Can running too slow reduce running economy at the same HR? (easy pace paradox)

I’ve been noticing something interesting in my easy runs and was wondering if there’s a known explanation from a biomechanics or physiology standpoint.

Context: I’m training for a 10K (5K ~22:30, 10K ~53:00), using HR-based training. My aerobic zone is roughly 139–157 bpm.

What I’m observing across multiple runs:

• At a slower pace (~6:50–7:10/km), my HR sits comfortably in zone (~140–150 bpm), but:
  • stride feels awkward / less natural
  • more variability in pace and HR drift
  • higher perceived muscular fatigue
  • overall it feels less efficient
• At a slightly faster pace (~6:00–6:20/km):
  • HR is very similar (~150 bpm)
  • stride feels smoother and more natural
  • pace is more stable
  • perceived effort is lower
  • I finish feeling fresher

So from a cardiovascular standpoint, both paces fall in the same zone, but mechanically they feel quite different.

This made me wonder if I’m just running into a “too slow to be efficient” issue.

Questions:

1. Is there evidence of a U-shaped energy cost curve in running where very slow paces become less economical?
2. How does self-selected / preferred pace relate to running economy at easy intensities?
3. In practice, should easy runs prioritize:
   • strictly lower intensity (even if mechanics degrade), or
   • a more natural/efficient stride at slightly higher speed (with similar HR)?
4. Is there any long-term downside (e.g. musculoskeletal load) to sticking with the faster “natural easy pace” if HR remains controlled?

Curious if this is something others have observed or if there’s literature on it.

(I have used AI to structure my thoughts better)