If you take someone that isn't heavily muscled and hit their bicep with a stick.....the the tissue will get slammed against the humerus as if you were smashing it between 2 sticks.
If you look at the picture Nevada MO Guy posted, you will notice another muscle beneath biceps brachii, the brachialis. This muscle is the major flexor of the forearm, when the forearm is in a flexed position, even slightly without load, the musculocutaneous nerve sits there in it's armoured glory, being cushioned by both flexors.
Will it hurt a bit if you hit it?
you betcha, but in a highly muscled person, not as much I would think.