Exercise produces little change in the proportions of fast and slow fibers in the muscle, this is because exercise does not change the types of myosin enzymes formed by the fibers. What exercise does do is change the rates at which metabolic enzymes are synthesized, leading to changes in the proportion of oxidative (fibers using oxygen for energy/aerobic) and glycolytic fibers (fibers not oxygen dependent for energy production/anaerobic) within the muscle.
With aerobic/endurance training there is a decrease in the number of fast-glycolytic fibers and an increase in the number of fast-oxidative fibers as the oxidative capacity of the fibers is increased.
With strength/anaerobic training fast-oxidative-fibers are converted into fast-glycolytic fibers. Fast-oxidative fibers have medium speed contractile ability compared to fast-glycolytic fibers. If regular exercise is stopped, the changes in the muscle that occurred as a result of the exercise will slowly revert back to their state before exercise began.
