among the fast-twitch fiber subtypes. To more clearly establish the influence of resistance training on muscle fiber composition, biopsies from the original study were analyzed biochemically for myosin heavy chain (MHC) composition by use of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and histochemically for fiber types by use of myofibrillar adenosinetriphosphatase activity. The results show that after training (n = 13), IIb MHC composition decreased (P < 0.05) from 19 +/- 4 to 7 +/- 1%. IIa MHC, in contrast, increased (P < 0.05) from 48 +/- 3 to 60 +/- 2%. These responses were essentially mirrored by alterations in fiber type distribution. The percentage of type IIb fibers decreased (P < 0.05) from 18 +/- 3 to 1 +/- 1%, whereas the percentage of type IIa fibers increased from 46 +/- 4 to 60 +/- 3% (P < 0.05). Neither I MHC composition nor type I fiber percentage changed with training. The control group (n = 4) showed no changes in MHC composition or fiber type distribution. These results suggest that heavy resistance training alters MHC composition in human skeletal muscle, presumably reflecting a change in genetic expression.