The muscle-up is a rare beast – an upper-body exercise that requires both pulling and pushing power and phenomenal core strength to boot.
Those unfamiliar with the move say it's just a pull-up combined with a dip, but a muscle-up is much more than that. It's an unparalleled assault to the upper body, and you have to be strong just to do one.
If you want to build huge muscles, you must continually challenge them by placing a greater demand upon them. If you stick with the same stress level month after month, you'll quickly reach a point where your body is used to the stress and won't need to adapt (i.e. grow) anymore.