Myosin heavy chain IIX overshoot in human skeletal muscle

The distribution of myosin heavy chain (MHC) isoforms, fiber type compositi on, and fiber size of the vastus lateralis muscle were analyzed by sodium d odecylsulfate polymerase gel electrophoresis (SDS-PAGE), ATPase histochemis try, and immunocytochemistry in a group of adult sedentary men before and a fter 3 months of heavy-load resistance training and, subsequently, after 3 months of detraining. Following the period of resistance training, MHC IIX content decreased from 9.3 +/- 2.1% to 2.0 +/- 0.8% (P < 0.01), with a corr esponding increase in MHC IIA (42.4 +/- 3.9% vs. 49.6 +/- 4.0% [P < 0.05]). Following detraining the amount of MHC IIX reached values that were higher than before and after resistance training (17.2 +/- 3.2% [P < 0.01]). Chan ges in fiber type composition resembled the changes observed in MHC isoform content, Significant hypertrophy was observed for the type II fibers after resistance training. Maximal isometric quadriceps strength increased after resistance training, but returned to pretraining levels after detraining. The present results suggest that heavy-toad resistance training decreases t he amount of MHC IIX while reciprocally increasing MHC IIA content. Further more, detraining following heavy-load resistance training seems to evoke an overshoot in the amount of MHC IIX to values markedly higher than those ob served prior to resistance training. (C) 2000 John Wiley & Sons, Inc.