Three things driving FTP:
- Your VO2max
- Your fractional utilization I.e. percent of VO2max that you can ride at for MLSS (influenced by VLaMax)
- Your mechanical efficiency at using oxygen to convert fuel into power output
How much the above reduce as you “detrain” varies across people - and probably largely influenced by genetics, and maybe past training history.
I think one thing that is not caught by a traditional FTP test is repeatability. I.e. if you did a few 20min FTP tests one after the other, how would the result change.
I’ve seen studies where VO2max reduces as the rider gets tired. I.e. a VO2max ramp test was done when fresh vs fatigued.
I also think fractional utilization decreases during successive efforts (eg during a race) due to gradually accumulating lactate - and this is more pronounced if you are less trained.
Apparently mechanical efficiency does not vary much, but I think it probably does vary more than people think as muscle fiber composition moves about with long blocks of certain kinds of training. It just hasn’t been measured as much as VO2max and VLaMax, hence it’s harder to identify if it moves around or not.
Another factor in this I think is what your limiter is on performance - oxygen delivery vs oxygen consumption. If your cardio system is your limiter, I think this means your FTP won’t vary as much with training, as, say someone with oxygen consumption (muscle) being their limiter. This one is some speculation on my part, but from what I’ve read, I think muscle mitochondria content and function in the muscle is more trainable than heart performance (stroke volume, max HR) and hematocrit (other than altitude training). I could be completely wrong on this, but Is something I’ve not yet found a study that explores this to set me straight.
Lots of stuff to spend hours researching on Pubmed!