The deltoid is a thick, triangular shoulder muscle. It gets its name because of its similar shape to the Greek letter ‘delta’ (Δ). The muscle has a wide origin spanning the clavicle, acromion and spine of scapula. It passes inferiorly surrounding the glenohumeral joint on all sides and inserts onto the humerus.
The deltoid is formed of acromial, clavicular and scapular spinal parts. Acromial part (middle fibres) abducts the arm, while the clavicular and scapular spinal parts play a significant role in stabilization, ensuring a steady plane of abduction. Additionally, the clavicular part (anterior fibers) can act as a flexor and internal rotator of the arm, while the scapular spinal part (posterior fibers) can extend and externally rotate the arm.
Lateral 1/3 of Clavicle (clavicular part), Acromion (acromial part), Spine of Scapula (spinal part)
Mnemonic: 'Deltoid helps you carry SACS'
|Insertion||Deltoid tuberosity of humerus|
|Innervation||Axillary nerve (C5, C6)|
|Blood supply||Deltoid and acromial branches of thoracoacromial artery, subscapular artery, anterior and posterior circumflex humeral arteries, deltoid branch of profunda brachii|
Clavicular part: flexion and internal rotation of the arm,
Acromial part: abduction of the arm beyond the initial 15°
Spinal part: extension and external rotation of the arm.
This article will cover the anatomy and function of the deltoid muscle.
- Origins and Insertion
- Blood supply
- Clinical notes
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.
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:
- 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.
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!
The deltoid muscle is innervated by one of the main branches of the brachial plexus, the axillary nerve (C5, C6).
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, which is a branch of the brachial artery, which is the continuation of the axillary artery within the arm.
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.
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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.
It is incredibly important to properly test the function of the deltoid muscle to accurately determine muscular or nervous injury. An inability to abduct the arm from a position in which the arm is resting at the side of the body does not indicate an injury to the deltoid muscle or the axillary nerve. An inability to initiate abduction of the arm (up to approximately 15 degrees of abduction) would indicate involvement of the supraspinatus muscle or the nerve innervating it, the suprascapular nerve.
To properly test the function of the deltoid and the axillary nerve, the arm must be beyond 15 degrees of abduction. Once the arm is in this position, the patient then pushes against resistance. If the muscle is functioning properly, contraction of the muscle should be felt near the acromion of the scapula.
Axillary nerve injury
As the axillary nerve passes posteriorly in the axilla, it runs around the surgical neck of the humerus. Fractures in this region of the humerus can, therefore, affect the functioning of the nerve, and consequently the deltoid muscle. In addition, the axillary nerve can also be damaged during dislocation of the glenohumeral joint or it can be compressed during incorrect use of crutches. Symptoms may include atrophy of the deltoid muscle, resulting in weakness and a loss of muscle tone, making the shoulder look flattened rather than rounded. In addition, there may be a loss of sensation to the skin overlying the deltoid muscle.
An awareness of the location of the axillary nerve location anteriorly is also important during intramuscular injections in the deltoid muscle, and during surgical approaches to the shoulder to avoid injuring the nerve.
Deltoid pain may be indicative of injuries involving other muscles that stabilize the glenohumeral joint, and other structures related to it. The subacromial/subdeltoid bursa, lies deep to the deltoid muscle, between the acromion and supraspinatus tendon.
In overhead motions of the arm, the greater tubercle of the humerus approaches the acromion, especially when the arm is internally rotated. The subacromial/subdeltoid bursa can become irritated and swollen as it can be repeatedly pinched between the acromion and greater tubercle of the humerus. The bursa may then become distended and cause pain underneath the deltoid muscle.