Other than Spellcasting, what ELSE causes Fatigue?

[FireHorse]

Originally Posted by philreed

Strict tracking of fatigue throughout a normal adventure would be dull.

Indeed, it would, which is why I would never bother doing so.

But I can easily imagine Fatigue becoming relevant during a protracted battle, when the flow of combat makes it impossible to stop and rest. And I can also imagine many ways to wind up in such a battle: sieges, running battles, etc.

ITL says "exertion" causes Fatigue — so all I'm wondering is, how many turns of sustained physical combat qualifies as enough exertion to cost some ST? If just running up a flight of steps will do it, how long can you keep swinging a sword before it begins to wear you out?