Your skeletal muscle contains two types of muscle fibers: fast-twitch and slow-twitch. Your fast-twitch and slow-twitch muscle fibers are separated into three distinct types. Slow-twitch fibers contain type 1 muscle fibers, while your fast twitch fibers contain both type 2a and type 2b. The percentage of each muscle fiber differs among everyone, and from one muscle to another in the same individuals, according to "Fitness: The Complete Guide," by International Sports Sciences Association.
Type 1 muscle fibers are your slow-twitch fibers, which have a very low force output and are highly resistant to fatigue. Those who engage in prolonged low-intensity exercise such as marathon runners, type 1 fibers are dominantly used. Fat is the predominant fuel utilized by type 1 fiber, which utilizes the aerobic pathway. Aerobic exercise increases your need for oxygen, such as long distance running. Once the exercise intensity increases, Type 2 fibers will gradually be recruited.
Your Type 2a muscle fibers are your intermediate fast twitch fibers. Meaning, Type 2a fibers can use both the aerobic and anaerobic pathways, making both carbohydrates (glucose) and fat the predominant fuel. Unlike aerobic, anaerobic exercises are short-duration and do not require oxygen. Type 2a fibers produce high force output for longer periods of time. In this way, they are more prevalent in activities than use a combination of Type 1 and Type 2 muscle fibers like middle-distance running or swimming.
Type 2b muscle fibers are your fast twitch glycolytic fibers. These strictly utilize the anaerobic pathway, making carbohydrates (glucose) the predominant fuel source. They are recruited for activities involving very short-duration with high-intensity burst of power such as sprints or near-maximal lifts. Due to their ability to fire rather quickly, they excel at producing powerful bursts of speed. But, due to the rapid firing of all the muscle fiber types, they have very low resistance to fatigue.
According to "Wardlaw's Perspectives In Nutrition" the individual differences in fiber-type distribution are partially accountable for producing elite marathon runners who could never compete at the level as sprinters. Genetics also play a large role in determining the proportion of muscle fiber types. Olympic athletes such as sprinters tend to fall into their sport based on their genetic makeup. However, with appropriate training, you can produce adaptations to your specific sport to develop those muscles within limits.