The type of muscles that develop as a guy transitions from teen/young adult to full fledged man. It is accompanied by a gain in strength and usually happens between the ages of 26-28. Man muscles aren't bigger, but they are rock hard and will allow men to beat much "stronger" young men in things like arm wrestling and give them greater endurance in repetitive hard labor.

It's the sweat that accumilates under big boobs

The muscle in the forearm that gets ripped as a result of jerking-off to often.