I see your 15 minute VO2max interval, and raise you a 20 minute VO2max interval. 
Anyone that has gone out too hard during a 10 mile TT then tried to hang on for dear life knows this feeling! (Pretty much everyone!?)
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@GoLongThenGoHome that is VERY interesting! Iâve only ever experienced that at intensities below ~lactate thresholdâŚabove that intensity the âlong componentâ of VO2 holds primacy for me. HR always goes upâŚthe only question is how fast. Ha!
Iâm not sure why you see the HR response that you do but itâs fascinating.
Search for âVO2max verification.â Youâll find this paper as well as dozens more stretching back to the 1990s.
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I have read that the body is always trying to optimise things. Whatâs the least amount of energy it needs to use to complete a task?
Now it maybe when when my brain receives the signal from the muscles âWe need more oxygen etcâ it employs a series of strategies. So first of all it tells the heart to beat faster. But then I start breathing more efficiently and bringing more oxygen in per breath. It means my heart doesnât need to beat as fast to deliver more oxygen.
Does the brain ultimately know how much oxygen the lungs can exchange per breath given the rate and force of the diaphragm etc? We clearly have multiple inputs signalling the brain. I ultimately see a sub conscious calculation going on somewhere , where it is measuring the feedback loops on muscle oxygen use, oxygen / blood exchange in lungs, and HR and stroke volume. Then it decides what to increase or decrease.
A bit like the ventilatory thresholds. You know where your breathing changes? Initially just increasing HR was enough and then you really need to ramp the breathing up. Well as those thereâs also those times where you start breathing harder initially when you up the intensity but then the breathing settles down. Maybe as the breathing settles it gets more efficient, and therefore Har can decrease and the muscle still gets some amount oxygen.
Other thoughts. 
Ok, I take it back. Half a decade later, turns out I CAN go on like that for about 15 minutes, sort of accidentally, no special equipment needed.
During a PR 5000 on the rowing erg, my HR reached >92% of max at about the 3min 40sec mark. The entire effort took a couple seconds over 18min. For 14.5min my HR averaged ~97% of max. Somebody else pointed this out to me & I have to concede itâs true.
Although if you had asked me to do that just for the sake of doing itâŚpsychologically, it would probably have been too big an ask.
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Arent Max HR different across different sports due to muscle engagement. I can imagine that max hr for rowing is significantly higher then max hr for cycling. And atleast that threshold HR will be significantly higher for rowing (like it typically is for running).
not to say that wasnât a killer workout.
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That makes sense to me. For instance, I would expect the maximum heart rate you could achieve by pedalling hard with ONE leg would be less than the maximum heart rate you could achieve by peldalling with TWO legs. Or, taken to the absurd, youâll achieve a higher heart rate cross-country skiing real hard than you will by doing index finger curls as hard as you can.
But, for me, there is not that much difference in HR between rowing and cycling.
Yesterday I made an all out 2 hour power attempt & for the final 40 minutes of the attempt my HR averaged a little over 94% of max HR. So very nearly the HR intensity of the rowing attempt at what was a much lower relative intensityâŚand no sprint at the end. I just think that, for me, there isnât much difference in max HR between rowing and cycling. (even though I agree that kind of defies intuition)