I’m heavily invested into Garmin’s ecosystem and like data in general. Every few rides my Garmin Edge 530 gives me a new FTP + Threshold estimate, and it’s in a ballpark of 240W/163bpm. I’ve just done Garmin’s FTP test protocol and it showed the same. When I do Zwift’s FTP test, TP’s FTP TTE or just a hard race on Zwift, Intervals.icu estimate my FTP/Threshold HR around 280W/173bpm.
I know Garmin uses HRV and other metrics to calculate their FTP estimate but most people say their FTP estimates from different platforms are within 10W or so. IMO, 40W diff in my case is a lot, and I want to understand why, which estimate I should trust, and what number I should derive my FTP zones from.
I think I’ve just found answers to my own questions.
Below is how Garmin and Intervals.icu calculate their Power/HR zones based on their own estimates of FTP. Interesting that Z1-Z5 for both power and HR match very closely (Intervals HR uses 7 zones but their Z5-Z7 is similar to Garmin’s Z5). It means that your times in zones will be very similar between the 2 platforms.
Re the question which FTP number to use as a reference point in workouts. I use the number from Intervals and it makes me working hard on my 4x4m 115% FTP intervals, for example. If I used Garmin’s number, this workout would be a breeze.
Unfortunately, you’re not showing us the most important part… If you have a high anaerobic capacity, Intervals will over-estimate FTP with the default setting for ‘eFTP min duration’. You may want to adjust that to at least 600s (10min).
Yes, I’ve got this setting at 300s as well but I don’t think my anaerobic capacity is disproportionally high (power curve below). Intervals give me approximately the same eFTP regardless whether I do a 6min climb, 20min FTP test or 35m FTP TTE test so I trust this estimate the most. I found it interesting and very useful that Garmin and intervals zones are almost identical even though their FTP estimates are very different.
Intervals.icu uses a generalisation of the “95% of 20m power” rule of thumb to estimate FTP and longer durations will get you a better number. I don’t know how Garmin does it. If you can still get VO2Max workouts done with the Intervals.icu FTP then it probably isn’t too low.
