[MaxWilson]

From each attack action. If you don't have some form of Quickness, that means that each turn you are in melee combat (and aren't Awed, or Repelled, etc.) you will take fatigue equal to your encumbrance. If you are quickened you will make twice as many attacks (or even more if you have Heroic Quickness as well) and take twice as much fatigue per turn. If you are naturally 0 Encumbrance then you will always REMAIN at 0 Encumbrance for melee purposes, no matter how much armor you wear.

There are other things (flying, spellcasting, stun damage, berserk, etc.) which can also increase fatigue. There's a thread called "Fatigue guide" or something which details it extensively.

-Max