The shoulder is a very mobile structure that is made up of a “ball and socket” joint created by the arm bone (humerus) and shoulder blade (scapula). This ball and socket joint is surrounded by four muscles collectively called the rotator cuff, which act to hold the joint together. There are many ligaments that further help to provide stability to the shoulder joint. The shoulder joint also has a bursa, which is a fluid filled sac that acts to reduce friction between the moving parts. The whole shoulder joint is surrounded by strong muscles like the deltoid, pectoralis and trapezius. All of these parts have to work properly together to create arm movement and generate strength.