Agreed on this, unless your doing some very short rest like 6on 3 off
At sub-threshold power (say, below 94%), Iām not sure Iād call three minutes rest āvery shortā. For example: most of the TR sweet-spot workouts, and all of the More Sweet Spot! team workouts, were built with 3-min rests between intervalsā¦ even the 3x30 and longer intervals.
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My sense is that on longer interval workouts, fitting the TR workout within a defined total workout time (90m or 120m) drives some of the rest interval selections more than any strong scientific basis.
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My guess is that 1, 2, 5 minutes or whatever doesnāt matter much in the large scheme of things. Do the work and go home. Donāt worry about a few extra minutes of rest or getting off the bike to take a drink.
I did 4x20 yesterday. Iām finally hitting the limits of my muscular endurance plus Iām four weeks into this block. My legs felt heavy though I could still push the watts. Iāve recovered pretty well over night.
Edit: was also going to say that I did a 2 minute maximal effort to seed the WKO5 model and busted the PD curve. WKO now thinks my ftp is +15 since the start of this block. Iāll have to do some more efforts fresh and a KM ftp test to confirm.
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3 min in a 3x30 is pretty short. The reality is you only need as long as you need to get mentally right when youāre doing sweet spot. 3 might be enough, 5 might be better. Whatever gets you through within reason. Rule of thumb might be to not exceed 20% of work time for rest intervals on SST and under. 2:1 work:rest can be common at threshold, but probably way too much at sub-threshold. I agree though a 3 min rest for a 6 min interval at threshold probably isnāt necessary.
I did some playing around with 5x5 on 1min rest at threshold last offseason and have done sets of 4x8s at 2min (the Seiler stuff), and those can be relevant, but I still think the longer intervals are where the money is made at threshold.
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YES.
I think that might be individual, and maybe come down to training experience and knowing when hard becomes too hard.
Iāve heard someone say, "do a little less, so you are hungry for moreā. That works for me, because I struggle with motivation. If every workout feels like Iām pushing my limits, I get mentally tired very quickly.
However, there might be people who want that feeling of exhaustion, and others who just donāt know how hard they can push, so would nornally bail out too early.
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The answer IMO is a little of both. Most of your training should leave a little on the table. OCCASIONALLY you should totally bury yourselfā¦ and then let yourself recover from it.
A lot of athletes have to overcome the mindset of trying to make every single interval set maximal. In all but the most special cases, that will lead to inconsistency, burnout, and plateaus or just lack of progress.
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Hey! I resemble that remark. 
You are hardly alone in that regard, believe me!
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Chiming in again. I did a vo2max mid-october but had to finish it prematurely due to the birth of our daughter and a subsequent covid infection. However, I still felt some of the positive effects of the block and tested my FTP at 340W (30min TTE). In subsequent weeks, training both threshold and sweetspot concurrently, I progressed with 2x22.5min, 1x50min, 3x20min for threshold and built up to 1x120min for sweetspot. I could not quite do 3x30min threshold with long rest periods, but 3x30min + 15min was still ok.
Now I did an intense vo2max block, after which I tested FTP at 350W (TTE 40min). This was early this week, and yesterday I already did 2x25min at 350W which felt manageable:
Doing threshold/SS work always feels very rewarding to me because the gains manifest pretty quickly from workout to workout. I have my first race in ten weeks, would be great to drive TTE to 60+ min before that! 
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Last workout done, legs felt pretty fatigued but pushed through.
Progression:
25+15min
40+10min
30+20+10min
At the same time doing sweet spot progression just one workout left targeting 90-100min of work.
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Almost all threshold progressions that Iāve seen have been simple and equal length intervals. I know Zwift and possibly TR include some varying lengths, but perhaps as much for interest as for science.
Iām interesting in these declining length intervals. Do you think thereās any scientific or physiological or psychological reason to favour them or disfavour them?
Does an interval below 10mins cease to be productive as a Threshold interval?
Does the declining duration help to keep the psychological strain down?
Is programming these just more complicated, so coaches donāt bother thinking past simple formulas?
What other factors are in play that Iām missing?
I do it purely for psychological reasons, threshold work is hard enough as it is any tricks that can make it easier Iāll take. Probably the most important thing is the total time in zone of the workout anyway.
I wonāt go shorter than 10min interval, I was planning 45min+15min for the last workout to make it up alpe du zwift in one interval but legs werenāt feeling that great.
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My guess is that in the large scheme of things, it doesnāt matter that much overall. If there was a study, it would show some not statistically relevant difference like the Seiler study. That study showed that 4x8s were ābestā but not by a statistically relevant difference thus one could conclude that any of the interval sets were just fine and all accomplished the same thing.
I have read or heard that it takes maybe a couple of minutes for the aerobic system to ramp up so that puts more intervals at a disadvantage over less intervals. Like 3x10 is 24 minutes time in zone if it takes 2 minutes to ramp up. 1x30 would be 28 minutes of time in zone.
I guess if youāre playing around the TTE limit, you can either pre-plan some bailout breaks with some shorter intervals to grow Time in Zone or just bailout in actuality when it gets too hard.
Probably some subtle ways to manipulate intervals and rest into a fixed period of time, but pre-planning that is probably big-braining it unnecessarily.
4x8 at what %?
Maximal power, aiming to have interval 4 as close as reasonably possible to interval 1.
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@ABG last month I did a progression 4x7- 4x8-4x9
I did what @Helvellyn mentioned. Average power for those intervals were around 105-107% ftp. It could be different for you. My main target was 105% but I went over
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No, just need to reduce the rest as well, sometimes called āfloatingā intervals in the running world.
From Marius Bakkenās website:
āWe would traditionally do two double thresholds with type 6-minute intervals in the morning and either 1-minute intervals (30 seconds rest) in the evening or alternated with 45 seconds intervals (15 seconds rest) ā plus one more intense workout (an āX elementā). Our shorter intervals in the evening would go slightly higher in intensity, usually right at or right above the anaerobic threshold, while the morning was lower.ā