Deltoid muscle

Structure

The deltoid has three functionally and anatomically distinct parts. The acromial part, sometimes also known as the middle or  central, is the largest and the strongest. It is a multipennate muscle. It arises as four intramuscular septa, which interdigitate with the three tendons at the insertion site (one each for the anterior, posterior and middle parts). The four septa are connected by short, strong muscle fibres.

The clavicular (anterior) and scapular spinal (posterior) parts are both unipennate and converge directly onto the inserting tendon.

Before jumping into the origins and insertions of the deltoid muscle, learn the main muscles of the upper limb with the following quiz. 

Origins and Insertion

The deltoid muscle has a very broad origin and a narrow base, thus creating its triangular shape. The three parts of the deltoid each have a different origin:

• The clavicular (anterior) part originates from the superior surface and the anterior border of the lateral third of clavicle.
• The acromial (middle) part arises from the lateral margin and superior surface of the acromion of scapula.
• The scapular spinal (posterior) part originates from the lateral 1/3 of the spine of scapula, on the crest.

To easily remember the three origins of the deltoid you can use a simple mnemonic!

Deltoid helps you carry SACS:

• Clavicle
• Acromion
• Spine of Scapula

The muscle fibres then run inferiorly towards the humeral shaft and converge to a narrow strong tendon. It inserts into the deltoid tuberosity located approximately halfway down the lateral aspect of the shaft of the humerus. 

Relations

The deltoid is a superficial muscle of the shoulder, thus it lies deep only to its overlying fascia, the platysma muscle and skin. Due to its superficial nature, the deltoid can be easily observed and palpated.

The deltoid overlies a number of other muscular structures: the rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis), the pectoralis major and the tendon of pectoralis minor, as well as tendons of coracobrachialis, both heads of biceps brachii and long and lateral heads of the triceps brachii muscle. The deltoid also covers the coracoacromial ligament, subacromial bursa, bony structures (coracoid process and proximal humerus), and neurovascular structures (the axillary nerve and anterior and posterior circumflex humeral vessels) of the shoulder region.

If any of the structures you encountered here seem difficult, why not take a look at the following study units, full of helpful videos and quizzes!

Innervation

The deltoid muscle is innervated by one of the main branches of the brachial plexus, the axillary nerve (C5, C6). 

Blood supply

As the deltoid muscle is rather large, it receives a rich vascular supply from various sources:

• Thoracoacromial artery (acromial and deltoid branches), branch of the axillary artery
• Subscapular artery, branch of the axillary artery
• Anterior circumflex humeral artery
• Posterior circumflex humeral artery
• Deep brachial artery (deltoid branch)

All arteries supplying the deltoid are branches of the axillary artery, except for the deep brachial artery (profunda brachii), which is a branch of the brachial artery, which is the continuation of the axillary artery within the arm.

Function

The deltoid muscle (acromial part) is the principal abductor of the arm at the glenohumeral joint. However, it can only do so, when the arm is already abducted beyond fifteen degrees. This initial part of abduction is produced by the supraspinatus muscle. The clavicular and scapular spinal fibers of the muscle guide the arm through the abduction motion.

Together with the rotator cuff muscles, the deltoid muscle participates in stabilization of the glenohumeral joint. When carrying heavy objects while the arm is fully adducted, the muscle will produce a line of force (static contraction) that prevents the inferior displacement of the glenohumeral joint. The deltoid also undergoes eccentric contraction when the arm is being lowered, or adducted. That allows adducting the arm in a controlled manner.

The clavicular (anterior) fibers of deltoid  act along with pectoralis major to produce flexion of the arm during walking or running motions. These fibers are also active during internal (medial) rotation of the humerus.

Did you know that our upper limb muscle anatomy chart lists the actions of the deltoid and all of the muscles of the upper extremity in easy to revise from tables? Get yours now!

In contrast to anterior fibers, the scapular spinal (posterior) fibers of deltoid act with the latissimus dorsi to produce extension of the arm during ambulation. In addition, these fibers will assist in external (or lateral) rotation of the humerus. This is important from a functional standpoint as strengthening the posterior fibers of the deltoid muscle can help to offset the tendency of the shoulder to become internally rotated due to poor posture.