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What are Fast Twitch Fibers?

Archana Khambekar
Archana Khambekar

In general, muscles are composed of two basic kinds of fibers, namely fast twitch and slow twitch fibers. Typically, fast twitch fibers are attributed with the ability to contract swiftly and with great force. Sprinting and lifting heavy weights are among the activities where these muscle fibers could be engaged. Training that works the fast twitch muscles potentially might help enhance muscle strength and capacity, thereby improving athletic performance.

Muscles usually comprise a mix of both slow and fast twitch fibers. Some parts of the body might have a greater or lesser number of the fast or slow twitch type of fibers depending on how the muscles in that particular part are used. Fast twitch fibers are also called type II fibers. They are classified as type II-a and II-b fibers. Type II-a, or intermediate fast twitch fibers, have the capacity to generate energy without using oxygen (anaerobic), as well as by utilizing oxygen (aerobic).

Muscle fiber is made of myofibrils.
Muscle fiber is made of myofibrils.

Fast twitch muscle fibers usually are larger in size and paler in color than slow twitch fibers. They contain enzymes that can burn glycogen, a form of carbohydrate kept in the muscles. This makes glucose available to the body when it needs energy for muscle contraction.

While fast twitch fibers may have the ability to generate great amounts of force or power quickly, they often tire sooner than slow twitch fibers. The latter have a rich blood supply that delivers nutrients and oxygen to produce energy required for muscle contraction. Slow twitch muscles tend to make a good amount of energy at a slower rate. Fast twitch muscle fibers are considered less proficient in energy utilization.

Sprinting relies on fast-twitch muscle fibers for speed and explosiveness.
Sprinting relies on fast-twitch muscle fibers for speed and explosiveness.

An aerobic activity that is repetitive, of low intensity, but continues for a longer duration, typically engages slow twitch fibers. Thus the slow twitch group of fibers could be used when running long distances, for example. Fast twitch muscle fibers normally come into play during anaerobic activities which can be more strenuous or need more power, perform for a shorter amount of time, and take up less oxygen. Fast twitch fibers could be employed in actions such as jumping or pitching a fastball.

Aerobic exercise uses slow-twitch muscle fibers.
Aerobic exercise uses slow-twitch muscle fibers.

Mostly, the proportion of fast and slow twitch muscles fibers that occur in an individual has a genetic basis. An intensive training program or workout may bring about a positive effect on muscle fibers over time. For instance, fast twitch fibers could be developed and enlarged with strength training or sprint training.

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Discussion Comments


@Mammmood - Since some of the fast twitch fibers do use oxygen, I believe that it helps if you drink water while you are doing your workout. As a matter of fact I think that you should drink a lot of water.

It will prevent your muscles from being dehydrated and I think it will keep you from getting tired much sooner too. I suppose you could drink the electrolyte drinks too but I don’t think that is required for regular workouts. Water should be fine for most people.


@MrMoody - I think that there is something you can do about it actually. For one thing, you can alternate between the workouts and aerobic exercises, thereby allowing you to switch between slow twitch and fast twitch fibers.

Also if you find that you’re getting any kind of twitch contraction in your muscles when you’re not working out, you may want to look at your diet. I would load up on vegetables, particularly vegetables that provide plenty of minerals, which your muscles need. This should reduce involuntary tremors.


The difference between fast twitch and slow twitch muscle fibers is interesting to understand. It explains to me what happens when I do workouts which according to the article engage the fast twitch fibers.

I usually get muscle tremors after a strenuous workout. I suppose the tremors are happening because they are fast twitch fibers. I am not a doctor but that seems to make sense.

One thing that has exacerbated the condition however is that I have mild tendinitis, so the twitching and the tremors are worse than what you would normally get. I don’t think that there is much I can do about it at this point. I just try to go easy on the workouts.

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    • Muscle fiber is made of myofibrils.
      By: Alila Medical Media
      Muscle fiber is made of myofibrils.
    • Sprinting relies on fast-twitch muscle fibers for speed and explosiveness.
      By: bonninturina
      Sprinting relies on fast-twitch muscle fibers for speed and explosiveness.
    • Aerobic exercise uses slow-twitch muscle fibers.
      By: .shock
      Aerobic exercise uses slow-twitch muscle fibers.