ISM recommendations require lactate, anything else is shooting in the dark IMHO.
Thru trial and error I’ve set the top of my Zone2 HR at 143bpm, which is about 89-90% LTHR. I ride by power and average HR is just below the top of zone2. Here is ride from 2 days ago:
about 100 minutes endurance with 10 minutes on either end for warm/cool down. About 74% IF and average 86% LTHR, however both power & HR slowly increase over that time (decoupling 0.2% so no decoupling). %LTHR is bottom right of the HR graph. The green band is 66-79% ftp.
And last night:
about 70 minutes at 77% IF and 88% LTHR.
Again thru trial and error I’ve learned that level I can sustainable do a two or three focused 90-110 minute sessions a week as part of a 2+ hour outside ride. Target is about 8-11 hours/week.
I’m going for performance while balancing recovery. Pushing higher loads requires more force on pedals, and seems to give me better endurance when it matters (epic events, Wed worlds, etc).
I’ll also add my HR is more “compressed” compared to others I’ve seen. A higher floor and lower ceiling. Threshold efforts are around 90% HRmax.
Hope that helps, I believe if you don’t go to the lab you need to experiment and figure it out for yourself.