But even if you train hard every day, you still might not be able to make your muscles perform as well as another person's. Athletes are not just made; they are born, too. Strength, power and endurance may be due in part to the distribution of fiber types within an individual's muscles. 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.
The training to improve strength, power and endurance of muscle performance is called resistance training (for example, free weights, jump-training and isometric training). Resistance training mostly increases the size of muscle fibers (hypertrophy). It is not clear whether training can increase the number of muscle fibers (hyperplasia). 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. Increases in strength, diet and improved cardiovascular performance can increase muscle endurance.