T Nation

Fiber Type Confusion


My professor at school believes that you can train your type IIb fibers and actually train to increase your type IIb. He acknowledges that aerobics and endurance training results in IIb-IIa shifts, but denies the fact that all types of training seem to increase the type IIa fibers.

I need some help. I've looked up several studies on isoform conversions and adaptations with humans and rats, and he still doesn't believe me. He says that the subjects in the studies aren't training intensely enough and that's why they shift to IIa.

He also denies the fact that sedentary and older people have more Type IIb than many trained athletes.

I have two questions:
It is well known that Type IIb fibers are the largest and have a potential for great growth, so at what point are these fibers converting to Type IIa? It seems like these IIb fibers are hypertrophied and then are converted to a bigger version of Type IIa fibers a couple weeks later?
When researchers take biopsies are they looking at a Type IIb that has the mitochondrial density and enzymatic capabilites of Type IIa, or are they looking at a fiber that is identical to the IIa without any trace of ever being a Type IIb?

My professor insist that it only looks like there's more IIa fibers, because they have hypertrophied and take up more CSA, in reality the same type IIb fibers are still there unchanged.

Can someone with more firsthand knowledge than me explain this clearly so that I can pass it on to my out of touch professor. Thanks!


Anyone??? Cressey? Berardi? Anyone out there


So let me see if I have this right:

IIB or not IIB, that is the question?

Sorry, I couldn't resist. Its been awhile since I delved deep into the semantics of fiber conversion. However, I seem to remember Waterbury talking about it. I would have to suggest that you ask him this question.

Take care,



2A and 2B are the strength fibers vs. power fibers, right? becasue i thought endurance fibers were like type 1...or maybe i'm just really sleepy.


I'll get to this ASAP.


IIb are the largest and potentially the most powerful. IIa fibers have the enzymatic qualities of IIb and type I. One lingering question I have is once the IIB convert to IIA do they lose some of that explosive power? Or, do they simply take on more aerobic capability associated with IIa?

Also, if all you did was 1RM training... or 1-2 rep explosive training, and took 4-5 minute rest periods, would that still stimulate IIb conversion or would you convert IIa to IIB. Everything I've read in the literature seems to point to converting towards the A isoform not the other way around (except for when you detrain or take time off that is).


IHGM, why not ask the prof for his references? Sometimes profs pull out the most obscure sources that we'd never otherwise find.

Then again, others simply talk out of their ass.


Ok.....time for Powda to go back to last semester and physiology of exercise (we're not all idiots at Arizona State by the way).

There are many types of muscle fibers; but you can primarily classify them as
1) Type I - Slow twitch, Slow Glycolytic, Red muscle. They are characterized by a high mitochondrial count and a large aerobic oxidative potential. They also do not have very high intramuscular glycogen stores. They about 110 msec's to "fire". These are basically for activities that last for I think over 90 seconds
2) Type IIA - Intermdiates, less mitochondrial capacity.
3) Type IIB or more recently called IIX - fast glyoclytic, white muscle (low blood supply) low mitochondria, high anaerobic (C6H12O6) Oxidative potential. "Fire" in 70msec. These are our powerful motor units.

Also remember that when recruiting Type IIB we are also recruiting Type I and Type IIA moto units. Interestingly enough in periods of long bouts of exercise requiring type IA muscle fibers you start recruiting higher motor units when you near fatigue.

Ok...now to answer your questions to the best of my ability:

It has been shown that there is a shift in the curve from Type IIB more units to Type IIA motor units following exercise. This is quite understandable actually...if you consider the old (non-Waterbury, average individual) doctrines of exercise. Most people don't train for power; they train with 3x8, or higher reps. This relies on the tpye IIA not type IIB motor units b/c the exercise lasts in the 30-90 second range normally. Figure 10 reps..2 seconds down..2 up...that's 40 seconds. At maximal exercise your body starts giving out after 5-10 seconds (try a Windgate test some day).
Thus it's more efficient for the body to have more Type IIA motor units b/c that's what's being trained and I would imagine that's what the study participants were doing. I doubt they had them powerlifiting.....
I haven't read the study though so this is hypothetical.

Your other questions I am unqualifed to answer.



Damn, just when I thought I was going to learn something about soluble vs insoluble dietary fibre... :-s


Here's the deal, only one study (that I know of) demonstrates an increase in the Type IIB(X) population. This study was performed with short, high intensity bouts of sprints. Here's the reference:

Jansson E, Esbjornsson M, Holm I, et al. (1990) Increase in the proportion of fast-twitch muscle fibers by sprint training in males. Acta Physiol Scand; 140: 359-63.

Unfortunately, no one has been able to replicate the results - and that's not good.

A general statement can be made regarding fiber types: the more they're recruited, the more endurance characteristics they'll exhibit. I hate to make that statement, but based on what's been published (up to this point), that's the case.

Actually, it makes sense, if you think about it. When we recruit a certain pool of MUs on a consistent basis, we're telling our body that they're important for our needs. As such, the body responds by upregulating metabolic qualities that allow for faster recovery (ie, more oxidative characteristics).

But, I'm not completely convinced that we can't increase our type IIB/X pool since many power athletes demonstrate huge populations of type IIB/X fibers (relative to normal). The question is: Were they born that way, or did their training enhance the type IIB/X fibers. Since NIH doesn't consider the answer integral to curing disease, no one knows.


Chad is right on the money. To add a few more things in the context of your questions:

In these studies, this problem is more than alleviated by the fact that the athlete's fibers are much larger and neuromuscular coordination is dramatically enhanced (just because you have the IIb doesn't mean that you're efficient at recruiting them).

The only way to increase IIb fibers is to detrain. This is one reason why some people have hypothesized that the way to create the ultimate sprinter is to train them on Earth and then send them into outer space (or put on bed rest) for a short period of time to allow for conversion back to IIb. In an ideal scenario, neuromuscular proficiency and muscle size would be preserved in this short period while optimizing fiber qualities. Any volunteers? :slight_smile:

I would encourage you to read everything from Robert Staron (Ohio University) that you can find on Pubmed.


Yeah I've read a bunch of Staron and everything pretty much points to a IIb- IIa shift. They even trained subject intensely and used a 1RM max effort day. The results... Hypertrophy in all fibers and a substantial shift from IIb to IIa fibers in just 2 weeks!

The detraining is a great method. I've recently detrained because of tendonitis and a ligament injury from power cleaning, man I'm making some great progress now, in both size and strength. It's just been 2 weeks since I've been back too.

I really want to know if the fibers, after shifting, show any signs of ever being a type IIb. Or, are they identical to IIa in every way. Does it appear as if IIa comes out of no where? Can a gradual shift be seen in intermediate isoforms that show the slow transformation in enzymatic qualities
I like the recovery theory, the muscle needs the mitochondria as well as the anaerobic capacity. This explains it in my book.