It’s a bit hard to tell with that short of a training history to go off of but…
You had been doing a lot more volume. You had a string of several weeks that were 13-22 hours and then it dropped significantly (probably lockdowns and colder weather) to about 4-6. Volume (even low intensity) plays a large part in VO2 so I’m not surprised that your power would fall. If you can, I would add some longer rides to the low volume plan while you are stuck inside. Long trainer rides can suck but I’ve found that with some form of entertainment (movie, TV, bike racing, youtube, etc) I can get through 3 hours without too much issue.
However, the caveat is only do more if you can handle the additional stress. You say you probably don’t sleep enough and have a stressful job. Both of those things can severely limit the amount of training stress you can handle as well as how well you adapt from it. Then add in the not so obvious stress of being stuck in your house during a global pandemic. It’s possible that those long outdoor rides you had been doing not only were good for your fitness but also for lowering your stress. So maybe you need to be more diligent about finding other ways to lower that.
And finally, maybe the ramp test just isn’t great at modeling your FTP. But without HR data its a little hard to see how the workouts correlate to one another. Do the threshold workouts at the ever lowering FTP feel easier and easier or do they seem to stay consistently difficult?
That was a big thing for me. I would constantly underperform in the TR ramp test and it would make me mentally nutty. I use to use the Joe Friel 30 min TT test and call that FTP, but I’ve also used another ramp test that does 2:30 steps and the high end of the ramp does not need to go that high and find it much better for me.
Thanks everyone for the really helpful feedback. Apologies for the late reply. A few late nights at the office…
Judging from all your responses it seems likely that the reasons for my fluctuating FTP must be a combination of:
fatigue
lower riding volume since the lockdowns / end of summer (as suggested by @mwglow15) . We used to do lots of long weekend and holiday rides, which are impossible to do when you’re on a 1-hour a day outdoor exercise curfew.
my focus on long endurance riding (and therefore lack of VO2 work) in the past few years, which means my FTP is high in relation to my VO2max (as explained by @ellotheth and @splash). This would explain why I can usually complete the threshold workouts at 245W without trouble but not the VO2 segments.
Going forward, I plan to do the following:
Move to a medium volume plan (as suggested by @holybinch) in order to up the volume / intensity somewhat, perhaps adding some time to weekend rides.
I will continue to use my current FTP setting (245W) and adjust manually according to how difficult the rides feel.
For my next ramp test, I will try to use a different method, such as the 20-minute test or the ramp test that uses 2:30 steps (as suggested by @onemanpeloton and @JT_34 ).
And to address the fatigue… I will make even more effort to prioritise sleep and reduce work-related stress. Come spring the outdoor riding will help as well.