A couple of ideas that center around Fatigue.
Perk - Push through the pain
sacrifice health to continue using energy once fatigue has hit. Include a possible regen penalty to even it out. You can literally run yourself to death, if not careful.
a sort of berserker idea.
fatigue animation -
Make the character look like it is skipping a step, and physically trying to catch its breath while fatigue is active. The “injured” animation may work for this.