When VO2max turns Anaerobic 💥

Just got done with a VO2 session. It was hard. Looked at my power and heart rate ex post facto and noticed something. A lot of time spent both in “anaerobic” power and heart rate values.

Is this a bad thing if the goal of the workout was to drive up aerobic adaptation by way of working at or close to VO2max?

Obviously, I could assign a lower power range for these intervals that stays within my Z5 level, but I’ve heard numerous folks say “Full Gas” and well, that evidently can put you into anaerobic territory. So I wonder.

I wondered the same thing a while back and found the following study in one of the threads on the forum:


The simple answer is that 4+ years ago Dr Coggan got rid of z5 and z6, and replaced with individualized power levels. Intervals.icu and others are still using the outdated original levels.

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But what are ilevels? No one over on the WKO Facebook group can answer. TARWAPM, 3rd edition calls it magic or something like that. Hand-waiving, basically. Sure, they may be individualized, but how?

Also, are ilevels proven to be better than the outdated methodology? Seems to me like it’s a selling point of WKO5.

OP asks a good question. How do we best delineate the work efforts over threshold? Are there physiological markers that can define VO2max vs anerobic efforts? Or is there even a need to go that far in analyzing, and just do the work you can do?

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I don’t think that burying yourself doing VO2max intervals at 125% and above is a good idea. It inflicts to much fatigue and and takes longer to recover from. Most people have a maximal stroke volume at 90-95% HR so I figure that accumulate as much time as reasonable in that domain. You can actually improve your VO2max doing loads of z2. Don’t get me wrong, there is a place for shorter intervals, but it is not the best thing for VO2max.


Appreciate your input, but this is part of my current plan. Z2 + 1-2 VO2max workouts per week.

Might double check your ftp setting in Intervals.icu, I noticed mine being off a lot of the time…

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The short answer is that doing on-off style workouts make that time in zone analysis pretty useless.

Look at the normalized power for each “block” of on / off intervals. You will probably see that it falls in VO2 or threshold.

The shorter 2-3 min intervals have a very large anaerobic component to them. They will target the muscles ability to clear lactate in addition to address your VO2max, a very useful ability in its own. If VO2max is the priority I’d go for longer intervals at 102-105% FTP.


I don’t know, but can tell you I got faster by following Carmichael Training Systems interval training using only an HRM. You don’t need WKO.

VO2max are called PowerIntervals in CTS terminology:



Again you don’t need WKO.

I think if you’re looking at time above 95% max HR ( > about 186 for you) as a proxy for time at VO2max, you are looking at time with HR in the ‘anaerobic’ zone. Most guidance I’ve seen is 15 - 21 min range depending on how well trained one is.

I basically only look at a chart from the Activity HR page and will drag my mouse around to settle on the > 95% MHR zone and see if I had the desired amount of time in that zone. I don’t care so much about the power as its just a means to elicit the state and there are different ways of going about it. I have also used this chart to identify which types of VO2max workouts work ‘best’ for me.

This is the HR time above zone chart for me from a Tour of Watopia stage. A bit different from a structured workout, but this is what I look at after a VO2max workout to judge how well executed it was. Normally looking for 15 - 20ish.

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It seems concordant, you spent about 17mins above FTP, and about the same time above LTHR.

Going back to basic energy systems:

  • Sprint energy is efforts up to ~15 seconds
  • Anaerobic efforts are trained using efforts roughly 30-120 seconds
  • Aerobic efforts are trained with easy spinning for hours (both metabolic and cardio), and 2-6 minutes efforts for maximizing the stimulus to your cardiovascular system

And couple that with Coggan stating that zone5 and zone6 are individual, and I hope the answer becomes clear.

For vo2max intervals your job as a cyclist is simply to go full-gas for 2-6 minutes, because rarely are you hooked up to a metabolic cart and measuring vo2 (oxygen consumption) in real-time and comparing to your max vo2. The amount of power required is individual. The heart rate response is individual. Suffer. Repeat. Establishing a foundation means repeating these efforts up to a total of 10-12 minutes. Then work your way up, with 24 minutes minutes being a good target.

With respect to power and heart rate, you can look for trends, but I wouldn’t pay attention to those zones because they are fixed percentages and not individualized.


My understanding:
VO2Max session are about Volume of oxygen, ie, breathing in and out. Not about power or FTP or HR.
Working at a very high rate of inhalation and exhalation improves your Heart Stroke volume, ie, pumping oxygenated blood.

High FTP helps you get to that very high breathing rate quickly, but once there, FTP is irrelevant, the training required is to stay at the high breathing rate for as long as possible for each interval, ie, as bbarrera has indicated maybe just 30 secs.

My experience is that the high breathing rate ( essentially hyper ventilating) can be maintained for a short while, until the body says no more, and during this period cadence and FTP drops continuously, hence your performance stats, but that doesn’t matter, as the training effect comes from the high exertion and breathing.

Paul Lausen’s research into this is interesting. He splits VO2Max into two categories, Needle type workouts, lots of very short explosive sessions, and secondly longer workouts of a few (2+) minutes. They improve two different aspects of performance, and also interestingly, that there is no measurable benefit of active recovery between intervals, ie, just recover as deeply as possible.


I’d concur with this.

Now it’s warm enough I’ve just this week switched to outdoor efforts for VO2 work. There are two hills not too far away that are perfect for the task.

First hill 6-7 mins, descend other side, second hill 2 mins, descend other side. Turn at bottom of second hill. Second ascent of second hill is around 7 mins from other side, descend. Second ascent of first hill from other side, 5 mins.

So 7 min, 2 min, 7 min, 5 min efforts. About 2 mins between each. Just went as hard as I could sustain. Resulted in peaks of 94%, 91%, 91%, 94% of max HR. Do I worry that I didn’t hit 100%, not at all. Just keep working hard up the hill, descend, tackle the next one. If the hills are the right length, and you don’t back off from the suffering, your efforts will work them selves out to give a high stimulus to boosting your VO2 max.


Sounds like a solid workout. The only thing I’d change is the rest. 2 min probably isn’t enough time to fully recover between those intervals


Here’s Monday’s VO2 session. We’ll call it 6x4. I was forced to do a hard start to get over an initial rise that lasted around 45sec.

Normally I do this work on flat ground as I don’t have anything longer than this climb. Versus VO2 work on the flats, the RPE and lactate burn was much higher do to more surges in power going up the steeper sections of climb.

Lots of time at or close to VO2max as well as an excellent W’ drainer. I was very happy with this workout…


Here’s today’s 5x5 session. Definitely hit some high heart rate values. I took an active recovery day yesterday after 6x3 on Monday and 2 hours of tempo on Tuesday, I felt really good today, but still noticed my heart rate was elevated at all wattages and zones. :man_shrugging:

Banked another solid session, RPE was noticeably not that high given all other metrics. The body is interesting. TR says 141 TSS / .94 IF and TP says 145 TSS / .95 IF for this ride. Seems way too high. Maybe my FTP is set too low at 325w?


I wouldn’t worry about it. This is my understanding of the process, but it might well be wrong!

While the power you made anaerobically doesn’t help develop your vo2max, the recovery from that power is an aerobic process and does contribute. Because the anaerobic effort is short (a few seconds), the recovery still falls into the time of the vo2max interval and adds to the load on your heart. Just think about doing a 5s sprint from a fully recovered state. During the sprint, you won’t be gasping for air, but afterwards, you will. If you now do the 5s sprint, and then back off power into the vo2max zone, the gasping-for-air will happen contributw to the vo2max gasping-for-air.

It is dictated by how long the descents take, though I suppose I could pause at the top or bottom. But I like the way it flows if I don’t stop.