How many types of fibers are there?

So there is always a lot of discussion on the forum about fiber conversion ect. With the break down being fast twitch and slow twitch and some intermediate fibers. But I have also read(In Fact and falacies of fitness) that there are many types of fibers like 12. So I take this to mean that there is more of a grey scale of fibers where you find a whole range of characteristics from purely fast to purely slow.
So my question is are there three types or is it fuzzier then that?

It’s always fuzzier than that. Categorizing things is just something we humans tend to do to make it simpler to understand.

Fibre “type” should more accurately be thought of as a continuum. The shortening velocity of a particular fibre is dependent on the ratio of particular myosine heavy chain isoforms. So to answer your question, going from slowest to fastest shortening velocity you have the following fibre types.
Type I
Type IC
Type IIC
Type IIA
Type IIB
As you can see this is more complicated than it is usuall presented. To further complicate things this is not the only way to classify muscle fibre types. You can also classify them as glycolitic and oxidative, or as red and white.
These terms and classifications are not necessarily analogous and strictly speaking should not be used interchangealbly.
hope this helps.

Alex Power

Firstpull would you be able to point me in a direction so that I could read up some more on what you have said? Amos

So how does this make you feel about the likelyhood of fibre conversion? More or less convinced?

More convinced and its a good conviction, because if fibre conversion is possible then doing the correct training will cause correct CNS changes which in turn will cause the correct fibre conversion . It means that training is not a waste of time, its not down to just pure talent.

I have read physiology studies that have identified bi-directional change this is where the extreme fibre types on either side convert to intermediate fibre BUT still keep their qualities. Apparently those who are more trainable have this bi-directional fibre conversion. Those who did the same training and had fibre conversion from fast to slow were less fortunate. The study did not id whether different training would have caused bi-directional change in those who were only able to achieve unidirectionality. Also relativley long periods of reduced volume but high intensity training also increases slow to fast conversion and apparently type IIB percentage is GREATER than intial levels .

You can even overcome fibre type make-up through enlargening the fibres. Example an athlete with 70/30% in favour of fast fibres has AN ADVANTAGE OVER AN ATHLETE WITH 30/70 in favour of slow fibre in say sprinting or weight lifting. If the 30/70 athlete undertakes a program that allows fibre conversion in favour of fast then he can reach the levels of the 70/30 athlete. Also he may still have the same fibre make up but highly hypertrophied fast muscle. He would only need to double the fibre size of fast twitch to become equal. That is why you may find that sports people in the same event have differing fibre make-ups but can achieve the same level of performance.

Most research that has shown conversion of one fibre type to another has shown a shift from type IIB towards type IIC. Most of these studies have been in response to volumous training loads. Some research has shown that during tapering of volume in preperation for competition there is an increase in expression of type IIB myosine isoforms. This seems to point to the importance of volume (both relative and absolute) in determining how your body respondes to demands imposed on it.

Alex Power

Do you have access to academic journals either online or at a local university? Let me know, this is where I do most of my reading.

The key phrase- both relative (appropriate to intensity) and absolute.

Possibly from a friend of mine. Where whould you suggest I start? Amos

First pull, what are your thoughts on the stage at which transitional fibers are no longer capable of being influenced one way or another. I ask because there are various thoughts on the subject which infer that by a certain biological age an athlete is no longer capable of re-educating transitional fiber composition.

Charlie, if transitional fibers CAN be re-educated and fiber switching is possible in a positive way to sprint training, what % of the general (human regardless of race/ethnicity) population could run say 10.2 secs with proper training in your opinion?

Charlie, if transitional fibers CAN be re-educated and fiber switching is possible in a positive way to sprint training, what % of the general (regardless of race/ethnicity) population could run say 10.2 secs with proper training in your opinion?

I have to say that in my opinion but I don’t think that age is as big an issue in this regard. Generally. It has also been demonstrated that as you age (35 and older) muscles tend to attrify preferencially in type II fibres. So as you age you generally tend to have less faster twitching fibres. That being said I don’t think age has much affect on training related fibre changes.

it has been shown that type I fibres don’t easily (if at all) convert to type II (the fast ones) fibres. So, I think that instead of worrying about how to convert type I into type II fibres people should be more concerned about managing volume (something Charlie has commited much of his writting to)in such a way that you don’t end up teaching all your type II fibres to behave like type I fibres. It’s important to recognize that fibre type is not the only determinant to speed and power production. Muscle fibre pennation angle as well as sacromere length have very important influences on rate of force production. Neurological factors also play an important role in rate of force production.

These may all be trainable factors. Sacromere length appears to be influencable by training and angle of pennation might as well (more latter). Neurological programming is also highly trainable, even if it just means learning proper technique.

I read one study that discussed changes in muscle fibre pennation angle in response to training, but since this research also involved a drug protocol I don’t think it’s in depth discussion is appropriate for this forum. That having been said, it does demonstrated that gross muscle architecture is not a fixed state. I think that it would be interesting to see more research on muscle architecture changes with training. I’ll try to find some, but I didn’t get the impression that there was much around. I’ll get back to the board on this issue.

Sorry for being so long winded.
Alex Power

Scandinavian Journal of Sports Medicine
Sports Medicine
Journal of Applied Physiology
European Journal of Applied Physiology
Journal of Sports Science
Journal of Applied Biomechanics
Medicine and Science in Sports and Exercise

If you have access to any (or all of these) that would be a good place to start. If this isn’t enough what you’re looking for, let me know.

Alex Power

FirstPull, I appreciate the detailed response.

I too am interested in the concept of sarcomere length and fibre pennation angle being potentially subject to change as a result of imposed training stimulus.

It is an interesting topic. Even if only from an accedemic standpoint, not that I wouldn’t like to put it into practice. I think that some time in the next week or two I’ll try to find some research on the topic. Hopefully I’ll find something.

Alex Power

Don’t worry about educating fibre, worry about educating coaches. that’s where results will come from.

i found this, although it talks about differences between sprinters and endurance runners…

“Sprinters also have larger muscles with longer muscle fascicle lengths, which are a measure of the number of sarcomeres stacked end to end (in series) in their muscles (Abe, Kumagai, et al., 2000). Sprinters, especially African-Americans, also have a different muscle shape with greater muscle thickness in the upper parts of the muscles near where they originate from the skeleton (Abe, Brown, et al., 1999; Abe, Kumagai, et al., 2000). The pennation angle, which is a measure of the angle at which the muscle fibres insert into the tendons, is also less in sprinters than in distance runners (Abe, Kumagai, et al., 2000)”
–> i repeat myself with this reference… :slight_smile:

hope it helps guys for some referencing!

Check out some recent work by Blazevich, A (2001, 2004). He found pennation angle changes in as few as 5-weeks of training. You could also search for Aagard from around 2001. These guys and Abe, T. are doing most of the research in the area of fasicle length and pennation angle at the moment.