Deltoid Muscle: Origin, Anatomy, Insertion & Functions

Deltoid muscle is a big, triangular, oval-shaped muscle that lies over the shoulder joints and glenohumeral joints. These muscles give the shoulders a round shape. Because of its placement and the large spacing of its muscle fibres, contraction of these muscles leads to a wide range of movement of the arm at the shoulder.

Keyterms: Deltoid Muscle, Muscle, shoulder joints, glenohumeral joints, clavicle, acromion, scapula spine, rotator cuff muscles

Also Read: Leg Muscles

---

What is a Deltoid Muscle?

[Click Here for Sample Questions]

The deltoid is a triangular shoulder muscle that is thick and strong. Its name comes from its resemblance to the Greek letter 'delta'. The origin of the muscle is broad, spanning the clavicle, acromion, and scapula spine. It enters onto the humerus and passes inferiorly around the glenohumeral joint on all sides.

The clavicle (collarbone), humerus (upper arm bone), and scapula (shoulder blade) are all connected to the shoulder blade. Because the deltoid is a superficial shoulder muscle, it is only as deep as its surrounding fascia, the platysma muscle, and the skin. The deltoid is easily visible and palpable due to its superficial nature.

The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis), the pectoralis major and its tendon, the coracobrachialis tendons, both heads of the biceps brachii muscle, and the long and lateral heads of the triceps brachii muscle are all covered by the deltoid. The coracoacromial ligament, subacromial bursa, bony structures (coracoid process and proximal humerus) and neurovascular structures (axillary nerve and anterior and posterior circumflex humeral vessels) of the shoulder are likewise covered by the deltoid.

The acromial, clavicular, and scapular spinal components make up the deltoid. The acromial component (middle fibres) abducts the arm, while the clavicular and scapular spinal parts stabilise it and keep it in a stable plane of abduction. The front fibres of the clavicle can also operate as a flexor and internal rotator of the arm, whereas the posterior fibres of the scapular spine can extend and externally rotate the arm.

Also Read: Types of Joints

---

Sections of Deltoid Muscle

[Click Here for Sample Questions]

The deltoid muscle has a triangle shape thanks to its large origin and narrow base. The origins of the three sections of the deltoid are as follows:

• The superior surface and anterior border of the lateral third of the clavicle form the clavicular (anterior) component.
• The lateral edge and superior surface of the scapula's acromion form the acromial (middle) part.
• The scapular spinal (posterior) component arises on the crest of the lateral third of the scapula's spine.

The muscle fibres then run inferiorly towards the humeral shaft, converge, and form a strong, narrow tendon. It goes into the deltoid tuberosity, which is about halfway along the lateral part of the humerus shaft.

---

Structure of Deltoid Muscle

[Click Here for Sample Questions]

The deltoid muscle is the shoulder's major muscle. The posterior circumflex humeral artery supplies it with blood. The posterior cord of the brachial plexus and other nerves, such as the axillary nerves, C5 and C6 nerves, make up the majority of it.

The deltoid muscle is made up of three unique groups of fibres, each of which causes the glenohumeral joint to move in a different way. These are divided into several sections that are generally referred to as:

• Clavicular part: Musculus deltoideus pars clavicularis is the Latin name for the Clavicular Part of the Deltoid Muscle. It begins in the clavicle's lateral third.
• Acromial Part of Deltoid Muscle: It comes from the acromial of the scapula and is also known as Musculus deltoideus pars acromialis in Latin (posterior).
• Deltoid Muscle Spinal Part: Also known as the musculus deltoideus pars spinalis in Latin, it comes from the scapula's spine.

All three points converge on the deltoid tuberosity, which is located on the lateral surface of the humerus' shaft.

Innervation: To carry out all of the movements, the deltoid muscles require a nerve to inform them what to do and when to do it. This muscle is innervated by the axillary nerve, which is known in Latin as Nervus axillaris. This axillary nerve receives fibres from the brachial plexus' C5 and C6 nerve roots.

Insertion: The fibres converge on the deltoid tuberosity, which is located in the centre of the lateral portion of the humerus shaft. The intermediate fibres run vertically, the anterior obliquely backwards and laterally, and the posterior obliquely forward and laterally.

The deltoid insertion is separated into two or three identifiable locations that correspond to the muscle's three places of origin, despite being conventionally regarded as a single insertion. An arch-like structure with strong anterior and posterior fascial connections flanking an intermediate tissue bridge makes up the insertion. It also produces extensions in the deep brachial fascia. The deltoid fascia also connects to the medial and lateral intermuscular septa and contributes to the brachial fascia.

---

Blood supply to Deltoid Muscle

[Click Here for Sample Questions]

Because the deltoid muscle is so large, it receives a lot of blood from a variety of places such as:

• 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)

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

---

Nerve supply to Deltoid Muscle

[Click Here for Sample Questions]

The axillary nerve supplies innervation to the deltoid. The axillary nerve emerges from the superior trunk, posterior division of the superior trunk, and the posterior cord of the brachial plexus, via the anterior ramus of the cervical nerves C5 and C6.

The deltoid muscle has seven neuromuscular parts, according to research. Three of them are located in the deltoid's anatomical anterior head, one in the anatomical middle head, and three in the anatomical posterior head. These neuromuscular segments are supplied by lesser branches of the axillary nerve and function in tandem with the pectoralis major and supraspinatus muscles of the shoulder girdle.

Surgical procedures on the axilla, such as those for breast cancer, can cause injury to the axillary nerve. It can also be harmed if the head of the humerus is dislocated anteriorly.

---

Functions of Deltoid Muscle

[Click Here for Sample Questions]

At the glenohumeral joint, the deltoid muscle (acromial portion) is the primary abductor of the arm. However, it can only do so if the arm is already more than fifteen degrees abducted. The supraspinatus muscle is responsible for the first part of abduction. The abduction action is guided by the muscle's clavicular and scapular spinal fibres.

The deltoid muscle, together with the rotator cuff muscles, helps to stabilise the glenohumeral joint. When carrying heavy goods with the arm completely abducted, the muscle creates a line of force (static contraction) that prevents the glenohumeral joint from being displaced inferiorly. When the arm is lowered, or adducted, the deltoid also contracts eccentrically. This enables for regulated adduction of the arm.

During walking or running motions, the clavicular (anterior) fibres of the deltoid work together with the pectoralis major to produce flexion of the arm. These fibres are also active when the humerus is rotated internally (medially).

The scapular spinal (posterior) fibres of the deltoid work with the latissimus dorsi to create arm extension during ambulation, in contrast to anterior fibres. Furthermore, these fibres will aid in the humerus' external (or lateral) rotation. Strengthening the posterior fibres of the deltoid muscle can assist to offset the tendency of the shoulder to become internally rotated as a result of poor posture, which is significant from a functional standpoint.

Also Read:

---

Things to Remember

• The deltoid muscle is a rounded, triangular muscle that runs along the upper arm and top of the shoulder. It gets its name from the Greek letter delta, which has an equilateral triangle shape.
• Tendons connect the deltoid to the skeleton at the clavicle (collarbone), scapula (shoulder blade), and humerus (upper arm bone)
• The deltoid originates from three places: the clavicle's lateral end, the acromion of the scapula at the top of the shoulder, and the scapula's spine.
• Each origin produces a distinct band of muscle fibres, with the anterior band originating at the clavicle, the lateral fibres at the acromion, and the posterior fibres at the scapula's spine. As they approach the insertion point on the deltoid tuberosity of the humerus, the bands blend together.
• By drawing the humerus towards the clavicle, the anterior fibres flex and medially rotate the arm. Reaching forward or tossing a ball underhand requires flexion and medial rotation of the arm, which moves the arm anteriorly.
• By pushing the humerus toward the acromion, the lateral fibres abduct the arm. When the arm is abducted, it moves away from the body, as when reaching out to the side. 
• The posterior fibres contract, extending and laterally rotating the arm by drawing the humerus toward the scapula's spine. As in reaching backwards or winding up to toss a ball underhand, extension and lateral rotation extend the arm posteriorly.