The muscles of your shoulder are strongly involved whenever you throw a punch. Your anterior deltoids, located on the front of your shoulders, work in synergy with your pectoralis major or chest muscles to drive your arm out toward your punch bag. Once you have struck your target, the muscles opposing your chest and shoulders -- specifically your middle trapezius, rhomboids and latissimus dorsi -- draw your arm back ready for you to throw another punch.
Extending your arm is primarily the job of your triceps brachii. Located on the rear of your upper arm, this powerful muscle snaps your elbow out so that you can reach your target. Your biceps brachii, located on the front of your upper arm, is responsible for bending your elbow as you pull your arm pack into your guard position.
There are a large number of muscles in your wrist that are either wrist extensors or wrist flexors. The extensors and flexors of the wrist contract statically and evenly on either side of your wrist joint to ensure your wrist is straight when your knuckles strike the punch bag. Boxers often wear hand and wrist wraps to help keep their wrists straight and minimize the likelihood of a hand or wrist injury.