But the HR is stable AND high and within the range which I have measured in lab and not maxed out or depressed.

You mention that I don’t get the training response I am looking for (although proxies such as hr and power is spot on). What response / stimulus do I get then?

How do you know your HR is not depressed? I'm not saying your lab results are incorrect, but max HR is not the same every day.

As an earlier post says, I would think you lowering your cadence would start to train vla (how efficient vs explosive your body is). Lower RPM intervals lowers your Vla and turns fast twitch/explosive muscles into slow twitch/endurance muscles. So, again, you're "fading" in the workout which is seen as your cadence lowering and you recruiting more of the muscular system.

Are you still increasing your Vo2? I'm not sure, honestly.

The first thing I'd do then is go look up AV Hills' definition of vo2max. IIRC he came up with that in the early 1920's and it's still spot on today. There is a part your missing in your posts that imo vo2max work should be targeting

I do not have my athletes do vo2 work for maximal O2 uptake. I want a different response bc that's the limiter in performance, not O2 uptake. Now I'm sure some coaches will argue with me but O2 uptake is rarely the limiter in human performance


BINGO


You are probably getting the responses your looking for but that's not the response you should be looking for. Conky gave you a HUGE hint here

Since you said you want to better understand the physiology I'm not spoon feeding you the answer. Yet you yourself pointed out a solution in post #11

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Brian Stover USAT LII
Accelerate3 Coaching
Insta

After reading this thread and attempting to evaluate my own Vo2 intervals (when i do them), I think they are just really hard intervals... they are certainly Vo2 power at around the 115-120%, but my HR might only just touch the 92% mark for a few seconds towards the end of a rep, and not even get that high for the first few. Based on a 3 minute 1:1 interval. I'll hold my 90ish cadence throughout.

But im not really sure how im going to adjust them going forward... getting my HR to pop up on the bike has never been easy for me, with running it will pop up to the roughly expected HR zones for the pace, but with cycling I feel like the muscles fail before being able to push the heart and lungs... for example stretching out those 3 minute intervals to 4-5 would be really really tough, and id fail later intervals almost certainly. If i dropped the power so i could stretch them out that bit longer, then I dont think the HR would get to where I need it either. I think ill need to either have harder starts to the interval in an attempt to spike the HR before then lowering to a more sustainable level and holding on... or lower the power slightly to say 108% and try to go even longer to maybe 7-10 minutes.

I had a quick look over my strava last year and only 1 session did i ever hold 92%+ hr for more than a few seconds (despite many sessions feeling like i was going to fall off the bike) .. and that was on a really steep and long local climb, about 1 hour 20 into my ride, where it was a case of push the pedals or stop.. i averaged 122% for 12ish minutes, with my HR hitting 92% just over 3 minutes in... but my cadence was 71 which is way below where i usually ride and it felt more like a weights session than cycling, so i assume i was tapping into the muscular system far more than ideally to train vo2. That was a max effort tho and i had to tap out at that point half way up the hill.

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@the.lazy.triathlete

https://www.strava.com/athletes/18691068

Not answering your original question, but what I think is happening (as I've seen it myself) - when you are in ERG mode the "smart trainer" is trying to keep the power steady, so when you slow your cadence it will increases resistance. However, it takes the trainer a few seconds (depending on the model) to detect and adjust to the lower cadence and so you are getting a small break of a few watts. That's why it feels easier as you gradually keep lowering the cadence.
I bet if you really worked hard and kept the cadence steady through the whole interval you would find that the avg. power for the interval is higher.
And conversely, if you start the interval with lower cadence and gradually increase it, you will end up a few watts higher for the interval than steady cadence.