Shoulder Muscles (Illustrations, with Rotator Cuff)

• The scapula and clavicle, which together form the pectoral girdle, and the proximal end of the humerus.
• Three joints of the shoulder:
• The sternoclavicular joint is where the sternum and clavicle meet.
• The acromioclavicular is where the acromion of the scapula and clavicle meet.
• The glenohumeral joint is where the glenoid cavity of the scapula and the head of the humerus meet. This is usually the joint we’re referring to when we talk about “the shoulder joint.”

The muscles of the rotator cuff cross the glenohumeral joint and contribute to its movements and stability. The glenohumeral joint is a highly moveable ball-and-socket joint that allows flexion, extension, abduction, adduction, medial and lateral rotation, and circumduction of the arm. Protraction and retraction of the scapula move the position of the glenohumeral joint, which provides even greater range of mobility of the arm (try it yourself – hold your scapula in place and move your arm – then allow your scapula to move, too – your reach is extended when the scapula rotates!).

• Many of the muscles of the shoulder are innervated by the brachial plexus.
• Shoulder separation occurs when the acromioclavicular joint is dislocated – this occurs when the ligaments that hold the acromion and clavicle are injured.
• Dislocation of the glenohumeral joint occurs when the ligaments that secure the humerus to the scapula are injured – this can damage the axillary and radial nerves, or tear the muscles of the rotator cuff.

We show the ribs, sternum, vertebral column of the axial skeleton, and the clavicle and scapula of the pectoral girdle; and, the humerus.

• Fibers of pectoralis major arise from the medial clavicle, the body and manubrium of the sternum, and the costal cartilages of ribs 1-6; they form a collective insertion on the humerus – specifically, on the lateral lip of the intertubercular groove (aka, intertubercular sulcus).
• If you activate your pectoralis major, you’ll activate pectoralis major – notice that your arm flexes, adducts, and rotates medially (think of hugging).
• The deltoid forms a cup-like cap over the shoulder; its fibers arise from the lateral 1/3rd of the clavicle, the acromion, and the spine of the scapula, and insert on the deltoid tuberosity of the humerus.
• Notice the small space between the origins of pectoralis major and deltoid on the clavicle – this is the clavipectoral triangle: the cephalic vein, which drains the lateral side of the superficial arm, dives deep via this triangle on its way to drain into the axillary vein.
• Deltoid in lateral view: we sketch a small scapula, clavicle, and proximal humerus in lateral view – label the spine and acromion of the scapula.
• The anterior fibers of the deltoid originate on the lateral 1/3rd of the clavicle; thus, these fibers flex and medially rotate the arm.
• The middle fibers arise from the acromion of the scapula; these fibers abduct the humerus (after supraspinatus initiates the movement).
• The posterior fibers originate along the spine of the scapula; these fibers extend and laterally rotate the arm (the opposite of the anterior fibers).
• Subscapularis lies in the subscapular fossa of the scapula; it inserts on the lesser tubercle of the humerus, and medially rotates the arm. It contributes to the rotator cuff.

We show the vertebral column and posterior skull with lines to indicate the superior nuchal lines and external occipital protuberance. We show the ribs, scapulae, clavicles, humeri (plural of humerus) and the proximal ulna (specifically, the olecranon process).

• Deltoid – now we can clearly see its origins along the spine of the posterior scapula.
• Fibers of trapezius arise from the posterior skull at the superior nuchal line and occipital protuberance, and along the midline of the neck and back via the ligamentum nuchae and the spines of vertebrae C7-T12.
• These fibers converge to insert along the superior edge of the spine of the scapula, the acromion, and the lateral 1/3rd of the clavicle (not visible, here, but see our tutorial on the muscles of the superficial neck).
• Trapezius is a large muscle in the superficial back that elevates, rotates, depresses, and retracts the scapula – thus, it helps to move the position of the glenohumeral joint and greatly increase the range of movement of the arm.
• Supraspinatus arises from the supraspinous fossa (the shallow depression superior – supra – to the spine of the scapula), and inserts on the humerus at the superior facet of the greater tubercle. Activation of this muscle helps to abduct the arm.
• Infraspinatus arises from the infraspinatus fossa (the shallow depression below – infra – to the spine of the scapula) and inserts on the humerus at the middle facet of the greater tubercle. Because of its more inferior position, contraction of infraspinatus laterally rotates the arm.
• Teres minor originates on the lateral border of the scapula and inserts at the humerus at the lower facet of the greater tubercle; like infraspinatus, it laterally rotates the arm.
• Teres major originates below teres minor on the lateral border of the scapula, and passes around to the anterior humerus and inserts on the medial lip of the intertubercular groove. Because of this anterior/medial placement, teres major medially rotates the arm and adducts it.
• Long head of triceps brachii slides between the two teres muscles (“teres” refers to their worm-like round and long shapes).
• The common tendon inserts distally on the olecranon process.
• The medial head of triceps lies deep against the lower half of the posterior surface of the bone.
• The long head arises from the infraglenoid tubercle of the scapula. This attachment site allows triceps brachii to provide accessory extension and adduction of the arm. It also creates important spaces where neurovascular structures travel to the muscles of the back.

Then, for completion, draw the lateral head of triceps brachii, arising above the radial groove on the posterior/lateral humerus. These are the muscles that help secure the attachment of the humerus to the pectoral girdle. We can remember these muscles with the acronym “SITS” – supraspinatus, infraspinatus, teres minor, and subscapularis. We show the scapula in lateral view, and label the acromion and coracoid process, superiorly (we can see a small portion of the body of the scapula between these projections).

• The glenoid cavity (aka, glenoid fossa) is where the head of the humerus articulates
• The glenoid labrum that surrounds the glenoid cavity. This “rubbery” structure helps deepen the cavity and hold the humerus in place.
• The tendon of the long head of biceps brachii merges with the glenoid labrum, superiorly, and that the articular capsule surrounds the joint.
• Anteriorly: The tendon of subscapularis.
• Superiorly: The tendon of supraspinatus (note that it is superior to the acromion).
• Inferiorly: The infraspinatus tendon.
• Inferior to this: The tendon of teres minor.

Notice that the inferior aspect of the articular capsule is not protected by muscular tendons – thus, this is an area of relative weakness. Be aware that we’ve omitted the bursae of the joint for simplicity – you can learn about these important fluid-filled sacs in the notes. The glenohumeral joint in coronal section: we show the acromion, clavicle, and glenoid cavity of the scapula. We show the ligaments that hold the acromion and clavicle together at the acromioclavicular joint – recall that this is where shoulder dislocation occurs.

• Articular cartilage coves the head of the humerus and lines the glenoid cavity.
• Synovial membrane, which lines the joint capsule and produces the synovial fluid that lubricates and circulates nutrients for the joint.
• Portions of the glenoid labrum are visible at the edges of the glenoid cavity.
• The tendon of the long head of biceps brachii passes over the head of the humerus and inserts on the supraglenoid tubercle of the scapula.
• Supraspinatus muscle traveling from the posterior scapula and sending its tendon to insert on the greater tubercle.
• The subacromial bursa (aka the subdeltoid bursa or subacromial-subdeltoid bursa, SASD).
• Middle fibers of the deltoid arise from the acromion and travel laterally over the humerus.
• Because the muscles of the rotator cuff travel within the small space of a highly mobile joint, they are susceptible to injury from overuse or trauma.
• Rotator cuff disorders can be due to inflammation (tendinitis) or tears of the muscles or their tendons, or to subacromial bursitis.
• Supraspinatus is most commonly involved in rotator cuff disorders; this is because of its anatomical position and the fact that it is relatively under-vascularized.
• NSAIDs and exercises that strengthen the rotator cuff muscles can help heal injury to the rotator cuff, but surgery may be necessary when the tendons are severely torn.