What is important is your ability to deliver energy where and when it is needed. You can improve nerve firing on muscles all you wish, but if the mitochondria in those muscles cannot deliver energy it does not matter.
4DP is an indirect measure of all of that including metabolism. So are HR zones, RPE, etc. They all have their advantages and disadvantages. For example, MAP is a stand-in for a whole bunch of biological processes. W’ is actually a better measure, but MAP is whole lot easier to understand and use for workouts.
Human biology is the reality. Everything is just a model that is more or less useful depending on the what you are trying to understand.
Not being able to turn the cranks is probably where you should be. That is why after both the MAP and FTP sections they have you get off the bicycle. Remember the penguins and tortoises?
I remember little from my last FF. They have you get off after the MAP effort and then the FTP effort to restore blood flow. There’s not enough time to recover from a leg blowout effort. I’ve done those and crawling around isn’t a joke. I needed assistance just to unclip and get off the bike. It’s called a stress test. I have cardiac issues and they want to make sure I’m not going to die on the road. FF comes nowhere close…unless you are really pushing it. You wouldn’t want to do something like HHNF with the numbers from that test.
I went back to my training data, and I noticed I have only done HHNF twice, both in 2021. My notes say that I set peak power numbers for several time durations. They also said I was wrecked by the time I was done.
I was doing the old HHNF. Surprise. The second 20 minutes was at 1.05%. Every ride I was getting a new FTP from TP. I went from about 230 to 260 in about six months. That wrecked me for years. I’m way more sensible these days…