What are the different types of muscle fibers?

Dr. Mehmet Oz, MD
Cardiology (Cardiovascular Disease)
Skeletal muscle is made up of two different types of muscle fibers: slow-twitch muscle fibers (red) and fast-twitch muscle fibers (white). You activate your white muscle when you do something explosive, like running upstairs. You activate your red muscles with steady exercise like walking around the neighborhood.

If you only work out one type of muscle, you’re only burning fat in 50% of your body, so it’s vital to work out both. Here are two easy exercises you can do for 10 minutes at home. Combined for 10 minutes, these two movements hit all your muscles:
  • Mountain Climbers: Give your white muscles a burn with mountain climbers. Start in a pushup position on your hands and toes, pulling your belly in tight while keeping your back straight. Then, bring your right knee up toward your chest, supporting your body with your left foot on the floor. Next, jump your right foot back to the floor, simultaneously bringing your left knee toward your chest like you’re climbing up a mountain. Start out slow to build coordination and endurance, then speed it up to get your heart pumping and work your white muscles.
  • Side Shuffle: Exercise your red muscles with a side shuffle. Stand with your feet slightly wider than shoulder-width apart and your arms bent at your sides. Bending slightly at the waist and keeping your belly tight, slide your left foot toward your right, then step to the right with your right foot. Repeat 2 or 3 times (depending on how much space you have -- don’t crash into a wall!), then reverse direction, leading with your left foot. Continue alternating, as the steady lateral movement works your red muscles.

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Marjorie Nolan Cohn
Nutrition & Dietetics

Muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers. Fast twitch fibers can be further categorized into Type IIa and Type IIb fibers.

Slow Twitch (Type I) - The slow muscles are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run marathons and bicycle for hours.

Fast Twitch (Type II) - Because fast twitch fibers use anaerobic metabolism to create fuel, they are much better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly. Having more fast twitch fibers can be an asset to a sprinter since she needs to quickly generate a lot of force.

Type IIa Fibers - These fast twitch muscle fibers are also known as intermediate fast-twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, they are a combination of Type I and Type II muscle fibers.

Type IIb Fibers - These fast twitch fibers use anaerobic metabolism to create energy and are the "classic" fast twitch muscle fibers that excel at producing quick, powerful bursts of speed. This muscle fiber has the highest rate of contraction (rapid firing) of all the muscle fiber types, but it also has a much faster rate of fatigue and can't last as long before it needs rest.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.