Muscles have a mixture of two basic types of fibers, fast twitch and slow twitch . Fast-twitch fibers are capable of developing greater forces and contacting faster and have greater anaerobic capacity. In contrast, slow-twitch fibers develop force slowly, can maintain contractions longer and have higher aerobic capacity. Your genes largely determine whether you have more of one kind of muscle fiber or another. Sprinters tend to have more fast twitch fibers.
Marathon runners tend to have more slow twitch fibers.
And the rest of us tend to have an equal distribution of both fiber types. It is not clear whether training can change the distribution of fiber types within an individual. Muscle fibers get bigger by having more muscle protein content, and that is achieved by making new protein and decreasing the rate at which existing proteins are broken down. These proteins include contractile proteins as well as the enzymes that are involved in various metabolic reactions. By increasing the strength of muscles, resistance training can also increase the power of muscles.