Your shoulder is composed of three bones. The humerus is your upper arm bone. The clavicle is your collarbone. The scapula is the shoulder blade that moves on your back. A prominent edge of the scapula, the acromion, forms the top of the shoulder.
The head of the humerus is round. It rotates in a shallow basin on the scapula called the glenoid. A group of ligaments, called the joint capsule, hold the head of the humerus in position. Ligaments are strong tissues that connect bones and provide stability. In other words, the joint capsule is responsible for holding your upper arm in place at your shoulder.
Four muscles at the shoulder form the rotator cuff of tendon that connects to the head of the humerus. The muscles allow the arm to rotate and move upward to the front, back, and side. A gliding membrane, the bursa, lubricates the rotator cuff tendons and reduces friction around them when they move.
You use your rotator cuff muscles whenever you perform overhead motions, such as lifting your arms up to put on a shirt or reaching for an item on a shelf. These motions are used repeatedly during sports, such as serving in tennis, pitching in baseball, stroking in swimming, and passing in football. Overhead motions may also be used for job duties, such as for construction.