Training induces a number of changes within skeletal muscle that are specific to

One reason why athletes train is to increase the capacity of their muscles to perform the work that their sport requires. Athletic training can be broken into two basic types: aerobic and anaerobic, depending upon the energy systems that are stressed by the specific training modality. In response to aerobic training, some of the fast glycolytic fibers are converted into fast oxidative fibers. This is accompanied by an increased mitochondrial density (size and number), an increase in the number of capillaries that surround each muscle fiber, and a decrease in the diameter of the muscle fiber (the last two would facilitate the delivery of oxygen to the active muscle fibers). For the slow oxidative fibers, they do not appear to be capable of converting to fast fiber types. However, the changes in the slow oxidative fibers to aerobic training would be similar to the fast oxidative fibers. In contrast, high-intensity anaerobic exercise will increase the glycolytic capacity of the muscle fibers (switch some of the fast oxidative to fast glycolytic fibers). At the same time, the density of mitochondria (size and number) will be decreased, the concentration of glycolytic enzymes will be increased, and the diameter of the muscle fibers will be increased. The increased skeletal muscle girth in an individual who regularly weight trains is related to an increase in the diameter of muscle fibers (increased myofibrils) rather than an increase in the number of muscle fibers.