Hand to hand combat without weapons its far more exhausting than any other form of fighting. Even if you have a rifle to muzzle thump somebody with, or butt stroke, you can fight for far longer than if the two of you end up grappled on the ground. This is due in part that the two of you are continually resisting the force of each others muscles, which means you are constantly under tension. Punches get weaker, your cardio resperatory system goes nuts, your legs don't move as fast, hits to the head blur the mind and cause vision imparements and headaches. God forbid you are the unlucky guy who has a rib broken by a solid blow. Hard hits can knock the wind out of you too, particularly if you are slammed. Even really good fighters I've only seen go for three, maybe four minutes before their endurance starts to suffer, and each man is just sort of guarding against the other hoping they'll give them an arm to break, they stop throwing punches and kicks, they breath heavily, and then hold each other almost as if cuddling on the ground.
Hand to hand combat is exhausting, I don't care how use you are to it.