"All out" efforts, whether they be short intervals, or long SST/FTP Test type efforts are as much (or more) a head game as a physical one.
I find that on days when I'm dreading the efforts, I usually struggle - but on days when I have the '
this is going to be a piece of cake' attitude going, I usually crush it.
I try to make sure I get myself into a positive mental state before doing these things.
Anecdotal story: Last Spring, at the end of a training block, my coach gave me a workout in Training Peaks that just said "SST Until exhaustion (!!!)". He scheduled this two weeks in advance, so I had quite a bit of time to ponder exactly what that means. I experienced lots of anxiety and trepidation leading up to this (this was my first time really working with a coach - so there was a little performance anxiety going on ).
I realize that some aspects of "exhaustion" can probably be measured in some scientific way if you are connected to the right equipment, but ultimately, if you aren't in some kind of sport science lab, it comes down to your brain telling you when it's time to stop.
I asked my coach what he had in mind in terms of goals for the duration. He wouldn't give me anything specific, but suggested to shoot for an hour. I created a two hour ERG block at the wattage he suggested (270), and just made up my mind that's how long I was going for. I figured when I got to the point of "exhaustion", I'd know and stop. But apparently I didn't. I ended up doing the entire two hour block in ERG.
There are lots of mind games going on when doing this kind of stuff. When I got to the 1 hour mark, I didn't really feel any worse than I did at the 30 minute mark, so I kept going. Once I got to the 90 minute mark, my HR was still about the same (~155), and I still felt about the same. At that point, I don't think you could have pulled me off of the trainer with a tow truck. It was about 95% determination and 5% physical effort.
The hardest part was trying to drink water and consume glucose at that effort level. Even on the trainer it's hard to do when your respiration rate is that high.
I've not done anything close to this since then, although he told me there are some more 'surprises' coming up in the next few weeks (at the end of this training block). "I can't wait!"