Here’s my theory: your heart may limit your VO2max, but it’s what your muscles can do that determines how often and even whether you can get there. Hard-start intervals get you to VO2max more quickly, but you burn out your muscles more quickly as well, and can’t do as many efforts. I’ve always figured that if you want force the system to adapt, you need to put it in extremis over and over and over and over and over and over again. Maybe I’m wrong about that, but it seemed to work for me.
Just wondering if you do an outdoor 6 × 3mins V02 but you are not hitting the power targets you know you can but you are obviously near max HR. Is this workout still beneficial. I’m talking about 10 to 15 watts short on what you can do.
Thanks!
Correct. When I wrote about this previously (above and elsewhere) I was not familiar with any literature showing longitudinal efficacy of hard-start/fast-start protocol on physiological (eg. VO2max) or performance (eg. TT) outcome measures. So it was a hypothesis/assumption that I (standing on the shoulders of giants) felt - still feel - comfortable with. Although I’m intrigued by the possibility that not all VO2 are equal for sport-specific adaptation.
Recently Ronnestad’s group has published more work on the subject but still I don’t believe there are any longitudinal interventions. It may be out there and I’ve missed it. If anyone knows, please post!
A recent presentation that has been linked here before
And two of the relevant studies
I am aware of at least two other groups that are at various stages of investigating the same question, so we’re in good company!
Jem
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Sorry for slightly hijacking this thread for my own interests… but the great discussion here inspired me to dig a bit more into the literature and synthesize some ideas about how metabolic intensity and mechanical workload might relate to VO2max trainability. Hopefully relevant to the conversation!
Quickest 3000 words I’ve written in a while 
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Empirical Cycling concludes the same thing on his recent VO2max podcast:
Historically much of the literature suggests that cardiac output , and more specifically stroke volume (SV) is the primary limiting factor
Having a few weeks of 80/20 training under my belt, I can say that it’s not as easy as it might appear on paper, mostly due to having to go HARD! for those hard sessions. It’s not 120%, it’s Hard% FTP. 
The subjects in both groups “ were instructed to perform intervals with their maximal sustainable work intensity, aiming to perform highest possible average power output during each interval session ” (Almquist et al, 2020)…this better reflects real world training, where an interval workout is typically performed as hard as possible, or close to it.
Also wondering if the training/adaptation benefits would be the same if the interval sessions are mixed. Instead of doing 10 weeks of intermittent style, what would result from doing 10 weeks of intermittent and hard-start and long/steady intervals? 
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I’d really like to know that too. I’ve been mixing them up lately. But my above threshold power is so bad right now that anything would be helpful. 
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One of my colleagues was supposed to investigate something like this over the summer, with a mix of HIIT (4+ min ‘classic’ VO2max) and SIT (sprint interval training; ie. 20-30sec all-out, ~5min recoveries). As a follow up to his meta-analysis.
But then COVID…
I’m currently writing a proposal for a fake-study design (in a fantasy land where I had all the time, resources, and willing subjects ) to compare hard-start to evenly-paced training.
I do think - and this is totally speculative/opinion - that mixing interval types within microcycles should be able to optimize training load across different limiters; central vs peripheral, etc. At the end of the article I give an example of performing a 30/15s (or SIT) workout to smash the legs on Monday, then say on Wednesday before the legs are fully recovered, add a hard-start long HIIT workout to give the cardiovascular system a bonus stimulus, while the legs can still handle the lower workload.
I’ve been doing this for the last 6 weeks w/ 2 VO2 sessions every 8 days - alternating 1x LIT, 1x Intermittent.
I’ve always done “classic” VO2 interval and find I respond better (acutely) to longer intervals than the 30/15 type. Six weeks in and I’m finding the same, my body is preferring (and "excelling"at) the longer 4-5min intervals (hard-start, declining power) vs doing 60-80-100 sprint style repeats. Perhaps that comes down to my individual muscle composition more than actual cardio factors. Accumulated fatigue might also be playing a role in executing short/short workouts “as hard as possible” vs just sustaining power. Definitely during the first couple of weeks the intermittent sessions were great for physical and mental variety but I couldn’t tell you if they have been more beneficial than the LIT sessions.
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phenotype has to play a role here
I suck at Tabatas and other intermittent intervals – I just can’t generate much power in 20-40 seconds. My power from 1 minute on down is pretty terrible, and always has been. I’ve not used INSCYD, but I must have a pretty low VLAMax. I suspect riders that are more on the high side of that will perform better and maybe adapt better to that kind of work.
4-8 minute efforts are much more my joint. but, I do better grinding them on the hills than spinning. my local VO2 hill is a mile at 6%. I get my best power standing for about half the interval and doing the whole thing in the big ring. I’ve been doing that hill for eight years, and I’ve tried spinning and grinding, and I’m a grinder.
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The podcast I just listened to below speaks to all of the information in this thread really well. I found some of the rest periods on the TR workouts a little too long, and as such, have opted to create my own with different rest periods to those on TR to target different systems etc.
I think if you start adding in energy systems into the interval discourse alongside power output etc. you get a more holistic view of the intervals and factor in an understanding of what you are trying to achieve.
Check it out. Great listening.