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The Difference Between Slow-Twitch and Fast-Twitch Fibers

Today's lesson is about skeletal muscle fibers. Learning the difference between slow-twitch and fast-twitch muscle fibers will give you a better understanding of how different exercises and workouts will benefit you.

muscle fiber type


To begin with, there are many different types of muscles in your body. Some work involuntarily, like the heart muscle…it works without you having to tell it. Our focus will be on the skeletal muscles – the muscles that move your bones, or your limbs and body around.


In short:

Your skeletal muscles are made up of 3 different kinds of fibers:

Type I – red fiber, SLOW-TWITCH muscles. They are slower-contracting fibers (weaker) but they are very efficient at using oxygen to create energy without lactic acid buildup. Think of these as "weaker" muscles with tons of endurance, and can go on forever. These fibers are used for high-endurance events like marathons.

Type IIa – these HYBRID fibers are a mix between Type I and Type IIb fibers, they have a very high contraction rate (making them very strong and explosive), and yet very energy efficient therefore making them very resistant to fatigue. They are found infrequently in humans and are very helpful for all types of physical performance.

Type IIb – these are the pure FAST-TWITCH fibers, white fibers, that contract very quickly (making them very strong and explosive) but they also tire out very easily.




Further explanation:

Every person has a different percentage makeup of the different muscle fibers in their body. These percentages are genetically determined and they will aid people accordingly. A "normal" person would be expected to have roughly 50% of type I muscle and 50% of type II muscle. It isn't surprising to discover that Olympic sprinters have 80% Type II muscle fiber and Olympic long distance runners have 80% Type I muscle fiber!

So as you can see, everyone already has a genetic makeup that makes their body more suitable for one type of sport or physical event over another. Finding out what your body is good at would be a great way to excel in your sport.

Do keep in mind that Type II fibers are not "stronger" than Type I fibers. Pound for pound, they are equal strength. Type II fibers are simply FASTER, meaning that they contract at a faster rate and can therefore handle a higher intensity workload needed to be more explosive, or lift heavier, or do more work.



Some final questions:

Q: Can you change your muscle fiber type by training?

A: There haven't been too many studies that support this but there has been some recent evidence that those with PURE fast-twich fibers Type IIb can transition to the "hybrid" Type IIa fibers with constant endurance training.


Q: Can I target different fibers with specifc training?

A: Most definitely! Plyometrics and weights can build your Type IIa and sprint training can build your super-fast Type IIb. There are many different exercises and routines to try.



Final Advice:

Never turn down an exercise or routine until you've tried it. There are so many people who won't do a particular exercise because they think there is no benefit. Very often, this turns out to be the missing key to taking their performance to a higher level. Try everything and just because it seems easy doesn't mean it isn't benefiting you.

{ 25 comments… add one }
  • Joyce Annn March 15, 2008, 5:39 am

    Could you answer these questions for me?

    Question 7:
    What is the most important factor in creating muscle strength:

    Set volume
    Gravitational force
    Exercise form
    Rest period between exercises

    Question 8:
    Fast twitch muscle fiber (white or TypeII) has a maximum speed of contraction rate of:

    42 mm/s
    50 mm/s

  • Harvey August 5, 2008, 3:43 pm

    Is there any difference between fast twitch and slow twitch muscle fibers in t erms weight?f

  • Rado August 8, 2008, 3:32 am

    muscle weight
    In my opinion, slow-twitch muscle is smaller whereas fast-twitch muscle is bigger – this in theory, should explain the bulky musculature in weight-lifters versus the lean muscle in endurance runners. I don’t know for sure though.

  • bean January 26, 2009, 8:14 pm

    re:muscle weight
    In response to rado, muscle bulk on weightlifters isnt from muscle fibre type, but the simple fact they are lifing weight to get bigger, whereas an endurance runner isnt bulking. A better example would be olympic sprinters compared to olympic marathon runners.

  • sasha February 5, 2009, 9:29 am

    is ugly
    sasha looks like jimmy

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    brianna love triston and ronal

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    clarissa loves jason

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    she luvvs saul a lot
    she doesnt wash her clothes at alll

  • lilly February 5, 2009, 9:48 am

    she does wash her clothes
    so do i she always does so do i

  • ingo March 26, 2009, 9:27 pm

    hi ,

    how can i avoid that my fast twitch fybres turn to slow twitch?


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    I love everyone
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  • cassie February 15, 2010, 9:13 pm

    mmm biceps

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    i love penis

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    ahmg.school is boring.

  • Reedy June 17, 2010, 9:39 am

    How can one detirmine the compsition of their muscle make up. ie. How do i know my fast twitch to slow twitch muscle ratio

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    im super gay

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  • PERIS September 14, 2010, 1:10 pm


  • Tommy Fidler September 29, 2011, 3:50 am

    changing to IIb
    is it possible to switch too mainly super fast twitch muscles? and how? and by doing lots of endurance and going too IIa will you get slower?

  • ferlina March 15, 2012, 9:21 am

    i have really understand that

  • ferlina March 15, 2012, 9:21 am

    and i love kelvin

  • Ehiaghe Imoloamen April 12, 2012, 6:31 am

    Which of these two types of muscle – Fast Twitch or Slow Twitch has more mitochondria than the other?

    Thanks for your speedy response!


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