You’re right, that’s my bad. I was remembering the phrase “exclusively anaerobic metabolism” which is wrong too.
Still, “relies primarily on type IIa muscle fibers” is egregiously incorrect. But still all of those statements of fiber type by definition of size principle and mixed MHC expression means they cannot be universally true for everyone. I’m trying to correct what are important distinctions on a very misunderstood subject.
So yes that’s my mistake, so let’s strike “exclusively”, but even “most” or “primarily” or as you say “which fiber type predominant” shows a lack of understanding that impacts training methodologies. It all depends on the velocity and force requirements as well as the individual’s reaction norm to the training environment, which muscle, and where on the muscle we’re looking.
Just sticking with the article’s histochemical categorization, if we look at a spot and you’re using IIa fibers, you’re also using all your type I fibers. If you have 80% type I fibers, odds are no, you’re not using IIa fibers at tempo. If you’re 50% IIa fibers at threshold and you’re only using 75% of muscle mass, that’s still only 25% IIa fibers being used, or 1/3 of active muscle mass. So “relies primarily on type IIa muscle fibers” would require both a very very high % of IIa fibers, but also a large amount of muscle mass recruitment.
Besides that IIx fibers exist in only small quantities in humans. The huge majority of studies on endurance or strength trained individuals will show 0-3%, which makes it exceedingly unlikely they’re ever recruited. After onset of most training programs, most fibers that would be classified as IIx shift to IIa.
Muscle fiber classification is no longer “semantics” at this point because of how much misinformation is out there, or people relying on classification schemes from 1994 and not 2020. It’s been 26 years, we know better now. Histochemical “IIb fibers” are classified as “IIx” in the modern era if we’re to keep from confusion between species, which we very much want to do. If we run sds-page of MHC of rat muscle homogenate next to human muscle homogenate, we see three bands for humans and four for rats. The shortest migration band for humans matches the second shortest migration of rats’ IIx MHC. Rats will have a fourth, even shorter migration in a band corresponding to their IIb MHC. Now if we start plucking individual fibers we will find a range of MHC expression per fiber, but how much of that is hybrid I/IIa or IIa/IIx, or if training shifts fibers to a more focused MHC type over another is very much still up for debate. But still no IIb anywhere to be found in humans.
Somehow, I don’t think trying to educate people is contrarian, but I am hasty and tactless sometimes and I apologize for that. I hope this clarifies things.