- According to Dr. Len Kravitz at the University of New Mexico, exercise causes muscle growth through a process called muscle hypertrophy. When muscles experience hypertrophy as a result of exercise, they grow, both in mass and in width (but not in length). It should be noted that the effects of exercise are caused by the muscle cells becoming slightly damaged, so that later, when muscle "damage" is mentioned, it is closer to what we think of as our muscles getting "tired." Muscles don't usually get bigger by developing more cells. Instead, the existing muscle cells get bigger. This happens because the muscle cells are able to adapt to the increased work that they need to do, which causes them to make more contractile proteins that help the muscle cells contract harder and for longer periods.
- When muscle cells become overloaded as a result of resistance-based exercise (such as weight lifting), they become slightly damaged. This causes the activation of a specific kind of cell called satellite cells. These cells, which are located on the outside of muscle fibers, work to repair and develop muscle cells. As a result, when muscle fibers become "damaged" as a result of exercise, the satellite cells repair them and then make more contraction proteins, which cause the muscle cells to be bigger and able to contract harder. Slow-twitch fibers (also known as Type I muscle fibers) have approximately five times the number of satellite cells as fast-twitch muscle (Type II muscle fibers).
- Muscle growth occurs as a result of muscle damage. Aside from its effects on the satellite cells, muscle damage also causes activation of the immune system, which leads to inflammation. One type of immune cell that becomes involved is called a macrophage, which works to remove debris from damaged cells and also secretes chemical signals, such as cytokines and growth factors. The growth factors stimulate the synthesis of new proteins in the muscle cells. The cytokines help activate and recruit the satellite cells, as well as cause other immune cells to come to the damaged muscle. These cells help repair the damaged muscle cells and break down unnecessary proteins, allowing the damaged cells to make new proteins that helps them get bigger and stronger.
