After more than a year with the 645M, I decided to performed the lactate test. first 3 "laps" of 4 minutes were alright. But when it asked for staying 3 min between 160 - 170 then it seems that my HR belt failed. I kept going faster and my HR dropped from 162 to 107 in first 2 minutes. At that point I've reached pace of 3:45 /km . I slow down and HR start going a bit up but not so much. I finished next 3 minutes lap at 138 when I was probably around +172 as I know myself quite well. What failed ? HR belt, lactate test within the Forerunner or my Heart :) ?
I will try again tomorrow
Try to run at least 5 minutes in each HR zone like 100-110, 110-120... You have to make some easy runs too so your watch will learn your zones...