Deltoid muscle

The deltoid muscle, also called the musculus deltoideus, gives the human shoulder its rounded shape. In animals like the domestic cat, it is referred to as the common shoulder muscle. The muscle has three distinct fiber groups: the anterior (clavicular part), posterior (scapular part), and intermediate (acromial part). Although its fibers are pennate, electromyography shows that the nervous system can independently coordinate at least seven subgroups within the muscle.

Some anatomists still use the older name deltoideus (plural deltoidei). The name comes from its resemblance to the Greek capital letter delta (Δ), and it is sometimes shortened in slang to "delt." A study of 30 shoulders found the average human deltoid mass to be 192 grams (6.8 oz), with a range from 84 grams (3.0 oz) to 366 grams (12.9 oz).

The anterior (clavicular) fibers start from most of the anterior border and upper surface of the lateral third of the clavicle. This origin sits next to the lateral fibers of the pectoralis major, and only a small chiasmatic space—through which the cephalic vein passes—keeps the two muscles from forming a continuous mass. The intermediate (acromial) fibers arise from the superior surface of the acromion process of the scapula. The posterior (spinal) fibers come from the lower lip of the posterior border of the spine of the scapula.

From these origins, the fibers converge toward the deltoid tuberosity on the middle of the lateral shaft of the humerus. The intermediate fibers run vertically, the anterior fibers go obliquely backward and laterally, and the posterior fibers go obliquely forward and laterally. Although traditionally described as a single insertion, the deltoid insertion actually has two or three distinct areas that match the muscle's three origin regions. This insertion forms an arch-like structure with strong anterior and posterior fascial connections flanking an intervening tissue bridge, and it sends extensions to the deep brachial fascia. The deltoid fascia also contributes to the brachial fascia and connects to the medial and lateral intermuscular septa.

Blood supply comes from the thoracoacromial artery (acromial and deltoid branches), the circumflex humeral arteries, and the profunda brachii artery (deltoid branch). The deltoid is innervated by the axillary nerve, which originates from the anterior rami of cervical nerves C5 and C6 via the superior trunk, posterior division of the superior trunk, and the posterior cord of the brachial plexus. Studies show seven neuromuscular segments: three in the anatomical anterior head, one in the middle head, and three in the posterior head. These segments are supplied by smaller branches of the axillary nerve and work with other shoulder girdle muscles, including the pectoralis major and supraspinatus. The axillary nerve can be damaged during surgical procedures in the axilla, such as for breast cancer, or by anterior dislocation of the humeral head.

Structures beneath the deltoid include the humerus, pectoralis minor, supraspinatus, infraspinatus, teres minor, subscapularis, pectoralis major, teres major, latissimus dorsi, coracobrachialis, biceps brachii, triceps brachii, anterior and posterior circumflex humeral vessels, axillary nerve, shoulder joint, rotator cuff, and coracoacromial ligament.

When all fibers contract together, the deltoid is the prime mover for arm abduction in the frontal plane. For maximum effect, the arm must be medially rotated, making the deltoid an antagonist to the pectoralis major and latissimus dorsi during adduction. The anterior fibers help the pectoralis major flex the shoulder and work with the subscapularis, pecs, and lats to internally (medially) rotate the humerus. The intermediate fibers perform basic shoulder abduction when the shoulder is internally rotated and shoulder transverse abduction when externally rotated; they are not used much during strict transverse extension (shoulder internally rotated), as in rowing, which relies on the posterior fibers. The posterior fibers assist the latissimus dorsi in shoulder extension and, along with the infraspinatus and teres minor, act as external (lateral) rotators, opposing strong internal rotators like the pecs and lats.

In humans, the deltoid helps prevent dislocation of the humeral head when carrying heavy loads. Its abduction function also keeps carried objects safely away from the thighs, as during a farmer's walk, and ensures precise, rapid movement of the glenohumeral joint for hand and arm manipulation. The intermediate fibers are most efficient for this role, though basic abduction movements (like lateral raises) are assisted by simultaneous co-contraction of the anterior and posterior fibers. The deltoid elevates the arm in the scapular plane, and its contraction also elevates the humeral head. To prevent this from compressing the humeral head against the undersurface of the acromion and injuring the supraspinatus tendon, some muscles of the shoulder girdle contract simultaneously.