Originally Posted by fuzzthebee

I train with a PT so I don't have to account for HR drift.

However, I only have a 4 bpm difference in average HR vs max HR during a 2 x 40' at 90% FTP. The first one is 153 bpm average, 157 bpm max. The second one is 153 bpm average, 157 bpm max. I reach 153 bpm around 5.5 minutes into the each interval. The last half of the second interval actually averages 91% FTP with an average HR of 154 bpm.

That seems pretty steady to me.