Getting worn out in battle.

[Saseal]

FFXV did something similar when you or the NPCs hit the secondary HP bar - it effected the moving animation as much as the standing (bent over, holding stomach, breathing hard, kinda staggeringish) - it would just be without the slowed movement speed that came with the animation, for FFXIV's case.

I think it's doable - just a standing/running animation swap (from standing straight to staggered/slumped) for the upper body in the least. WHM doing a BLM skill (sleep, etc) or SCH doing a WHM skill swaps class animations when casting, etc, already. Sprint does this already (as someone mentioned, which made me remember xD). If done right, I don't think it would impede movement but still give the aesthetic of struggle..