[Scott Johnston]

Andrew:  Thanks for writing in with your question.  This is very unusual to see AeT and AnT so close together.  I normally see, in the very aerobiacally fit, a spread of 5-8%.

From your consistent historical data, I would say you have done a good job on the tests with only one possible point:  Have all your AnT tests been uphill at the same grade?   This could matter because running uphill requires more muscular endurance. So, local muscular fatigue in the legs could potentially limit your HR on the uphills.  The steeper the hill, the more the ME and local muscular fatigue effect.  A 9% grade may produce this effect depending on your specific leg ME.  I hope this makes sense.

It is entirely possible to have thresholds this close together for a person will a very well-developed aerobic base.  A good test of whether you’ve nailed these zones is: Can you train daily at AeT (165-168) and recover well within 24 hours?  Or would it be too fatiguing to train your normal volume at that intensity?  With AeT and AnT only 3-4% different, I would expect that training in Z2 would not be possible long-term without risking overtraining.  Z2 is just too dang hard, too close to your maximum sustainable effort to be sustained day after day after day.

I hope this helps.