[Seth Keena]

Hi Henry,

Thanks for writing in.   There are many subtleties that can play a part in slowing pace over time at the same HR. Running calorie deficit or chronic fatigue in the legs and or globally can be some reasons. You might try to cut volume by 50% for a week or 10days , calorie balance for time period, and observe what happens to your AeT pace. If you see it come up, it was likely you were in need of a rest week. You may have been over training (beyond your capacity for too long) and the only way to know this is to resume the same volume as you were and watch what happens to AeT pace. If it slumps again, it’s likely this load is too much and you need to train fewer hours.

If you’re not already, adding a few strides and/or sprints to the week could help this situation. Enforce signaling for greater leg speed and power that translates to steady state aerobic pace. Basic strength in the training is almost always a good means to maintain power for stride length.

Above are some approaches to working through this, but there are more. Likely no one perfect answer either. Also, do another treadmill test at 135bpm; no use in prolonged speculation if you can do a workout and a free test all in one and walk away with the information you really need.

Hope this helps,

Seth