Three months ago I determined my AeT using an outdoor test to be 157 bpm. Since I am starting a new training plan for mountain running, I tried to do an indoor test with a treadmill set to 2 deg incline wearing a chest-strap HR monitor. Since the treadmill is limited to 60 minutes, I warmed up on an indoor running track for 15 minutes, then used the first 5 minutes on the treadmill to get my HR back up to about 160 bpm and calculated the drift from minute 6 onwards using two halves of 27 minutes.
What stumped me was that twice my HR actually decline significantly, the first time just between the 13th and 16th minutes it went down to 142 and then later between the 52nd and 55th minute again down to 145. Apart from these two dips the HR seemed to hover around 160 going up and down 3 bpm. I left the incline and the speed constant.
However, when looking at the data of the two halves, I found no drift 🙁
The first half of 27 minutes measured from 06:00 till 33:00 gave HR of 158 bpm avg (min 132, max 166 bpm) whilst covering 4.24km at avg pace of 06:24 min/km (min 07:02, max 05:48 min/km). And the second half of 27 minutes from 33:00 till 59:00 gave an avg HR of 158 bpm (min 141, max 164 bpm) whilst covering 4.22km at avg pace of 06:23 min/km (min 07:04, max 05:51 min/km).
So I want to know: are those two three minute dips in HR a known phenomenon? I wanted to target 160 bpm but after initial stabilization for about three or so minutes it ended up lower. Should I redo the test or can I guestimate AeT for the purpose of setting zones with the data above?
Thanks for your patience with answering yet another HR drift test question!