I figured this was a bug. Even when you're too tired to swing your weapon, you still gain exp from it.
R>attack
You are too fatigued to be able to attempt that maneuver right now.
[You feel very healthy, are somewhat tired, in a fair position, and are completely balanced.]
[Roundtime: 2 seconds]
You gain a new rank in Greataxe.
[You gain 18 experience points.]