The Deltoid is a powerful and triangular muscle located at the shoulder region; as the name implies, the deltoid has the shape of an inverted Delta of the Greek alphabets; the deltoid is part of the six scapulohumeral muscles that pass from the scapula to the humerus and act on the shoulder joint. This is a very important muscle with the primary action of abduction of the arm (abduction in anatomy means the movement of a part of the body away from the middle part). The Deltoid muscle consists of anterior and posterior parts or fibers and a lateral part (or the intermediate fibers). The six scapulohumeral muscles include the deltoid, teres major, supraspinatus, infraspinatus, subscapularis, and teres minor these are relatively short muscles that pass from the scapula to the humerus and act on the glenohumeral joint (shoulder joint).

Deltoid Muscle Anatomy

The Deltoid muscle is the thick and powerful muscle that forms the rounded contour of the shoulder. This muscle has three (3) parts that can act separately as well as together giving this muscle the ability to carry out four actions with the primary action being the abduction of the arm (which is the movement of the arm away from the central part of the body). The parts of the deltoid muscle are the clavicular parts (anterior deltoid), acromial parts (middle deltoid), and spinal parts (posterior deltoid). These parts of the deltoid are actually the divisions of the muscle fibers that have the ability to act separately or as a whole. When all the three fibers (parts) act simultaneously, they contract and cause the arm to be raised away from the body a process called abduction in anatomy, and this is the main function of the deltoid muscle; the anterior and posterior deltoid actions are just like guy ropes that help to steady the arm as it is abducted.

The deltoid alone cannot initiate abduction when the arm is at rest (that is, when the arm is fully Adducted) because the line of pull of the deltoid muscle coincides with the axis of the humerus (the bone of the arm) and thus pulls directly upward on the humerus instead of lateral pull and therefore cannot initiate or produce abduction unless the supraspinatus muscle helps by initiating the abduction to about 15 degrees before the deltoid takes over. Abduction can also be initiated when a person leans to the side.

Deltoid Muscle Location

The deltoid is located at the lateral part of the shoulder region and covers the shoulder joint like a cape; the deltoid has convex shape because the upper end of the humerus lies below it. This muscle can be tested by abducting the arm against a resistance this causes the muscle to contract and can be seen and felt.

Deltoid Muscle Origin and Insertion

The origin of the deltoid muscle is from the anterior border and upper surface of the lateral one-third of the clavicle and from the whole of the lateral border of the acromion and also from the inferior lip of the crest of the scapular spine. From the lateral border of the acromion, there are four ridges that may be seen that are caused by the four fibrous septa that pass down into the muscle. The spaces between the septa are filled with fleshy muscle fibers attached to contiguous septa.

The point of insertion of the deltoid muscle is on the deltoid tuberosity on the lateral aspect of the humerus. This is a V-shaped area on the humerus that also has a central vertical ridge. These limbs of the V-shape region and the vertical ridge form the points of insertion of deltoid. The anterior and posterior fibers, arising from the clavicle and the scapular spine are unipennate fibers (the fibers run in the same direction) that converge on the anterior and posterior margins of the deltoid tuberosity with greater range of movement.

Deltoid Action

There are several actions of the deltoid muscle with the aid of the supraspinatus muscle, the deltoid abducts the arm using the middle part (the multipennate acromial fibers); the anterior deltoid helps the pectoralis major in flexing and medially rotating the arm; the posterior deltoid assist the latissimus dorsi in extending the arm and act as a lateral rotator. Another action or function of the deltoid is its ability to act as a shunt muscle by resisting the downward displacement of the head of the humerus from the glenoid cavity.

Deltoid Muscle Innervation

The Deltoid is supplied by the Axillary nerve (C5 and C6) from the posterior cord of the brachial plexus. The axillary nerve supplying the deltoid contains the fifth and sixth cervical segments of the spinal cord, hence the numbers C5 and C6, the main supply is the C5 segment that is why it is in bold face. The nerve runs transversely round the back and lateral side of the surgical neck the humerus giving off numerous branches that enter the deltoid muscle in radial directions thus, splitting the muscle vertically does not damage the nerve supply. The surface marking the nerve supply to the deltoid is along a transverse line about 5 cm below the tip of the acromion.

Any damage to the axillary nerve causes paralysis of the deltoid and such a person cannot move the affected arm to the side. Also, a damage to one or more of the listed spinal cord segments or to the motor nerve roots arising from them can also cause paralysis of the deltoid muscle with accompanying atrophy. Injury to the axillary nerve may occur as a result of fracture to the surgical neck of the humerus because the nerve passes inferior to the humeral head and winds around the surgical neck of the humerus. The axillary nerve may also be damaged by dislocation of the glenohumeral joint and by compression from the incorrect use of crutches. As the deltoid atrophies, the rounded contour of the shoulder disappears giving the shoulder a flattened appearance with a slight hollow below the acromion; there may be additional loss of sensation over the lateral side of the upper part of the arm which is the area supplied by the superior lateral cutaneous nerve of the arm.

Blood supply of Deltoid muscle

The blood supply of the deltoid is from the posterior circumflex humeral artery and also the posterior circumflex humeral vein that enter the deltoid by passing behind the surgical neck of the humerus.

Clinical Use of Deltoid Muscle

The Deltoid muscle can be used as a site for intramuscular injection this is done on the lateral aspect of the bulge of the shoulder, no more than 4 cm below the lower border of the acromion, as the anterior branch of the axillary nerve curls forwards round the back of the humerus 5 cm below the acromion.