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Although it may seem simple, stretching your arms over your head requires a complex, coordinated movement of your whole shoulder girdle, termed the scapulohumeral rhythm. Many muscles are involved in this action, including the rotator cuff muscles on your shoulder blade, the deltoid muscle on your outer shoulder and the trapezius and serratus anterior muscles on the back and side of your ribcage.
The shoulder girdle consists of three bones: the collarbone, or clavicle, the shoulder blade, or scapula, and the upper arm bone, or humerus. The round head of the humerus fits into a socket on your shoulder blade just under a ledge of bone on your outer shoulder called the acromion. The sequence of the scapulohumeral rhythm varies depending upon whether you take your arms out to the side and overhead, an action called abduction, or forward and overhead, which is termed flexion. To raise your arms fully overhead, your shoulder blades must upwardly rotate, orienting your shoulder socket vertically and allowing the humerus to clear the overhanging acromion.
The four muscles of your rotator cuff attach to your shoulder blade. Their tendons wrap around the head of the humerus, blending with the connective tissues around the joint. The rotator cuff muscles stabilize the shoulder joint by pulling the head of the humerus into the shoulder socket. One of them, the supraspinatus, helps the lateral deltoid abduct the arm. Two others, the infraspinatus and the teres minor, externally rotate the humerus when you abduct the arm, helping the head of the humerus clear the acromion. The fourth rotator cuff muscle, the subscapularis, internally rotates the humerus when you flex your arm overhead.
The deltoid muscles on the side of your shoulders do most of the heavy lifting when you raise your arms. They originate on your collarbones and shoulder blades and insert onto your upper humerus. Each deltoid muscle has three sections, or heads. The front, or anterior head, flexes the shoulder, and is active when you bring your arms forward and up. The lateral head abducts the shoulders; it lifts the arms out to the side and up. The rear, or posterior head, takes your arm backward and is not directly involved in reaching your arms overhead.
Two muscles -- the trapezius and serratus anterior -- are responsible for upwardly rotating your shoulder blade. The trapezius originates from the back of your skull and the spine of your neck and ribcage. It inserts onto your outer collarbone and shoulder blade. The upper part of the trapezius lifts the outer corner of your shoulder blade, while the lower part pulls the inner shoulder blade down. When both parts work together, they upwardly rotate the shoulder blade. The serratus anterior originates along the inner edge of your shoulder blade. It passes under the shoulder blade and inserts onto your upper nine ribs. Its job is to pull the scapula around the side of the ribcage, assisting the trapezius with upward rotation.