T Nation

My theory on fiber conversion, whats your opinion on my opinion?

I believe there is not really a fast twitch IIa or IIb, just a fast twitch fiber. When you train your body’s fast twitch over time starts to form mitochondria and capillaries. So people say that your IIb fibers begin to convert to IIa. I believe it was the same muscle fiber all it did was grow mitochondria and capillaries so it didn’t necessarily convert to a different fiber. The same with IIa fibers, when you detrain(completely stop) or start using less and less sets or reps, your body has no reason to hold all the mitochondria and capillaries so it begins to get rid of it. Then people say your IIa converts to IIb, but I think your fast twitch fiber just got rid of the mitochondria and capillaries. I think labeling fast twitch fibers like IIa and IIb is just an easier way of labeling your fibers then explaining how your fast twitch adapted to the training. Ya’ll probably could care less about this stuff, do ya’ll think this might be correct?

DJM…Actually, I not only think that your opinion on this subject matter is incorrect, I know it is.

DJM, have you a thorough understanding of muscle morphology, how fiber types are labled, and the like? Have you studied this in your lab? Have you reviewed the peer-reviewed literature and extensive texts covering this topic? What is the basis for your theory?

I know I’m coming across quite abrasive, DJM, but you can’t just “come up with” an opinion or some theory on something that’s so well-studied and known.

The reason I came up with this theory is because I do not see how a IIa can convert to IIb or vise versa. So I figured they just named a fast twitch fiber IIa if it had a lot of capallaries and IIb if it did not which through training the fiber can lose or gain capallaries changing to IIa or IIb, but it still being the same muscle fiber. Can anyone explain how a muscle fiber can change?

I dont think that you actually lose the mitochondrian, just that it atrophies with lack of use. Charles Poliquin stated in his website, that IIb fibers will take on the properties of IIa or even Type I if given enough time. I.E. becoming more oxidative and less prone to hypertrophy. If you think about all the survival mechanisms of the body, it would lead you to think that the body would keep your Type IIb’s in case they are needed.

I have to disagree. With all the years of science and tests put into this subject, I don’t think they would label them as diffrent just to make it easier.

DJM, again, you cannot simply come up with theories without any scientific evidence or proof. To me, it seems like you’ve done little to no actual research.

Here is a short excerpt from Designing Resistance Training Programs by Fleck and Kraemer:

Unfortunately, I have not the actual references on hand, as I currently only have a Xerox copy of this particular chapter.

Here’s an excerpt from later in the chapter:

Let’s see if I can chip in before Timbo pulls his hair out!

I’m going to assume that you know that there is little evidence to suggest that training of any sort can convert Type II fibers to Type I (or vice versa).

That said, let’s consider for a moment the (sometimes subtle) differences between Type IIa and IIb that are most applicable to this discussion:

Type Iia (fast oxidative glycolytic): intermediate/low quantity of aerobic enzymes, high quantity of anaerobic enzymes, intermediate capillary density, low myoglobin count, and intermediate mitochondria density.

Type Iib (fast glycolytic): low quantity of aerobic enzymes, high quantity of anaerobic enzymes, low capillary density, low myoglobin count, and low mitochondria density.

Here are your skeletal muscle adaptations to aerobic exercise:

-Increased myoglobin content
-Increased mitochondrial density/volume
-Increased oxidative enzymes

The human body will always seek to adapt to a given stimulus in order to meet the demands placed on it. In this case, it does so by increasing its oxidative capacity via these three mechanisms.

If you’d like to read more, a good journal article to check out would be the following:

Andersen P, & Henriksson J. Training induced changes in the subgroups of human type II skeletal muscle fibres. Acta Physiol Scand 1977 Jan;99(1):123-5.

Hope this helps.

ryan, yeah I reread what I said, what i meant was by saying type IIa just meant more of the capallaries and IIb meant less of the capallaries but still the same muscle fiber.