40/20’s self-experiment 112% FTP work / 55% rest, result unexpected

I was watching the Seiler talks on his YouTube channel and noticed there was a study where the short intervals in constant series worked just as well as blocks of intervals broken up by rest in terms of time spent at 90% HR max.

I built up a workout and started pedaling. My recent estimate of 6 min power at 112% FTP was clearly way too low because I didn’t reach failure as expected. I didn’t even hydrate or take in calories because I thought it would be a short workout without rest.

Due to ERG, the interval Watts fall slightly under 112% FTP but this was surprisingly effective in accumulating time in VO2 range without high RPE. I’ll have to try again with a season best 6 min power number.

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TiZ and actual VO2 aren’t the same thing.


Well you went on like that for about an hour? That’s pretty good work!

Here’s one more thing to think about…if you want an approximation of what % VO2max you’re working at keep track of your resting heart rate, note your heart rate during the interval. If the difference between your heart rate during the interval & your resting heart rate is 95% or more of the difference between your peak heart rate and your resting heart rate then you can be pretty sure you’re in that ‘95% of VO2max’ working zone that sports scientists seem to like so much!

(working HR - resting HR)/(peak HR - resting HR) >= 95%

I always like to think of VO2max as a physiologic condition. Definitely I’ve been ‘at VO2max’ at work rate/power levels less than what most would consider Zone 5. :smiley: But, for the most part, a set of 40/20 intervals at 112% of my FTP is probably not going to result in much time at VO2max for me. It might be different for you! Take a look at your heart rate and give it a think…

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Yeah it was a 20 min warm up and I had the test end at 1hr30min. I thought that was plenty extra to prevent me from passing the test.

5 years ago, I had VO2 testing and my power at VO2 was 338 Watts I believe. I don’t have access to that testing anymore but back then my power at VO2 was 1.15x my VT2. I’m less fit now, but I imagined 1.12x my FTP based on a 30+ min test would get me working.

The main surprise to me is that this didn’t feel that bad for the TSS/XSS/TIZ it generated.

You have to do HIT harder otherwise it is a waste of time. These were closer to 130%, but it’s not a hard target. Go hard enough that it starts to feel like threshold… or just go ‘hard’ until you settle into something that feels like threshold.

Since I haven’t tried this before, I think attempting it by feel would require multiple sessions and trial and error. If I go too hard in the first x number of work intervals, I might tank before I can get an adequate number of intervals. I’ll make it harder next time. The Seiler talks had athletes working around their 6 min max power in either 30:15s or 40:20s. I built that up but my recent 6 min power estimate is probably too low.

The intermediate recoveries let you keep working. You’re power will drop over the course of the interval, but you’re still getting what you want out of the session.

I tried this again at higher watts to reach failure. Target 328w didn’t feel hard enough after 12 mins into the set, so I bumped it to 351w.

I looked at my two attempts on Xert and this MPA metric seems like some kind of voodoo magic. I didn’t reach failure on the first attempt and the MPA matched that observation. On the second attempt, I failed when MPA matched interval wattage. I only looked after the workouts, not during.

I’m intrigued by 40/20’s to failure as an open ended test similar to a Ramp. Asking someone to self pace VO2 intervals correctly seems like a tall order. How the heck would I pace a Ronnestad 3x13x30/15?

When HIT workouts call for “all out efforts” that seems incompatible with repeated reps/sets. After a flying 200 attempt, I used to have to lie down with my legs in the air and stay off the bike for 15 mins. They really mean max watts while completing the entire m workout… that takes a very accurate estimation of one’s current ability.


That’s because it is. Zert simply adjusts things after the fact so that failure coincides with MPA = 0. This is how they avoid impossibly negative results, like you see with W’ in Golden Cheetah or dFRC in WKO5.

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Not exactly. MPA = 0 would mean you’re dead. :frowning: Power > MPA is what defines a breakthrough.

I see now that the MPA = power at the point failure for the workouts I had breakthroughs in (KM’s long ftp tests to failure).

My ride yesterday showed power > MPA but no breakthroughs were recorded.

It seems like my prior breakthrough with 40/20’s was based on an average power for 20 mins?

I can only speculate how the platform is estimating breakthroughs using short on-off intervals but I noticed them in workouts aimed at reaching breakthroughs.

PS: I appreciate how XSS grades this kind of effort (relative to TSS)!

Xert breakthroughs must be more than 5 seconds.

It doesn’t work like that - Xert doesn’t really care what you average power was for a given effort - all it does is work out when you MPA is below the actual power. If you do that for more than 5 seconds you get a breakthrough and the fitness signature is adjusted to suit.

The fitness signature power curve (in green) is a representation of the max steady power you could hold for each duration based on your current signature. You can see in the graphic that you posted that the MMP curve for the workout is a bit below the green line at the 20 minute mark and may well not have affected your MMP curve due to the on-off nature of the effort. Luckily Xert can identify these types of efforts.