The human shoulder joint works by acting as a ball and socket coupling that provides extreme mobility to the shoulder. The shoulder joint aids in the forward and backward movement of the shoulder and enables the arm to freely rotate in and out or to extend toward and away from the body, states the United States National Institute of Arthritis and Musculoskeletal and Skin Diseases.
The human shoulder comprises three bones: clavicle, scapula and humerus, or more commonly known as the collarbone, shoulder blade and upper arm bone, respectively. The shoulder joint, otherwise referred to as the glenohumeral joint, consists of only the humerus and scapula. This joint is further surrounded by muscles, tendons, cartilage and ligaments, notes Healthline.
The rounded head of the upper arm bone forms the "ball" part of the shoulder joint. This fits into the shallow, dish-like portion of the shoulder blade called the "glenoid," which serves as the "socket." A thin, smooth membrane containing synovial fluid provides lubrication for the space between the ball and socket to facilitate joint movement. The group of muscles and tendons enclosing the shoulder joint forms the rotator cuff, which reinforces the shoulder joint. Two fluid-filled sac-like cavities called bursae enable the bones, muscles and tendons to easily slide past each other.
A fine balance of stability and mobility exists in the shoulder joint. In exchange for the widest range of motion in the human body, this particular joint is unstable and highly prone to overuse and injuries.