I've done over/unders as a cyclist because they can simulate bike race situations pretty well. I'm not sure if they're the right protocol for an IM. As with anything, it needs to be viewed in the context of overall training. Tangentially, FTP is fine, but not a very good measure of IM specific fitness. The 20min test is like 10% of the bike leg length, it's a good ballpark but quite non-specific. You could raise you FTP via VO2's and 20-min focused workouts and see little improvement in IM bike time. The bike leg of an IM should ideally have zero lactate accumulation (high production, but no accumulation, like <2.0mmol, depending on the athlete of course)

Now, are over/unders good for IM? There's probably better options. Over/unders (for me at least) are a pretty fatiguing workout, I feel them the next day and they definitely impact any workouts in the next 36-48h. If I'm doing a workout that leaves me recovering for that long it needs to provide a really good stimulus towards what I'm working on. For an IM that's ~4h bike power (LT1, basically). 3-4x40min would be a better option with similar recovery. or 3x60 going up in power, something like 85/90/95%FTP. The run is those are both 3.5-4h workouts, OU could be 60 min.

If you have generalized fitness gains on the table or are far away from upcoming races (months), then they could be a good option, even if for nothing more than providing a change of pace. Personally I prefer "peak/fade" over traditional over/unders. Sprint all-out for 20-30sec, then 8-10min at threshold. It stresses the same lactate clearing ability in a way that (for me) is less fatiguing.

For Ironman I'd maybe do intervals where you do 20 min as 20 seconds hard, 3:40 threshold x5, but that's just as much for variety as anything else. As the other poster already said, for threshold/tte work you're much better off just working on sustained power.

I would certainly recommend working on top end/VO2 for Ironman, but that's better accomplished with a focused VO2 block doing something like 4-5 reps of 4ish minutes done as hard as you possibly can, 2-3 times a week, for a 3 week period.

Over/unders are awesome for anaerobic capacity in trained individuals, but the studies that show aerobic benefits were primarily conducted in untrained subjects, so the benefits are a little suspect for trained individuals.

Coach gave me 20min reps (3x) alternating 3min @ FTP/2min sweet spot last week. Close to impossible to recover @ SS. It was a killer.

Considering I can do 3x20min @ ftp that doesn’t sound too hard. Sounds like your ftp is too high.

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My Strava | My Instagram | Summerville, SC | 35-39 AG | 4:41 (70.3), 10:05 (140.6) | 3x70.3, 1x140.6 | Cat 2 Cyclist

You don't always have to hit 100% of possible output. Most days are way under that.

The FTP model falls apart at longer durations. I know plenty of very good cyclists who can't hold their "95 of 20minFTP" for anything near an hour. The VLAMax and LT2/LT1 model does a better job of explaining it.

Imagine Blu/Walmaley/Skipper doing 3x20@(95%of20minFTP) vs. Cav/Bolt/Laveisen. One group would sail through this workout and the other would be in their heels for a week, it they could even finish it.

It's just the specificity argument again. Once you've peaked, or nearly peaked your genetic potential then specific without towards your goal is the only thing left to do

Wouldn't 95% of FTP put you above LT1? Certainly below LT2, but I don't think I can remain < 2.00mmol at 95% FTP, maybe 80-85%? Or am I just really un-fit?