The shoulder joint is made up of three bones: The scapula (shoulder blade), the humerus (upper arm bone), and the clavicle (collarbone). The shoulder joint is actually two joints working in unison to create the wide range of motion. The acromioclavicular (AC) joint is located between the acromion and the clavicle at the top of the shoulder. The second joint is the shallow ball and socket of the glenohumeral joint. The glenoid is the outer socket of the scapula into which the ball of the humerus fits. Arm movement is further enhanced through the ability of the scapula to slide along the rib cage. The synovial membrane surrounds the glenohuneral joint and secrets synovial fluid into the joint to maintain lubrication.

The muscles governing motion are primarily the four rotator cuff muscles. The supraspinatus connects to the upper portion of the humerus from the scapula. It’s function is to provide adductive (lateral upward motion) capability to the shoulder. The subscapularis connects the shoulder blade to the front of the humerus. Contraction of the subscapularis governs inward rotation of the joint. The infraspinatus and teres minor connect the rear of the humerus to the scapula to provide external rotation.

The deltoid is the second major muscle group of the shoulder. It is divided into three sections. The anterior (front) portion lifts the arm to the front. The medial deltoid lifts the arm to the side. The posterior (back) deltoid is responsible for backward motion of the arm.