The Difference Between Slow-Twitch and Fast-Twitch Fibers

by Rado on September 27, 2006

in Power Training

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… read them below or add one }

Joyce Annn March 15, 2008 at 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 at 3:43 pm

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


Rado August 8, 2008 at 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 at 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 at 9:29 am

is ugly
sasha looks like jimmy


heyy February 5, 2009 at 9:31 am

brianna love triston and ronal


clarissa February 5, 2009 at 9:32 am

clarissa loves jason


lilly smells February 5, 2009 at 9:44 am

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


lilly February 5, 2009 at 9:48 am

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


ingo March 26, 2009 at 9:27 pm

hi ,

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



Jay December 8, 2009 at 6:46 am

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cassie February 15, 2010 at 9:13 pm

mmm biceps


kassie February 15, 2010 at 9:13 pm



BETH February 15, 2010 at 9:14 pm

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ahmg April 1, 2010 at 2:00 pm

ahmg is boring.


Reedy June 17, 2010 at 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 at 1:10 pm



Tommy Fidler September 29, 2011 at 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 at 9:21 am

i have really understand that


ferlina March 15, 2012 at 9:21 am

and i love kelvin


Ehiaghe Imoloamen April 12, 2012 at 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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