Why try to come up with a (probably) wonky Vo2Max/FTP conversion when you can just do a 20min FTP step test with your trainer and get an answer?

What I'm curious about is... just how hard is 170% of V02max? Can it be expressed as ftp?
As you noted, I can do an FTP test, but don't have a way to measure my V02max.

To a large extent I'm just wondering if what I do when I say I'm doing Tabatas is anywhere near hard enough. I suspect it's not.

Just go really hard? Quick Google says a round last 4 min. That would need to be REALLY REALLY hard.

It's hard to determine because work rate above ftp is so individual. And there is no such thing as a vo2max power. Perhaps it was the maximum power measured in a ramp test?

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A person will not be able to do 20 seconds of 170% of VO2max over and over again like the typical tabata intervals prescribe.

You'd find it difficult to do 170% of FTP as well. I do that type of interval often (Madison Training), a good place to start would be 150% of FTP depending on the work/rest interval. Typically I'll do :45/:45 but :20/:20 or :20/:30 is just as difficult.

Then the object of a tabata is not so much the power but holding your HR in the VO2max range and getting a short rest while keeping it in that range over and over again. It's a fun workout. Another variation is the Ronnestad which is 13X:30 second at VO2max and :15 off, 6 min off then 2 more rounds

If you want to train your abilities at VO2max, I would start off at 5X3 min at VO2max (~120% of FTP) with 3 to 4 min rest, then work that up to 5X5 min

There's a range, it's called fractional utilization. Most people have an FTP between 70% and 90% of their VO2 max. So your Tabatas would be 127-153%FTP. Not very helpful.

What is helpful is to know the point of Tabata style (or microburst VO2max) workouts. A tried and true way to raise VO2max is to spend time near or at VO2max. 90-95% of max HR correlates close enough to VO2max level exertion. You need to be going very very hard to utilize 100% of you body's oxygen use capacity. So hard that it's unsustainable beyond a few minutes. One way to do this is sets of 3-8min efforts at 90-95% MHR. I've had coaches say that the first minute doesn't count as "time in zone" and you want to target 10-15 minutes "in zone". More than that and you can't go hard enough, less and you don't get as big of benefit as you can for the fatigue cost. The lactate buildup during these workouts is quite high and that limits their length.

Tabata's come in as an easier way to achieve this. 5x4min at VO2max is a hard session, especially if you're not used to it. Instead of doing 4 min steady you do 10-15min of 2:1 work:rest sprint/easy. The sprint seconds should be as hard as you can go. The point of them is to create such oxygen debt that even with the rest period your body is still at VO2max. The rest periods allow a little bit of lactate clearance and oxygen uptake. Your HR when done properly should be >90% max after a few minutes.

TL:dr - For tabatas don't set a power target, go as hard as you can.

"fractional utilisation" applies to oxygen consumption, not power. And vo2 at ftp just means what your slowtwitch fibres' max demand is. All power outputs above FTP rely on a fast twitch fibre contribution, and will all result in fatigue and therefore reaching "vo2max" eventually. If you have a lot of fast twitch oxidative muscle mass, you can manage vo2 intervals well above FTP; if you're more of a tempo time triallist then you won't be able to.

I don't know why so many people and coaches overcomplicate this by not seeing the simple biology causing the "science". We simply have oxidative muscle fibres that fatigue at different rates. If you activate all of them at once (eg at the last minute of a 6min piece, or mid-HIIT set with insufficient recovery), you are at VO2max. If you only activate the ones that can work at steady state, then you're at your "threshold fractional utilisation". If you cruise around in zone 2 then you're only using the most efficient bunch of your slowtwitch fibres.
Bo point using generic vo2 rules of thumb as if the human body is a black box with divine biochemistry formulae. Use your fatigue curve. Not percentages based on the blood lactate of 17 untrained subjects in a norwegian lab in 1993

Your math on fractional utilization is backwards, looks like you multiplied instead of dividing? Think of it as a ratio, if FTP = 80% of VO2max, so VO2max = 125% of FTP, and therefore 170% of VO2 max would be 170% * 125% = 212.5% FTP. Probably better to approximate VO2max for this usage by maximum recent 5 to 6 minute power best, which isn't perfect but probably a better guess.

Quite aside from using this number or going all out, the original Tabata protocol is brutally hard. 8x (0:20 at 170% VO2 max, 0:10 rest) adds up to 4 minutes at ~113% of VO2max, or ~140% threshold. It's bad enough to try and average that straight through, but surging is harder. Yes, I've tried both. If I remember right from the original study, I don't even think most of the participants were able to finish even the single set?

Aside from a different definition of fractional utilization what difference are you postiting?

All power outputs rely on all muscle fibers, to different degrees. Z1 has almost zero anaerobic or fast twitch involvement. Work above VO2max for more than an minute or so by definition needs to have 100% aerobic utilization. W' sort of gets at this tangentially.

The body doesn't just run aerobically until it runs out, lactate still can build up at sub -vo2max power. I feel like we're just saying the same thing in two different ways.

The fatigue curve (I assume you mean something like the power curve, x=output time y=power) would be good to use for vo2 workouts, but OP obviously does not have one, since they're asking for conversion to FTP.

Regardless of all the underlying why, the how for Tabata type workouts is to go as hard as you can for the "on".

This is where I think most people go wrong. Power meters are great, but they miss the time component. If what you’re saying were true, the step duration of a lab ramp test wouldn’t matter. But we all know that both lactate and vo2 results will be very different in a 1min step test versus a 6min step test. You’re not going to reach VO2max within one minute at 140% nor four minutes at 110%.

I don’t think there’s any evidence at all that you activate all muscle fibres at all intensities, unless you do an easy ride at 25rpm or use manhole covers as swim paddles. After all, it’s force, not power, that determines instantaneous fibre usage, and it’s power over time, ie energy demand, that determines the metabolic fatigue side.

Yes. It's super simple.

do a 30 min all out test on the trainer. 45-50 would be better

then a few days later do a 6-7 min all out test on the trainer.

You'll have your vo2 power and roughly your threshold power (probably need to subtract 2-5w from the 30 to get something you could truly hold for 50-70min)

Once or twice I've done 30 minutes then a few days later came back and did 50 or 60 or 70 min to see how the differences. If you do a 30, 55 then a 70 min all out TT over a period of ~ 10d you're going to notice that suddenly holding your old FTP for 30min is something you kinda shrug your shoulders and laugh about.

Understandably, most reasonable people won't do that, and I can't blame them after doing it myself a few times. In my 30 years of coaching it seems most people also underestimate what really long duration intervals can/will do for you and that most people would rather do shorter, higher intensity intervals hoping for the same sort of benefit. That's not really how it works though as emceemanners alluded to.

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