Does a backpack work any muscles surrounded by your body?
Answers: Anytime you include weight to your body you are working the muscles a touch harder than normal. Your backpack will mostly imagined affect your upperbody muscles, your back muscles, and even your leg muscles. Probably not ample to build muscle though.
Sure. It works your legs, carrying the extra weight, and your lower wager on to keep your posture correct. Maybe your shoulders a bit if you own to "shrug" them to keep it on.