I also want to add I tried 2x5 minutes at 112% yesterday and it seemed to work out, although I was really digging in the last 2 minutes of the second interval. So I think my true VO2 Max lies somewhere around the 115% mark.
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Reducing the intensity is always the way to go here - what percentage of FTP youâre doing the intervals at doesnât matter: VO2max is a physiological state not a specific power output. Being able to spend as much time above 90% of VO2max is the crucial ingredient. Once you find that level you should extend rather than increase the power.
Studies have shown that the optimum time for VO2max intervals is 4 x 8 minutes so think about that the next time youâre gutting it out after 3 minutes!
Mike
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So youâre suggesting to just knock down the intensity a few percent to complete the workout? I donât know why I donât resort to this when Iâm in the middle of a workout, maybe because Iâm just broken by that point haha.
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Itâs always good to try to make an educated guess as to how hard you can go before you start. Learn something from what youâve done before and make that best guess.
The trouble with turning it down during the session is, as you suggested, that youâre probably too far gone by the time you turn it down.
Iâd say aim low and turn it up as you go but donât underestimate how hard the last interval is going to be just because the first one was easy.
Mike
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I would slap a âit dependsâ caveat on this, as in it depends at the %FTP you are riding.
Iâd love to watch someone attempt an 8 min 120% interval vs an 8 min 110% interval.
The standard issue seems to be 5x4 min @120%. Variations grow from there.
The optimasation of VO2max intervals is based around elevating your physiological response to >90% of your maximum oxygen uptake (VO2max) for as long as possible. Itâs up to you to find out what power output you can do 4 x 8 minutes at.
Shorter, harder intervals will have you spending much less time at >90% VO2max.
For me, with my FTP based on the ramp test, that level is at about 108% but I had to play around a little to get there.
There are other methods to extend the time at >90% VO2max: 30/15 intervals are very effective and Iâve just started reading up about descending 30/15 intervals which compensate for increased oxygen uptake as the intervals go on, by reducing the intensity of each step as you get further into the set.
Mike
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This is the main point of VO2max training.
If it takes ~90 seconds @~120% to enter true VO2 territory, then 8 min 108% intervals may not actually provide the same time @>90% VO2max than more intense and shorter intervals do, as in it might take a greater percent of that 8 minutes (slower rate) to enter true VO2 than it would during a 4min@120% interval.
Kind of the reason why you do TWELVE 30/15s vs ONE 4/4 â shorter/less intense intervals will take longer to stimulate the conditions required for adaptation.
Unless tested in a lab for true VO2max and conduct a different ramp test, interval levels are always going to be best guesses, at best.
A nice little test protocol with transfer equations for VO2max are presented here. Can be done nicely in the basement at home:
However, knowledge of VO2max still doesnât tell what the best way for training is. This notion of âmaximizing >90%â is not without dispute. A Coggan is one of those questioning it.
From a practical point of view Iâd say itâs actually more important âjust to do itâ instead of wondering about the very specifics. Many people actually fail with not doing these workouts at all because they hurt. A lot.
Not a big fan of intermittent intervals. Yes, they are easy on the mind but in the end you train for performance. And performance is hammering up a 5min hill or closing the gap in a group. Yes, there are physiological differences with short intermittent intervals (more reliance on fast twitch fibres), however, I consider the mental aspect more important. Enduring the suffering of 5min efforts is a skill to be learned. Raise the pain tolerance. Intermittent intervals hurt as well (in the end) but it is not the same as when doing longer intervals. HOwever, with longer intervals there is the story of âjust to do itâ.
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Spending time at high percentages of VO2max or at VO2max does not necessarily give better adaptations since the time you can spend there is severely limited.
The optimised intervals of 4 x 8 minutes comes directly from this paper by Steven Seiler at al: https://www.researchgate.net/publication/51543724_Adaptations_to_aerobic_interval_training_Interactive_effects_of_exercise_intensity_and_total_work_duration
The 108% of FTP was from my own personal experience of trying these intervals but might not be right for you.
Mike
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Having just referenced a paper as to why 4 x 8 minute intervals are better, Iâm now going to say that short 2:1 repeated intervals may be even better than that. Iâm not sure of why Seiler has dismissed these but the work done by Prof. Bent Rønnestad suggests that the shorter intervals in longer groups can be even more effective.
Unfortunately the paper isnât freely available but they appear to have tested subjects ability to hold power at pVO2max (along with tests of other key indicators) and despite doing intermittent intervals vs steady state intervals, they were generally better at holding power. Not all of the subjects reacted in the same way to the stimulus, which Rønnestad points out quite clearly in the presentation.
As ever you mileage may vary.
Regarding the psychology of knowing that you can hold a certain power for 5 minutes, I have no doudt that actually training that way will have benefits. Whether you need to do it all the time may be the question you need to ask.
Mike
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Understood, however, the time to ACHIEVE true VO2max conditions using lower intensity intervals is much longer than shorter, higher intensity intervals.
But they arenât better.
The most optimal interval would provide the highest gain with the least amount of resources (time, energy, etc.)
The study shows the 8min intervals increased VO2max by 8%.
The 4min intervals provided a 5% gain â 62% of the 8min gain in 50% of the time.
32 mins = 8% (100%) gain = 0.25%/min
16 mins = 5% (62%) gain = 0.315%/min
4min intervals give 26% more VO2max gain/minute than 8min intervals.
(Pretty sure both the TSS & kJ/Cals were higher in the 8min group than the 4min group.)
Thus, by design, 8min intervals are not the most optimal, that is, the cost:gain is not linear.
(Notes:
Interval RPE between the 8min and 4min groups was ~8%, so the efforts werenât that dissimilar.
The 8min group was the only test group to contain all male subjects, the other two groups contained females â how did this effect the results?
The HRpeak of the 8min group was upper threshold zone; 4min HRpeak was upper VO2max zone. More time spent training closer to/in LT would explain the decrease in lactate in the 8min group vs unchanged in the 4min group.)
Having said that, would be great to see a study examining equal time in intervals vs equal number of intervals.
How would 8x4min intervals stack up against 4x8min intervals?
(caveat: end of day post; please check all maths. 
)
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Whatâs important here are the absolute gains not the relative ones: The sessions were done at the maximium tolerable intensity for the prescribed intervals so adding more to make up for the shortfall of the shorter intervals would have been impossible.
8 x 4 minute intervals would have to be carried out at a much reduced intensity, very close to that of the 4 x 8 minutes intervals but the additional rest period would reduce the amount of time at >90%VO2max.
Mike
This is a current thread which is quite active which debates the same thing
FWIW I struggle with 6* 3 mins at 120% and nearly always fail the last one. I find erg mode gets me through further but once I start grinding I struggle to go much longer
Just wanted to add that Iâm quite happy to be wrong - just trying to interpret the information as best I can.
Good to have these discussions. I really enjoy them.
Mike
That would be my recommendation. I really struggle with the VO2 sessions at 120%. Iâm 42 and this may have something to do with it (fast twitch etc) but completing the session is the most important thing. I often ramp hard VO2 max sessions down to 90% or less if Iâm in bad shape!!
If there were multiple studies proving the overwhelming efficacy of 8 min > 4 min intervals, I might be a believer.
Well, thatâs the trouble with all of these studies. There just arenât enough of them.
What was interesting in the study by Prof. Bent Rønnestad is that although his study showed that the 2:1 intervals were better in general, there were subjects that did not respond well to them, responding better to the continuous effort intervals.
Youâre mileage may vary, as they say.
Mike
These studies are trying to find small differences in effectiveness that likely vary by individual and where that are in their training cycle (Ronnestad points this out in his video). So itâs not too surprising that thereâs not a conclusive answer.
The fact that we are doing some form of VO2max training is 90% of the benefit - and suffering 
Well, that is 100% correct.
Not as much fun to discuss thoughâŚ
Mike
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Letâs take a look at the chart below of a VO2max workout (the athlete is a UCI MTB pro). This was a solid VO2max workout completed during the first week of an intensive VO2max cycle. It is a 7 x (4 minutes on, 3 minutes active rest) interval setâŚNote the time above 90% of VO2max: 17:35âŚthe athlete was full on VO2max for almost 5 minutes.
~8x4min is obviously achievable and without âa much reduced intensityâ. As well, Iâm doubting that the 8 min interval would ever deliver you into âfull on VO2maxâ territory.