The shoulder is one of the most mobile joints in the body. It flexes, extends, abducts, adducts, and rotates the arm. This amount of movement comes with a price, though: the shoulder is not as stable as the other joints. The muscles surrounding this vital joint accommodate the movement and necessary stability of the shoulder.
The trapezius is one of the largest shoulder muscles. It has three sections. The upper part supports the weight of the arm, the middle retracts the scapula, and the lower part rotates and depresses the scapula. This large muscle originates vertically from the occipital bone at the back of the head through the cervical and thoracic sections of the spine and attaches to the clavicle and scapula.
Another of the main shoulder muscles is the deltoid. It lies over the top of the shoulder and has three origin points, one from the clavicle and two from the scapula. It inserts into the humerus, the bone at the top of the arm. The deltoid flexes, rotates, abducts, and extends the humerus, depending on which fibers are activated.
The pectoralis major is large, and one of the most significant chest and shoulder muscles. It is thick and fan-shaped, originating from the clavicle and sternum and inserting into the humerus. Depending on which head of the muscle is involved, the pectoralis major flexes, adducts, rotates, or extends the arm.
The serratus anterior is one of the smaller shoulder muscles. It originates from the front of the upper ribs and inserts into the scapula. There are three sections to the serratus anterior: the superior, intermediate, and inferior. This muscle allows the arm to rotate forward and pulls the scapula forward over the back of the rib cage. It is also vital for elevating and upwardly rotating the arm.
The rhomboid major and minor are important back and shoulder muscles. The rhomboid major originates at the thoracic vertebra, and the rhomboid minor originates at the lower cervical and first thoracic vertebrate. Both insert into the scapula and retract and rotate it. In some anatomical cases, these two muscles are fused into one.
The rotator cuff is made up of four muscles: the infraspinatus, trees minor, subscapularis, and supraspinatus. These muscles work together to stabilize the shoulder joint and are also involved in the abduction and possible lateral rotation of the humerus. The supraspinatus is most commonly torn when the rotator cuff is injured, and there is extensive research into this muscle.
The pectoralis minor is one of the smaller shoulder muscles. It is triangular and sits directly underneath the pectoralis major. Together, they form the front wall of the axilla or armpit. It attaches to the third and fifth ribs and the scapula. This muscle rotates the scapula downward and internally.
Another of the large shoulder muscles is the latissimus dorsi. It originates from the seventh thoracic vertebrae through fifth lumbar vertebrae, scapula, lower ribs, and the iliac crest in the pelvis and inserts into the humerus. In addition to extending the arm, it also participates in adduction and medial rotation.
Biceps brachii shoulder muscles have a long and a short head. They originate from separate parts of the scapula, then come together to form a tendon that inserts into the forearm at the ulna. Though it crosses the shoulder and elbow and is visible when flexing, the biceps brachii does not move the upper arm. It flexes the elbow and rotates the forearm.
The triceps brachii is another muscle that originates in the shoulder but does not move it. The triceps has three heads: a long head that originates at the scapula, and medial and lateral heads that originate from the humerus. This muscle extends and retracts the forearm and stabilizes the shoulder at the top of the humerus.
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