Your muscle fiber type mostly determines the type of athlete you are. If you are strong and fast but get tired quickly you probably have a greater percentage of fast–twitch (type II) muscle fibers. If you can run long distances without getting tired you most likely are made up of more slow-twitch (type I) muscle fibers. If you are equally good at both strength and long endurance type activities; however, then you might have an equal proportion of both type I and type II (50/50) muscle fibers.
Skeletal muscle is made up of two basic fibers with each of them having different contractile and metabolic abilities. The two basic types of fibers are slow-twitch and fast-twitch. Their names give you a little bit of a clue at what they are good at.
Slow-twitch fibers are slow to contract and have a high number of mitochondria (the engine of the cell) and are good at burning oxygen and fat as fuel. They are your aerobic muscle fibers. While fast-twitch are faster to contract, can store phosphates and glycogen in them which makes them more glycolytic. They burn carbohydrates as fuel.
Because of their make up slow-twitch fibers do not fatigue as easily as fast twitch fibers. Slow-twitch fibers are responsible for longer endurance type activities like walking or jogging moderately for long distances, while the fast-twitch fibers are recruited during high-intensity activities like sprinting, weight lifting and any burst of activity. This is the why aerobics burns fat and weight lifting burns carbohydrates.
Everybody has both fiber types but genetics determine what your ratio is. Most people are about 50/50 but some athletes have a greater amount of one type, which can be an advantage in a particular sport. If you were a power lifter who needs quick bursts of strength it would be to your advantage to have a greater percentage of fast-twitch fibers.
Research has shown that athletes can skew a portion of the muscle fiber type in a desirable direction with the proper training program. For example, a marathon runner who runs long distance all the time can have a greater percent of slow twitch fibers by making some of their fast-twitch fibers act more like slow-twitch fibers. The body is so good at adapting to whatever stimulus you give it so use it to your advantage. If you want to get fast and strong, train those type II fibers with short bursts of exercise like sprints. If you want to burn some fat, take long walks and train your type I fibers.
Let me know what type of athlete you think you are.
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Best – Mike Cola
Muscle fiber Types