I'm going to probe those statements a bit, mostly in the hope that you -- or anyone -- might be able to provide me with more information on the subject. You and I have discussed this "all roads lead to Rome" business in the past. And that was very valuable to me as it led to a bunch of subsequent research and learning.

FWIW: I ride for health benefits alone. Any improvements that I make to my cycling performance are secondary.

There are good reasons to believe that improvements in the Zone 2 energy system may have significant health benefits.

As with anything physiological, it seems reasonable to expect that the best way to improve the Z2 energy system is to tax it. And, if one is at all short on training time, to tax it maximally. That, usually, is how physiological adaptations are encouraged.

It is clearly the case that all of the body's energy systems are in play, in some proportion, at all levels of energy expenditure. All roads do lead to Rome in this sense.

What is not clear to me, is whether or not the Z2 system continues to be taxed maximally at levels of expenditure above the Z2 threshold.

As an example, I consider the top of my Z2 range to be about 180W. If I ride at 280W, does that mean that I'm doing 180W Z2 and 100W glycolytic, as I would wish? Or might it be 60W Z2 and 200W glycolytic which I would consider to be a problem?

Regarding the fragility of Z2, it is my understanding that, while one can bounce in and out of Z2, the rebound back in would take some time. I feel as though I recall reading something to the tune of 20min - 30min. Someone ought to fact check that however.

This makes sense to me since most of the body's signaling systems are based on the accrual and depletion of chemicals. Somewhere, there's a reservoir of something that builds up when you ask for more energy. As that reservoir fills, it triggers the greater involvement of the systems that produce energy more quickly. Surely, reversing that process is not an instantaneous thing.

Whether or not this kind of fragility affects one's riding choices is probably a function of the kind of ride being undertaken. For example:

1) Six hour ride with a single, half hour climb in there someplace? No sweat, most of that ride will still be maximal Z2.

2) One hour training ride. A climb at the beginning may, effectively, make it so that almost none of the ride is maximal Z2. Most of my rides are under 90 minutes. I address this by sticking to Z2 heartrates for all but the last 15 minutes or so.

3) A longer ride in the mountains. I live in the Rockies and there are plenty of rides around here where you're basically climbing or descending 80% of the time. Given a 20 min - 30 min lag time in getting back to Z2 once you've gone significantly beyond it, one can imagine rides where you wind up spending almost no time maximally Z2. I address this by using a bike with a very low gear ratio that allows me to Z2 spin my way up pretty much anything. Then I'll strategically choose a few hills to climb where I do ride at a higher intensity but I have enough space around those big efforts to spend some meaningful time in Z2.

I've been wrestling with sorting this stuff out for the better part of a year. My intent here is to facilitate further discussion, not have my comments interpreted as being authoritative in any way. I'm definitely a physiology tourist.