Muscles of the Shoulder and Arm

By David Terfera and Shereen Jegtvig from Clinical Anatomy For Dummies

The muscles that provide the movement for the shoulder and upper arm include the anterior muscles in the front, the posterior muscles in the back, and the shoulder muscles that are sort of in between. Four anterior pectoral muscles move the pectoral girdle and arm from the front:

Pectoralis major: This muscle has two heads, or sections of muscle tissue, and covers much of the front of the thoracic cage. The clavicular head attaches to the medial portion of the clavicle. The sternocostal head attaches to the sternum and first six costal cartilages. It inserts into the lateral lip of the intertubercular groove of the humerus. The pectoralis major flexes, adducts, and medially rotates the humerus.
Pectoralis minor: This muscle is found in the anterior wall of the axilla, underneath the pectoralis major. It attaches to the 3rd through 5th ribs and the coracoid process of the scapula. It helps to stabilize the scapula.
Subclavius: This small, round muscle is located just below the clavicle. It anchors and holds the clavicle down during movement.
Serratus anterior: This muscle is attached to the lateral portions of the first eight ribs and the medial border of the scapula. It helps hold the scapula against the back.

The posterior axioappendicular muscles (or extrinsic back muscles) attach the pectoral girdle to the thoracic wall and posterior thoracic cage:

Trapezius: This triangular muscle covers the back of the neck and the upper half of the posterior thoracic cage. It attaches to the base of the skull, the nuchal ligament, and spinous processes of the 7th cervical vertebra and all 12 thoracic vertebrae, and runs to the clavicle, the acromion, and the spine of the scapula. The trapezius has three portions: the superior part elevates the scapula, the middle part retracts the scapula, and the inferior part depresses the scapula while lowering the shoulder.
Latissimus dorsi: This V-shaped muscle covers much of the thoracic wall below the trapezius. It attaches to the spinous processes of the bottom six thoracic vertebrae, the iliac crest, the thoracolumbar fascia, and the bottom four ribs. It also attaches to the humerus. The latissimus dorsi extends, adducts, and medially rotates the humerus.
Levator scapulae: This muscle runs deep to the sternocleidomastoid and trapezius muscles. It attaches to the transverse processes of the first four cervical vertebrae and the medial side of the scapula. It raises the scapula upward and rotates the glenoid cavity downward.
Major and minor rhomboids: These muscles run underneath the trapezius, from the spinous processes of the 7th cervical vertebra through the 5th thoracic vertebra to the scapula. They pull the scapula toward the vertebral column.

Six scapulohumeral muscles attach to the scapula and the humerus, all working to move the glenohumeral joint:

Deltoid: This muscle gives the shoulder its rounded shape. It originates at the outer third of the clavicle and the acromion and spine of the scapula and inserts into the deltoid tuberosity on the humerus. It has three sections. The anterior part flexes and medially rotates the humerus. The middle part abducts the humerus. And the posterior part extends and laterally rotates the humerus.
Teres major: This muscle originates at the bottom part of the scapula and inserts into the intertubercular groove of the humerus. It adducts and medially rotates the humerus.
Supraspinatus: This muscle originates at the supraspinous fossa of the scapula and inserts into the greater tubercle of the humerus. It helps the deltoid abduct the humerus.
Infraspinatus: Originating at the infraspinous fossa of the scapula, this muscle inserts into the greater tubercle on the humerus. It laterally rotates the humerus.
Teres minor: This muscle originates at the lateral part of the scapula and inserts into the greater tubercle of the humerus. It adducts and laterally rotates the humerus.
Subscapularis: Originating at the subscapular fossa, this muscle inserts at the lesser tubercle of the humerus. It adducts and medially rotates the humerus.

Patients who complain of shoulder pain commonly have problems that fall into one of four categories:

Inflammation often affects the rotator cuff. Such conditions include bursitis, and tendinitis.
Instability includes partial or complete shoulder dislocations that happen when the head of the humerus is forced out of the glenoid cavity. This dislocation is usually due to traumatic injury, but it can also happen from overuse.
Osteoarthritis happens when the cartilage of a joint wears down. It causes pain, swelling, and stiffness and usually worsens over time.
Fractures usually involve the clavicle, but the humerus is also subject to several types of fractures that can occur at the head, tuberosities, surgical neck, or shaft.

Shoulder pain can also be due to serious health conditions such as tumors, infection, and problems with the nervous system or referred pain during a heart attack.