Example - LTHR = 172
Coggan Z2= 118-143
Friel Z2 = 141 - 152
I'm going to say that Coggan's method appears to be more of a 'Baseline' where Friel's seems to actually fluctuate in line with your LTHR.
For me 130 HR is NOT zone 2.5 ... 146... yeah... that seems legit. Funny that they both seem to agree on the top of zone 3 (Coggan 162, Friel 160)...
Thoughts? There is a big difference between 130 and 150!