The shoulder is the most mobile joint in the human body. The joint includes four tendons. The purpose of a tendon is to hold muscle to bone. Together, these four "rotator cuff" tendons stabilize the upper arm bone to the shoulder socket and allow a wide range of motion in the shoulder.
Any swelling, inflammation, tearing, or bony changes around these tendons causes pain when a person tries to move the arm above the head, behind the back, or straight out in front.
The tendons pass underneath a rigid bony arch in the shoulder. The most common cause of shoulder pain is when the tendons become trapped under this arch. The compressed tendons become inflamed or damaged, a condition called rotator cuff tendinitis. This can occur from general wear and tear as you get older, an activity that requires constant shoulder use like baseball pitching, or an injury.