So… this isn’t entirely correct.
Your FTP is NOT the power you can maintain over an hour… FTP is something that, in a well trained athlete, translates to roughly the power that you can maintain at a steady state effort for an hour but that’s not actually what FTP is.
FTP is functional threshold power, a measure of your anaerobic threshold level… which is the point at which an effort causes your blood lactate to accumulate faster than it can be metabolized via the cell to cell lactate shuttle. It’s the functional threshold between an aerobic and anaerobic effort. Actually measuring this involves blood sampling and respiratory exchange testing every few minutes at increasing outputs.
The common one hour FTP measure we all know is sort of a poor man’s equivalent meant to bring a reasonably comparable field testable measure to everyone without access to a traditional lab but it introduces a lot more wiggle room. If you’re a well trained athlete, then your 20 minutes max multiplied by 0.95 will correlate pretty well with your lab tested FTP… if you’re a twice a month weekend warrior who rides the couch harder than you ride the weekly club ride, then it’s likely closer to a 20 minute max multiplied by 0.85… if you’re totally untrained and hopping on a bike for the first time in ten years, you’re gonna be closer to a 20 minute max times 0.75.
So, for a highly trained and tested athlete like Ganna doing a 50km TT… his lab tested FTP is the exact spot at which his physiology will start to accumulate more lactate than it can shuttle and create fatigue induced power losses. For the rest of us playing along at home with power meters and a strava account, it’s a ballpark measurement with a lot of wiggle room and your average cyclist isn’t going to be able to hold their 20 minute times 0.95 power for a full hour.