I am under the impression that popular media says “zone 2 AND hiit” and NOT just zone 2 as this post says.
ISM says that zone 2 is the optimal intensity for building mitochondrial function and fat oxidation, and high intensity is needed to train the glycolytic system. For example, he starts around the 1 hour mark here:
I’m not sure how this has (or should have) much relevance for a cycling training community.
ISM is wrong.
Directly from the abstract “ We conclude that current evidence does not support Zone 2 training as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity.”
Too many people fell for his claim that substrate oxidation is equivalent to aerobic adaptation when it is not.
« Zone 2 training for members of the general public ». The study is not aimed at cyclists.
It is relevant to anyone who incorporates z2 in their training but more relevant to low volume cyclists. It also does a good job of summarizing much of the existing literature and that is relevant to higher volume cyclists as well.
A good point the article makes is what I also quoted above regarding mitochondrial and fatty acid oxidative capacity. That is definitely relevant to anyone who trains.
Did some of you find the study online? I can’t find it for download unless you have academic access.
Classic “zone 2” is just endurance riding. The more the better. There is little controversy about that.
ISM identified an area around 2mmol of lactate as the his zone 2 which for everybody else is tempo. Maybe his the same as fatmax? It’s just spicy endurance.
My guess is that you could do 10 hours of endurance or 8 hours of endurance + spicy endurance intervals and probably reach the same fitness. Pick your poison.
Pogacar is the only world beater using this method (if he’s even using ISM’s methods still). The rest of UAE didn’t make miraculous transformations.
Here’s the .pdf if anyone wants it but is unable to download. I have academic access…
s40279-025-02261-y.pdf (1.1 MB)
So reading this, I don’t think there’s anything surprising or counterintuitive. They are challenging the idea that Z2 is some magical mitochondria-stimulating thing, and that you should ONLY do Z2 for optimal mitochondrial function. I think we all know this is not correct.
As I understand it, the purported benefit of Z2 is that it provides some aerobic adaptation while being sustainable and easy to recover from. Of course you still need high intensity sessions. Quote from the paper:
“Conflating the training habits of endurance athletes and the optimal exercise dose for improving mitochondrial capacity in non-athletes may be misplaced for two reasons: (1) endurance athletes perform high volumes of both low- (Zone 2) and high-intensity training [12, 13, 16], making claims of a causal relationship between low-intensity training and mitochondrial capacity tenuous and (2) the total training volumes undertaken by endurance athletes, often > 20 h per week [17], are substantially greater than physical activity targets set by public health guidelines [18]. These caveats make it challenging to confidently infer that Zone 2 training is optimal for eliciting improvements in mitochondrial capacity, especially in populations performing total training volumes consistent with physical activity guidelines (i.e.,~150 min per week).”
Translation: 3h/week of Z2 may not, actually, make you Pogacar. 
Z2 vs intensity is like asking should you just eat carbs or protein, you need both.
The ratio just depends on how much volume you do. But simple boring truths just don’t generate clicks the same way.
Yeah that was my point.