I based mine on “All day HR” which was seen on some epic ‘in it to finish it’ rides during the first year of cycling (2016).
For example this day with 8+ hours of climbing for the 5 climbs:
Average HR at average temp for the 5 climbs:
- 137bpm at 44F
- 140bpm at 65F
- 141bpm at 74F
- 137bpm at 81F
- 135bpm at 65F
Similar data for other long days of climbing.
In 2020 I tried lower 128-134bpm HR target, and then this past year I’ve experienced greater fitness gains by going back to targeting 136-142bpm and shooting for an average of 138bpm on my endurance rides.