What type of heart rate monitor were you using to track this? The fluctuations seem pretty minor. The last 25 minutes not moving from 133 strikes me as more odd. Also, do you have a TrainingPeaks account, and could you provide a public link to the workout?

Good – optical heart rate monitors tend to be less effective than chest strap monitors. From what you’re describing, it sounds like you can put your aerobic threshold at ~130, and ignore the fluctuations. Heart rate is a valuable metric, but remember that it’s a proxy. The heart’s response to the demands you put on your body is not precise, so some variation is to be expected. One of the coaches might have more insights to share.

Hey smatty,

Regarding the 2nd 15 mins of your test when you had a higher HR than the first: after doing my own AeT test on the treadmill recently (and after monitoring my HR for the past two years) I now know that it takes me a solid 20-25 minutes to properly warm up before my HR will stabilize at a given pace and grade. It wasn’t until about minute 25 on my treadmill test that my own HR stabilized during my treadmill test, and it too stabilized at a lower average HR. Next time I will warming up for a full 25 minutes. This could be something for you to try.

Regarding the constant HR reading:
I wear a Suunto Smart Sensor HRM, a chest strap model like the Dual. Don’t know if it behaves at all like the Dual, but the Smart Sensor has given me a lot of grief. I am now on my 3rd one, as the first two would show very erratic readings (despite me diligently and routinely cleaning it after each use, as per instructions). This included ‘sticking’ to one HR for a long time and showing no variation (despite increasing or decreasing intensity notably). It also included readings in the mid 200’s (!) when my carotid pulse indicated I was in the 140’s. The first unit was replaced by Suunto because of this erratic issue and the second one would have too, but I let the warranty period lapse so had to purchase the 3rd. If this is or becomes a common problem with your HR data and monitor, you could send an email to Sunnto support. They may assess the data and if deemed a technical issue, may warranty your HRM (if it hasn’t expired).

Good luck.