You want to be stronger if you are a weightlifter, you want to be able to throw a blistering fast ball if you are a baseball pitcher or you want to be able to finish strong at the end of a 26-mile race if you are a marathon runner. The power of muscle contraction is how fast the muscle can develop its maximum strength. Muscle endurance is the capacity to generate or sustain maximal force repeatedly. 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. 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.