VO2 MAX vs FTP training

well, to be fair, vo2max is pretty accurate in WKO4 or 5 these days.
Can also get it from INSCYD.
I’ve never used the lab setting, but heard that if you don’t have the same tester, the tests can be unreliable.
But yeah totally agree, FTP makes more sense for zones at threshold or below, then things really get interesting and should use WKO I-Levels!


I am no doctor or physio but a quick google shows plenty of published material discussing “Type IIB Fibers” :man_shrugging:

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They exist in other species but not us. We’ve known since 1994, thanks to sds page experiments, that what was thought to be IIb was actually IIx when human muscle fiber homogenate was compared to rat. Smerdu, american journal of physiology I think. Besides that, the article talks about certain fiber types exclusively doing some types of work. This violates the size principle, but also has training implications. :frowning:

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IIb fibers, or IIb myosin?

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Actually, there is an interesting discussion over on the intervals.icu forum about estimating MAP. Also, the latest podcast from Scientific Triathlon has Seb Weber discussing the new protocols that INSCYD uses for non-lab based VO2 max, FTP and other such parameters.


Thanks - That episode has been downloaded for a while, but not had the chance to listen yet. Will do so today.

It’s a new one – up this week.

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Yep, that’s the one. Time goes slowly when you’re homeschooling 2 kids and working a FT job… :ok_hand:

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This is the correct answer. Researchers discovered that humans have type IIx MHC (myosin heavy chain), rather than than the previously thought type IIb MHC. But the the muscle fiber is still classified as type IIb. So type IIb muscle fibers are made up of predominantly type IIx MHC. It’s semantics really, as the fiber type IIb still does what we thought it does, the biological makeup is just different than previously thought. It’s just a naming system. He’s just being a contrarian.

Here’s the research paper he was quoting: https://journals.physiology.org/doi/pdf/10.1152/ajpcell.1994.267.6.C1723

No it doesn’t. Did you even read it?
"relying almost entirely on type I (slow twitch) "
“riding at this intensity begins to recruit some Type IIa muscle fibers.”
“activates more Type IIa muscle fibers than lower intensities”
“relies primarily on type IIa muscle fibers”
“type IIb fibers doing most of the work”

Literally never once states the fiber types are exclusive. It describes which fiber type is predominant, which is correct. It’s a spectrum, and the proportion of fiber type activation will change with exercise type, which is what the article says. Stop trying to be contrarian.


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.


WOW! This thread went waaaay above my head, but thanks for the interest! LOL!

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Love the nuance here. Can I ask a basic question since we’re on the topic?

Do human muscle fibers exist along a spectrum of “slower” to “faster”, or is there more of like a bimodal fiber distribution that we call I and IIa/IIx?

Any fundamental references I should read up on? Thanks!

The fibers do indeed exist on a spectrum. There can be lots of fibers that are only I, IIa, or IIx. Intermediate or “hybrid” fibers exist, but it’s very much up for debate how many fibers are hybrids and where they generally shift with training. This is of course MHC. When you do histochemical staining, you’re staining for the ATPase domain, and it’s hard to get a sense of nuance there. For references, check out anything by Jimmy Bagley, but I’d be remiss to suggest anything on muscle physiology without mentioning Walter Herzog.


If you want to get in to 2020, nobody runs SDS-PAGE any more :wink:

This is a really useful answer, and addresses one of the huge misconceptions about the idea of VO2max power.
It’s also worth considering the relationship to your lactic acid production/processing. I prefer to define FTP as ‘the output at which you are producing lactic acid at the same rate as you can process it’. Therefore, above FTP, you begin to accumulate lactic acid, and below FTP, you can keep the amount stable.

Isn’t the pKa of lactic acid below 4?

All this talk of lactic acid is making my head hurt.

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Indeed - where’s a hydrogen ion when you need one… :rofl:

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…making my legs hurt

On eHarmony or match.com, looking to replace their ex lactate?