Anaerobic Threshold Test Gone Terribly Wrong

Howdy folks!

I’m about 6 weeks into the 16 week Big Mountain training plan, and I just took the Anaerobic Threshold Test as described on Training Peaks. I followed the set up instructions pretty much exactly. My set up was this:
* slowly warmed up for 15 minutes, increasing speed until I broke a sweat (hit about 3 mph). Incline was set to a 15% grade this entire time.
* At this point, I increased the intensity to around 4.5 mph, keeping the same 15% grade. After around 10 minutes, I realized this speed was unsustainable so i bumped the speed a bit back down to around 3.8 mph.

What’s strange is that, this entire time, my heart rate did not even CLOSE to my AeT of 160 bpm (determined by a MET test in a lab). In fact, according to my Garmin, my average heart rate for this effort was 108 bpm. You should know that I was trying – HARD. Especially when I saw my heart rate not getting close to AeT, I bumped the speed back up a bit. No good.

What I found was that by far, what felt like my limiting factor in this test was my calf/ankle endurance. when I was going 4.5 mph, my heart rate was around 125 bpm, but my calves were absolutely on FIRE.

Does this mean my limiting factor is my calf/ankle endurance, and my aerobic system is leagues ahead of what my calves/ankles can handle going uphill? And if so, how should I adjust my training to correct this? Add as much vert as possible to all my aerobic base building runs?

I should note that I, for the most part, notice this similar effect when I do my long “Zone 2 Hike on Hilly Terrain”. I usually do these in tall high rise stair wells with a bunch of vert gaing over several hours, and my HR is much below my AeT while my legs slowly give out first.

Thank you so much for any help and advice! Cheers.