Not sure what you are disagreeing with. Point I am making is that with a power meter you can prevent the case that I am describing in the next sentence.
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This is my thinking from combining bits and pieces of information over time, but happy to be explained otherwise:
VO2 max is when you are consuming the most O2 and converting that into power. That happens clearly at a level above Threshold and is very roughly at 120%-125% of Threshold power.
If you go very hard at the start and you fall back to Threshold power towards the last intervals, then you are not at the maximum rate of using O2 turning it into power. You would still be breathing much harder then normally at Threshold power, as if it was a VO2 max effort, but that would be more related to the build up of CO2 and your body wanting to get rid of it than the O2 requirement.
I’d tweak that slightly and say that it’s not that VO2 max equates to 120-125% of threshold, but that that’s a good number for doing VO2 intervals as it’s hard enough to get you into a state of VO2 max fairly quickly, but “easy” enough to allow you to stay in that state for a sustained interval. Doing 110% of threshold would get you to VO2 max eventually but it’s going to take a while so it’s not very efficient training. And 150% of threshold is going to get you there very quickly but you can’t stay there for long. So the purpose of hard start intervals is to go off a bit harder than 120-125% to get you to VO2 max faster, then drop the power to enable you to sustain that state. And as a happy coincidence, going off hard then settling into a more sustainable power correlates very well to a lot of race situations.
Interestingly on a recent podcast Chad said that you should be aiming to work at a high percentage of VO2 rather than VO2 itself. Might need to try and find that and go back for another listen as that was news to me!
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Hypothetical question: 12x2min at 125% vs 5x5min at 115% in general? And for rider with high anaerobic contribution specifically?
EDIT: I am trying to decide which progression to follow for upcoming block.
Yep, 1:1 ratio. But I guess your reply to @cartsman post actually already gave me answer – I personally spend more time at Z5 HR with longer intervals.
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I’d say it partly depends on what kind of racing you’re going to do. E.g. if you’re doing events with hills that are going to take >2 minutes to climb, I’d do a progression which extends the length of the VO2 intervals. Whereas if you’re doing punchier races where it’s about accelerating and recovering then those shorter harder intervals are more specific.
If there’s no particular event in mind, then as a more anaerobic rider I’d be inclined to do the longer intervals to nullify your anaerobic contribution (you can “cheat” your way through shorter intervals with a high anaerobic contribution). Other options would be the hard start intervals and/or keeping recoveries quite short.
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Thank you. I don’t race at all, only doing ultra-distance stuff (24h+) for my own pleasure. VO2max is just “necessary evil” to raise the roof 
Don’t forget the option of 3 or 4 8-minute intervals with a 2-minute hard start. Hurrah! 
Ok, challenge accepted, week after next will go for it, PL-wise it should become slightly less Stretch workout 
(2min at 121-125%, 6min at 106-110%, 5min recovery)
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if I did the math correct ((239*.2)(260)+(239*.1)(660)) each interval draws down 14.3kJ from your anaerobic ‘battery’ and if you have a small enough ‘battery’ then you might fail to complete the first interval. And if your ‘battery’ is say 10kJ, then it would be a bad interval design, for you.
Thanks for that. So I was doing a bit more digging and this is what I make of it: I think that VO2 max is tied to a particular theoretical intensity, which is the power produced when using max amount of O2 without any power being produced anaerobically. It is just that the power produced is always a combination of aerobic/anaerobic, if you are fresh and just at that theoretical VO2 max power, there is still a considerable amount of power produced anaerobically. So you would not be at max O2 consumption, but would get there eventually when you get tired and anaerobic power decreases. This then ties in to your explanation that picking 125% makes anaerobic power decrease sufficiently fast to get you to VO2 max in a reasonable time. Also it is probably a good amount above the theoretical VO2 power, such that anaerobic power does not need to fully come down to zero in order to achieve VO2 max.
Had to swallow big chunk of humble pie: got new AI FTP with accompanying PL drop, grabbed lowest productive PL VO2max workout to get taste of 8-min intervals (Ryan Peak, 106% of FTP, no hard start) and managed only 2 intervals
Need to do some work before even thinking about hard start. Anyway, it revealed weakness (muscular endurance) that may make it less beneficial at current fitness level to chase 8min intervals. Duration in Z5 HR for recent VO2max workouts:
- 6x2: 84s
- 4x4: 241s
- 5x5: 727s
- 2x8: 71s
Considering drift, probably can’t compare directly but still, it seems 5x5 would give biggest bang for buck (for me personally at current fitness level)?
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The 4x8min, maximum effort across all intervals prescription is brutal.
You are simply aiming at maximum time in zone.
Many, get the progression wrong.
Do not be attached to 8mins. It’s arbitrary, it means nothing to our bodies.
Just aim at more time each session.
For example…
Week 1
2x8
Week 2
2x8
1x4
Week 3
2x8
1x6
Week 4
3x8
Add time in any logical manner. There is no failure if you can’t do 4x8 etc. Some find V02max intervals much harder than others.
Personally, I think I’ve managed a ‘perfect’ 4x8 a total of once. I’m a very anaerobic athlete and can easily cook myself going even slightly too hard in V02max intervals.
Get after it…
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Thank you, seems reasonable. Although, I am actually not chasing 4x8 per se, just trying to figure out most beneficial “time at VO2max HR vs time spent” progression for coming 3 weeks (VO2max twice a week) before going back to base phase.
Came up with (Tue/Thu, Sat Z4, rest is filled with real low Z2)
w1: 5x5 110-112%, 4x6 112%
w2: 5x5 112-113%, 5x6 112%
w3: 5x5 113-116%, 6x6 112%
Will adjust on the go, depending on feeling/recoverability. During following base, doing on the bike only Z2+Z4 and continue experimenting with different VO2max intervals on the SkiErg. I guess there it will be totally different as more muscles are involved.
I recommend Paul Laursen research see his You Tube and literature. I think longer threshold workouts, ie, several minutes of sustained high effort have their place. But his R&D indicates high reps of very short but intense HIIT are better, eg, 5 sets of 20s/20s x 8 +4min inactive recovery