Another question for the coaches - what happens after the 8th minute of a V02max effort that decreases it’s training efficacy?
There is nothing special about 8 minutes (although that is a bit long for vo2 max intervals imo). Just like there is nothing special about 5m or 20m or any other specific interval length.
There are different physiological process involved for different interval lengths and intensities of course but it’s not a hard switch from one moment to another.
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Oh I think there might be. There is a reason most prescribed VO2Max intervals are for 8 or fewer minutes.
I’m not a coach, but I’ll have a wild guess.
At ~120% of threshold, muscular fatigue & the buildup of H+ probably limits output to the point that this power number can’t be maintained for the majority of people.
Also with VO2max intervals, your muscles are burning like crazy AND you’re breathing like a fish out of water (if you’re doing it right). Sounds like hell. 
Edit: Keeping them shorter is probably more to do with workout/plan compliance than actual training benefit. I feel like I reach VO2max in a 20’ FTP test & hold it for about the last 10’, & that by definition is ~105%. Not something I want to do every week.
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What do you think that reason is?
Like @roleypup said:
If you are doing classic vo2 max work you want to get as much time as possible at vo2 max, intervals need to be long enough to get you there but not so long that you blow up completely.
The classic vo2 max workout is a 5x5m set, it’s horrible but very effective.
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@Corey could you chime in here? 
I doubt there is a hard switch at 8 minutes because we don’t really have hard switches like that. Even if there was, everyone is different so there would be no reason why the hard switch would be in the same place for everyone.
There is a sliding scale in terms of benefit and once the intervals get too long they stop being VO2 max intervals and start being threshold intervals. Which is basically what @Twowkg says, you want to accumulate as much time as possible at VO2 max and if you go up to say, ten minutes, you will only be at the lower end of VO2 max, and then likely only for the last part of the interval.
I’m not sure what you mean here. But let’s try one possible interpretation and assume you meant to write “Why is the length of VO2max intervals capped at 8 minutes?”
If that’s your question, then the answer is simple: that’s what the typical power-duration curve looks like. I had one small climb in Japan that took me around 6:50–7:30 minutes to complete. This was in the middle of a ride, so I wasn’t 100 % fresh. My PBs for each season (when I wanted to attack that climb) was consistent when I scaled with my FTP: it was always around 117 %. (I was so consistent, I could use it to help validate my FTP.)
However, when you train VO2max, you don’t want to empty the tank in one sitting. Longer VO2max intervals range from 6 x 3 minutes to 5 x 5 minutes at typically 120 % FTP. Of course, you can tweak these numbers as well as the rest periods in between.
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**There are no magic intervals **
With any intervals you need to balance the following:
- What is achievable (your power duration curve)
- Not generating too much fatigue
- Having enough and the right tore of stimulus to trigger the intended adaptation
- Time constraints the user has
- Difficulty to execute the workout properly
So short answer is intervals longer than 8 minutes don’t get enough adaptation for the amount of fatigue and mental stress involved. And they may not actually be VO2max intervals if they become too long.
The following numbers are ballpark correct, for some people, but VO2max work should always be customized to your own PD curve.
Let’s say that someone could do 10 min at 110% (not many can). It’s highly unlikely that they could do more than two of those intervals within an hour workout. And they would need 20 minutes or so between intervals because each one is brutal. The first 2-3 minutes of the interval would not have the elevated heart rate and breathing being targeted. So total time providing VO2max stimulus is around 15 minutes. And the fatigue would be really high. And motivation and pacing would be issues. Not ideal.
Whereas with 5x5 with the rests are shorter (5 min or so) and the intensity is higher (let’s say 115%). They would reach VO2max breathing quicker, say 1 minute into each interval. Total time at VO2max is around 19 minutes (higher intensity gets to VO2max state quicker). Pacing and motivation are better than the 10 min intervals. Fatigue is likely lower. Total workout time is about the same.
The 5 minute intervals are going to provide more stimulus because they are both higher intensity and longer time in zone for the workout. It’s also less clear if the 10 minute intervals should even be considered low VO2max or high threshold. Especially if the athlete has a low (30 minute) TTE at FTP. There are no hard lines between zones especially above threshold.
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There isn’t anything special about 8 minutes or any other time.
It’s about time in zone that you can support and what your goal is. Do longer intervals and it limits your ability to do more in the same workout at a high quality. if you’re doing 4x5, 5x4, etc. - you’re getting more time at or close to your VO2 Max in aggregate for the workout and depending on your goals - a better total training stimulus.
Just keep in mind that there’s a difference between the VO2 Max Power Zone, and Training or Riding at your VO2 Max. Not the same thing.
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I agree with a lot of what’s being said here. 
Riding in the VO2 Max power zone, or close to/at true VO2 Max, is extremely taxing and uncomfortable, and I can imagine that attempting to do 8-minute intervals wouldn’t be sustainable for even one session, much less multiple sessions throughout a training block.
You could ask the same question about anaerobic intervals. Why not push those out to 4-5 minutes like VO2? I don’t know.. Try it! 
Structured training is all about finding ways to get the right amount of stimulus through targeted intervals. You’ll likely be able to get much more total quality time in each session doing x5 intervals than x8s. This is why on/offs or microbursts were created. Stacking up all of those short intervals during workouts like 30/30s adds up over the duration of the workout, but as a whole, it isn’t as taxing. The intended training stimulus for those workouts is different, but the logic is similar.
With all that being said, we do have plenty of x8 interval workouts. We call those “long supra-threshold” workouts, and here are a few examples.
These can actually be really productive workouts, but they are tough! As others have mentioned, though, the power targets with these are lower because that’s just how a power curve works for most of us. If we could do 120% of FTP for 8 minutes over and over, we’d likely be getting paid to ride our bikes. 
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That’s savage.
Even with this much rest, I expect that most people will fade.
A theoretician’s answer would likely be yes, as riding above threshold will eventually lead to max O2 update. But a practitioner would say no, because that’d be an insane way to get to VO2max for the reasons you have given.
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Can you share some insight on how they are used in training plans?
In my experience, I’d get one of these types of workouts during a polarized block where (for reasons I don’t understand) the progression was no longer done on interval length (4 x 8 —> 4 x 10 —> 3 x 16), but by intensity (4 x 8 @ 100% —> … —> 4 x 8 @ 105 %). So those workouts were used like a threshold workout.
In TR parlance, search for “four 8-min” in Workouts / VO2 Max / Long Suprathreshold. There are lot of them, with Workout Levels ranging from 7.8 to 10.8.
Once I worked up to 8-minute intervals, I now do prefer them. It is slow burning discomfort, reaching pretty high but this I can cope with. As beginner, it used to be other way around, I liked on/off stuff more (a la 2-3x 12x30/30). I guess it depends what kind of pain one becomes used to. Regardless, Garmin VO2max estimates reach roughly same peaks during focused block.
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My FTP as determined by me is 235-240W. TR AIFTP just put me at 236W on Sunday. Intervals.icu says 240W. Let’s just use 240W to keep the percentages lower.
Saturday I did 266W for 10 minutes. That’s 111%. And I did that sometime during my 4 hour, 60 mi ride with 3600 ft of climbing (distributed over many 10-20% grade hills). I did 264W for 10 minutes a week ago in a hard group ride.
I’m just a naturally a good anaerobic athlete. I often complete hard 2-3 hour rides with an IF of ~1. That’s usually lots of time tempo and threshold with plenty of above threshold efforts. It’s probably related to my massive thighs.
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Fatigue. The longer you go, the more threshold like the workout becomes, and the less you’ll be able to actually achieve and sustain an effort level that elicits VO2max.
There’s a range of intensities (all of which above threshold) at which an athelete can generate enough power that the body hits VO2max. It takes longer to reach VO2max with a lower power level. If power is too low, you’ll simply fatigue before achieving much if any time at VO2max.
You can reduce time needed to achieve VO2max by doing a hard start, and then dialing power back. These are particularity brutal, but a nice middle ground if you can recover from it. I’ve burnt myself out more from this workout variation than anything else I’ve ever done, by a good margin though.
They can actually be included in a handful of our plans, from Polarized, like you mentioned, to Criterium, XCO, and several of the triathlon plans, among others.
These types of efforts serve as a really stable way to reach peak aerobic uptake, which differs from hard-starts, which are another common path you could take to get there quickly. These longer efforts are good for mental training too, as they’re often just harder than you want, and they last a reasonably long time (~6-12 minutes).
You’ll use these skills in situations like breakaways/bridges, and climbs in the range of ~10 minutes.
They’re definitely somewhat specific, but also really productive. 
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When you do the 2 x 8 min FTP test you are riding at 111% for 8 mins, a 10 min recovery, then repeat. I don’t think you’ll need twice as much recovery if you extended out to 10 mins and dropped intensity.
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You are likely referring to the output of this paper
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