Protocol Online logo
Top : New Forum Archives (2009-): : Cell Biology

Muscle Fiber Types are FICTIONAL, or is it not? - (May/16/2013 )

Hi everyone,

Can difference in muscle fiber types be actually physically seen if muscle being cut and put under microscope? Muscle fibers will not look the same for sure (size, color, etc), because of their different capacity of motor neurons, mitochondria and capillaries, etc. But does this give a right to separate muscle fibers into types?

As we know: If muscle is stimulated in particular way (growth or endurance training) - organism will adapt by raising or lowering the number of mitochondria, capillaries or motor neurons in order to perform in given specific external challenges. Hence, if you need more power to lift more weight - body has to grow muscle size, in order to do that it needs fast muscle contraction, which is achieved with bigger impulse coming from motor neurons. That's why it starts to add more motor neurons (hopefully it increases size as well). On the other side (for endurance training), body requires more oxygen, which comes with capacity of capillaries, myoglobin and mitochondria. Mitochondria also provides energy/ATP.

Therefore, there are but one muscle fiber, which change because of required adaptation. It is just that capacity of all functional important parts of organism affect it's shape and color of appearance. In this case created TYPES of muscle fibers (I, IIa, IIb, IIx) are there just for easier theoretical navigation, like 2+2, or is it not? Like Type IIa muscle fiber is in-between type I and type IIb. But what if muscle fiber is in-between type IIb and type IIa, or even somewhere higher or lower (if you know what i mean here). Then there can be type IIc, IId... and on. Thus, because of the wide spectrum of available capacity levels there can be many more muscle fiber types. Or is it not?

This is only my thinking and opinion. I would like you to correct me or prove me if I am wrong. As I am not sure to 100% here.


Yes. Their are definetely different muscle types. We can successfully classify them as type I or II because each one is mechanically and functionally different. For instance, Type I are classified as slow twitch muscle fibers. This is because they are used for long duration contractions. They are vascular and have the capacity to produce the required energy for contraction at that particular site. You can also classify them on a molecular basis. Type I contains Troponin I, the protein that binds calcium and causes a shift in the tropomyosin filament. Type II is the opposite. It is highly anaerobic, used for explosive movement and contains Troponin II. Their are many other differences between type I and II, but you get the idea.

The further classification into type IIa, IIb, etc... is also necessary. These types have specific functions that do not totally conform to "just type II," requiring them to be a subset. If this does not make sense, think about epithelial cells. The distinct between the types of epithelial cell is minimal, but a distinction must be made due to their function and overall structure. These two concepts are very analogous.

I suggest this book. It will clarify all of your questions. If you do not want to buy it, I believe I saw this book on a pirate ship...

Edit: Dead Link


Thank you for your reply and suggested book to read. Unfortunately, for some reason, I am not able to open the link you provided for this book (error of some sort). Could you write title of it please?

Furthermore, regarding fiber types: various people are born with specific amount of different muscle fiber types, but they are able to convert one to another, aren't they? Conversion does change not because of the changing types, but all organic units (I mean mitochondria, capillaries, etc.) that are required for this specific type to function in its specific way it is converted into. In other words, it does not convert the type, but it adds/removes those organic units, which makes it to be called this kind of type. To be more precise: organism does not convert type I to type II, but it converts fiber in the way it can be called type II. Am I correct? .

I think I kind of got it. Thank you.

Moreover, About this type IIa, it is in-between of other two, which have each part of both. Can it be called a hybrid type in the way? And why there is another one called type IIx (IIa is not enough being different?) and can there be more than already known types?


There is no conversion between types. The types are defined base on there molecular composition, resulting in different functions. Each muscle in your body can either be composed solely of one type (sartorius muscle) or they can contain a combination of the two. Every Homo sapien is born with a relatively consistant number of muscle fibers. Depending on your demographic and centuries of evolution, you may have more of either type of muscle.

They are not hybrids. They are subsets. Think of the analogy of epithelial cells. Each subset is different and needs to be classified as such.

Is this alright?

Boron and Boulpaep, Medical Physiology


Thank you very much.