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Muscles are made up of hundreds of thousands of fibers that allow daily movement and athletic activity. Slow twitch muscle fibers, also known as Type I fibers, are responsible for endurance. This type of muscle keeps twitching while engaged in activity, unlike fast twitch muscle fibers, which produce more power but do not remain moving for very long. Slow twitch muscle fibers are useful for aerobics, swimming, running, and cycling, all of which require long periods of endurance.
Slow twitch muscle fibers can fire for longer periods without getting fatigued than fast twitch muscle fibers can. This is because they can use oxygen more efficiently to generate more fuel for activities that require endurance. Muscles use adenosine triphosphate (ATP) as a source of energy. ATP is created by the metabolism of oxygen in the bloodstream. Slow twitch muscle fibers complete the ATP-generation process better than fast twitch, or Type II, fibers.
Both fast and slow twitch muscle fibers play an important role in strength training. When lifting weights, more repetitions with a lower weight will engage the slow twitch fibers, resulting in more muscle endurance but less strength. Women often prefer this type of strength training to avoid a bulky look. Fewer repetitions with more weight engage the fast twitch fibers, which help with building more strength.
Most people have close to an equal amount of slow twitch and fast twitch muscle fibers, although some people are naturally born with more of one type than the other. Individuals with more fast twitch fibers are usually better at anaerobic activities that require a lot of power over a short period of time, such as weightlifting and sprinting. People with more slow twitch fibers may be better at aerobic exercise over longer periods of time than people with an equal amount of fast and slow twitch fibers and people with more fast twitch fibers than slow.
Some muscle fibers can change from fast twitch to slow twitch during training. While scientific studies have yet to arrive at a definitive answer for how and why this occurs, many test subjects have experienced an increase in slow twitch muscle fibers while participating in endurance training. Muscle fiber type and amount are usually only a significant factor in athletic performance at the professional level. A proper diet, sleep schedule, hydration, and exercise routine is often more important than muscle fiber type in determining training or athletic success.