The shoulder is one of the most complex and flexible joints in the human body. This overview will help you understand its key components and how they work together.
The shoulder consists of three main bones: the humerus (upper arm bone), scapula (shoulder blade), and clavicle (collarbone). The shoulder contains several joints, including the glenohumeral joint, which is the main shoulder joint, the acromioclavicular (AC) joint where the collarbone meets the shoulder blade, and the sternoclavicular joint where the collarbone meets the breastbone.
The soft tissues of the shoulder include various muscles. The rotator cuff consists of four muscles that stabilize and move the shoulder: supraspinatus, infraspinatus, teres minor, and subscapularis. The deltoid is the large, triangular muscle covering the shoulder, while the biceps and triceps are arm muscles that cross the shoulder joint. Tendons connect muscles to bones, with the rotator cuff tendons being particularly important for shoulder function. Ligaments connect bones to other bones, provide stability to the joints, and help prevent excessive movement.
Other important structures include the labrum, which is a ring of cartilage that deepens the shoulder socket, the bursa, which are fluid-filled sacs that reduce friction between moving parts, and the joint capsule, which is a sleeve of tissue that encloses the shoulder joint.
The shoulder’s unique design allows for exceptional range of motion, including forward and backward movement, rotation, moving your arm away from and toward your body, and circular movements. The rotator cuff muscles work as a team to keep the ball of the humerus centered in the socket, control arm movement, and provide stability during motion. The deltoid muscle provides power for lifting and moving the arm, while the rotator cuff guides and controls these movements.
Understanding this anatomy helps explain why the shoulder is so mobile, why proper form during activities is important, why some shoulder problems are common, and why different parts of the shoulder can affect each other. This complex arrangement of structures working together allows us to perform countless daily activities, from reaching overhead to throwing a ball.
The humerus, more commonly known as the “funny” bone, is the long bone of the upper arm. The ball-shaped head of the humerus sits comfortably within the glenoid cavity of the scapula to form a ball-and-socket joint.
The clavicle, a horizontal S-shaped bone, connects the upper limbs to the trunk of the body. The shoulder’s range of motion heavily relies on this bone’s ability to oppose outside forces that move the upper limb toward the thorax.
Commonly referred to as the AC joint, the acromioclavicular joint is formed by the connection of the clavicle and the acromion (the highest portion of the shoulder blade). Together, these bones provide the ability to lift the arm above the head.
The articulation of the head of the humerus into the glenoid socket of the shoulder blade forms the glenohumeral ball-and-socket joint of the shoulder girdle. The glenohumeral joint is the most mobile in the body as the ball-and-socket joint configuration allows motion in several directions.
The sternoclavicular joint is created from the connection of the clavicle and the sternum (breastbone). While this is a lesser-known shoulder girdle joint, it is responsible for clavicle movement along three different planes.
Surrounding the upper portion of the humerus is a group of four muscle-tendon pairs known as the rotator cuff. While the rotator cuff is primarily responsible for securely stabilizing the humeral head within the shallow glenoid socket of the scapula, it also contributes to the shoulder girdle’s range of motion.
The glenoid socket of the scapula is lined with a cartilaginous tissue ring known as the labrum. This ring is an attachment point for the biceps tendon and several shoulder ligaments. The labrum also stabilizes the glenohumeral joint by deepening the glenoid cavity to better hold the humeral head in place.
Between the bones and surrounding soft tissues are a number of small, fluid-filled sacs that secrete a viscous fluid responsible for reducing friction with shoulder joint movement.