The body has approximately a quarter of a billion skeletal muscle fibers. Most muscles are made up of a mixture of both slow and fast twitch muscle fibers. Most of us have an equal number of both, 50% slow- twitch and 50% fast -twitch.
Slow-twitch muscle fibers use large amount of energy slowly so that you can work out for a long time without getting tired. They make energy from the oxygen in your blood and that is why these muscles appear very pink. The muscles in your back that help you maintain posture are mainly slow-twitch muscles. All sorts of endurance activities such as marathons, cross country skiing, and biking rely on your slow-twitch muscles.
Fast-twitch muscle fibers use small amounts of energy quickly. These muscle fibers contract quickly and get tired quickly. The muscles that move your eyes are fast-twitch. You use your fast-twitch muscles jumping to catch a ball, sprinting for the bus, or free weight lifting.
Have you ever noticed how much stronger a sprinter looks than a long distance runner. This is because fast-twitch muscles grow bigger giving sprinters those unbelievably strong looking thigh muscles. You can perform exercises specifically designed to help one type of muscle fiber over another. To increase your fast-twitch fibers which improve your speed, power and agility, workout with plyometrics and do lots of sprints. To improve your slow-twitch muscle fibers do cardio workouts and weight training.
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Elite athletes usually have a disproportioned amount of slow or fast-twitch fibers. For example an Olympic marathoner may have 80% or more slow-twitch fibers, where as an Olympic Sprinter may have 80% or more fast-twitch fibers. This out of balance muscle fiber helps them to excel at their individual sports. If you find your body more suited to a specific sport if could very well have something to do with your ratio of slow to fast-twitch muscles. But don’t be discouraged if have a regular 50-50 distribution. There are other things that contribute to athletic success such as, hydration, mental attitude, nutrition and rest.