I’m in the first few weeks of the Intro to Ultra training plan and trying to determine my AeT heart rate–without much luck thus far. I’ve done 2 tests now at increasingly higher heart rates and each time I’ve basically failed to get almost *any* drift.

For reference, about 6 months ago, I completed a half marathon in 1:30 at around 172hr. The data below is admittedly from a wrist monitor on my watch–but it’s been pretty consistent without much variation, so I’m hesitant to ascribe any problems to that. On the lower end, I’ve also done a handful of longer runs at around 145-150, with very little drift.

Should I do yet another test? Call the top of Zone 2 at 170 and just train at closer to 155? Any suggestions would be helpful!

Here are the numbers, and some screenshots for reference:

First test:
Pa:Hr: .84%
avg Hr: 159
avg pace: 8:12

Third test
Pa:Hr: .27%
avg Hr: 170
avg pace: 7:23