@davidbrush: The thing to be careful of is that you don’t inadvertently test at your anaerobic threshold because that will behave in a similar fashion.
The big difference is that with an AeT drift test, your average HR could rise further but it doesn’t. At AnT, your average HR can’t rise further because of the mix of intensity and duration.
I was skeptical of the drift test too, but I have more faith in it after comparing it to gas exchange tests. Here are some examples from athletes that I coach:
* Athlete H did a drift test that suggested an AeT HR of ~150. He later did a gas exchange test that suggested the mid-140s. The test was not our protocol, and his AnT HR is much higher, so we continue to use ~150;
* Athlete V did a drift test that suggested an AeT HR of ~130. A later gas exchange test that was done according to our protocol suggested ~135. His AnT / AeT gap is huge and training at ~135 is not very taxing, so we started using 135.
Done properly, the drift test will give you a reliable heart rate to base your training off of.
In the post that you linked to, I think you’re trying to be precise with something that can never be. Heart rate is a reflection of global stress, so unless all of the other stresses in your life are static, you’re splitting hairs to try and determine the difference between say, 135 and 140. Within five, perhaps even ten beats, it just doesn’t matter. For example, a ten-beat difference on 140 bpm is ~7%. So if you train ten beats too low, you’re getting 93% of the benefit. Close enough!
Only when AnT / AeT equals <= 1.1 can small changes in heart rate reflect significant changes in load. (And at that point it makes sense to use something that can be precise like pace or power.)