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.

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