What is that “zone of power output” that you only spent 43 seconds in during your 3x15 intervals?
Whatever your heart rate trends to when going at FTP after say 15 or 20 min, is you FTHR. This HR will be lower than the HR you maxed out in the ramp test.
I am quite confused about this subject. My HR zone based on HRmax it would be xx - 150 bpm. Using LTHR it would be much higher ( - 160 bpm ). I’ve tried 3hr endurance rides on both method.
Did it with LTHR, of course feels harder to my body, and it takes longer time to recover. and it really effects my performance to do the HIIT next days.
While did it with HRmax feels much easier, but I don’t know if the stimulus was enough or not.
Ignore HR. Ride to power. If you don’t have power, learn RPE.