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The deltoid muscle is a large, rounded, triangular muscle located on the uppermost part of the arm and the top of the shoulder. It is the muscle that forms the rounded contour of the human shoulder and is also known as the 'common shoulder muscle'. The deltoid muscle is responsible for a wide range of movements of the arm at the shoulder, including abduction, flexion, and extension. It also helps to stabilise the shoulder joint. The deltoid muscle is particularly susceptible to injury in athletes who perform a lot of overhead arm movements, such as baseball pitchers, tennis players, and weightlifters.
| Characteristics | Values |
|---|---|
| Location | Shoulder |
| Shape | Triangular |
| Parts | Three |
| Function | Movement of the arm in different directions, abduction, flexion, extension, stabilisation of the shoulder joint |
| Blood supply | Axillary artery |
| Innervation | Axillary nerve |
| Conditions/Injuries | Adhesive capsulitis, axillary nerve palsy, bursitis, rotator cuff tears, shoulder impingement syndrome, shoulder separation, strains, tendonitis, enthesitis, calcific tendinitis, myositis, infection, tumours, chronic avulsion injury |
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What You'll Learn
Deltoid muscle anatomy
The deltoid muscle is a large, rounded, triangular muscle located on the uppermost part of the arm and the top of the shoulder. It is named after the Greek letter delta, which is shaped like an equilateral triangle. The deltoid muscle is the muscle that forms the rounded contour of the human shoulder. It is also known as the 'common shoulder muscle', particularly in other animals such as the domestic cat.
Anatomically, the deltoid muscle is made up of three distinct sets of muscle fibres, or parts: the anterior or clavicular part (pars clavicularis), the posterior or scapular part (pars scapularis), and the intermediate or acromial part (pars acromialis). The deltoid's fibres are pennate muscle. However, electromyography suggests that it consists of at least seven groups that can be independently coordinated by the nervous system. The three parts of the deltoid each have a different origin: the lateral end of the clavicle, the acromion of the scapula at the top of the shoulder, and the spine of the scapula. Each origin gives rise to its own band of muscle fibres, with the anterior band forming at the clavicle, the lateral fibres forming at the acromion, and the posterior fibres forming at the spine of the scapula.
The deltoid muscle has a very broad origin and a narrow base, creating its triangular shape. The anterior part attaches to the lateral third of the clavicle, the lateral part attaches to the acromion process, and the posterior part attaches to the spine of the scapula. The deltoid muscle fibres run laterally and inferiorly towards the humerus and converge into a narrow base that attaches to the deltoid tuberosity of the humerus. This is the “insertion” of the deltoid muscle. The deltoid is a superficial muscle that overlies the glenohumeral joint and is one of the six intrinsic muscles of the shoulder.
The deltoid muscle is supplied by the thoracoacromial artery (acromial and deltoid branches), the circumflex humeral arteries, the profunda brachii artery (deltoid branch), and the deep brachial artery. It is innervated by the axillary nerve, which runs around the neck of the humerus after exiting the axilla. The axillary nerve is sometimes damaged during surgery or injured by anterior dislocation of the head of the humerus, which can lead to paralysis of the deltoid muscle.
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Deltoid muscle injuries
The deltoid muscle is a large, triangular-shaped muscle that forms the rounded contour of the human shoulder. It is also referred to as the 'common shoulder muscle'. The deltoid muscle is divided into three distinct sections: anterior or clavicular (front delt), middle or acromial (side delt), and posterior or scapular (rear delt). It is a powerful muscle that helps move the arms in different directions and is used in many athletic activities.
Deltoid strains are graded based on their severity, with Grade 1 being a minor tear causing mild pain and Grade 3 being a complete tear with severe pain and dysfunction in the arm. The most common cause of deltoid strains is overuse of the muscle without adequate rest, leading to discomfort, swelling, and loss of function. In some cases, a forced eccentric contraction of the shoulder, such as during weightlifting, can lead to a deltoid strain.
To diagnose a deltoid strain, a physical examination is necessary to assess the mechanism of injury and the patient's symptoms. Range-of-motion and strength testing are also performed to confirm the diagnosis and rule out other possible injuries. Treatment for deltoid strains depends on the severity, with Grade 1 injuries requiring minimal treatment and Grade 3 strains needing three to four months of recovery, including rest and careful management.
In addition to strains, other deltoid muscle injuries include rotator cuff tears, shoulder impingement syndrome, shoulder separation, tendonitis, and adhesive capsulitis (frozen shoulder). The axillary nerve, which supplies sensation to the deltoid muscle, can also be damaged during surgery or due to trauma or overuse of crutches, leading to shoulder weakness or numbness.
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Deltoid muscle strengthening exercises
The deltoid muscle is a large, triangular-shaped muscle that forms the rounded contour of the human shoulder. It is comprised of three distinct parts: the anterior or clavicular part (commonly known as the front delt), the posterior or scapular part (commonly known as the rear delt), and the intermediate or acromial part (commonly known as the side delt). The deltoid muscle helps move the arms in different directions and protects and stabilizes the shoulder joint.
Wall Pushes
Stand next to a wall with your back and elbow straight. Gently push your arm against the wall as hard as you comfortably can without causing pain. Hold for 5 seconds and repeat 10 times on each side, ensuring it is pain-free.
Dumbbell Arm Raises
Using a suitable dumbbell weight, perform arm raises in the anterior, lateral, and posterior directions. Repeat 10 times on each side, ensuring it is pain-free.
Resistance Band Exercises
Using a resistance band, perform forward raises, lateral raises, and pullbacks. Repeat 10 times on each side, ensuring it is pain-free.
Flexion, Internal Rotation, and Horizontal Adduction
For this exercise, a therapist or partner is needed. Flex your elbow to 90 degrees and have the therapist apply resistance as you abduct your shoulder. For the anterior deltoid, the therapist stands behind you and holds your shoulder and forearm. To stretch this muscle, perform the reverse action by doing extension, external rotation, and horizontal abduction without letting your torso rotate.
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Deltoid muscle innervation
The deltoid muscle is innervated by the axillary nerve, one of the main branches of the brachial plexus. The axillary nerve originates from the anterior rami of the cervical nerves C5 and C6, via the superior trunk, posterior division of the superior trunk, and the posterior cord of the brachial plexus. The deltoid muscle receives a rich vascular supply from various sources, with all arteries supplying it being branches of the axillary artery, except for the deep brachial artery (profunda brachii), which is a branch of the brachial artery.
The axillary nerve supplies sensation to the deltoid muscle. Compression of or damage to the nerve can occur during surgery, traumatic injury, or overuse of a crutch, leading to shoulder weakness or numbness, especially around the deltoid muscle. The axillary nerve is also sometimes damaged during surgical procedures of the axilla, such as for breast cancer. It may also be injured by anterior dislocation of the head of the humerus or during shoulder dislocation or fracture of the surgical neck of the humerus.
The deltoid muscle has seven neuromuscular segments, three in the anatomical anterior head, one in the anatomical middle head, and three in the anatomical posterior head. These neuromuscular segments are supplied by smaller branches of the axillary nerve and work in coordination with other muscles of the shoulder girdle, including the pectoralis major and supraspinatus. The axillary nerve plays a crucial role in the function of the deltoid muscle, and its damage can lead to paralysis of the deltoid muscle and deltoid muscle dysfunction.
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Deltoid muscle blood supply
The deltoid muscle is a large, thick, triangular shoulder muscle. It is an intrinsic muscle of the shoulder region, and its shape resembles the inverted Greek letter delta. The deltoid muscle has three functionally and anatomically distinct parts: the acromial part (or side delt), the clavicular part (or front delt), and the scapular spinal part (or rear delt). The deltoid muscle is a key player in shoulder stability and motion, and it is a common site for intramuscular injections, including vaccinations.
The deltoid muscle receives a rich vascular supply from various sources. The deltoid receives blood from the thoracoacromial branch of the axillary artery. The thoracoacromial branch originates from the second part of the axillary artery, which lies posterior to the pectoralis minor. The thoracoacromial artery travels alongside the cephalic vein in the deltopectoral groove. The deltoid muscle also receives minor contributions from the posterior humeral circumflex and profunda brachii arteries.
The anterior circumflex humeral artery supplies the anterior part of the deltoid muscle, while the posterior circumflex humeral artery supplies the posterior and middle parts. The profunda brachii artery is a branch of the brachial artery, which is the continuation of the axillary artery within the arm. The deltoid muscle is also supplied by the circumflex humeral arteries and the thoracoacromial artery (including its acromial and deltoid branches).
The deltoid muscle is innervated by the axillary nerve, which arises from the anterior rami of the cervical nerves C5 and C6. The axillary nerve is a terminal branch of the brachial plexus posterior cord. The anterior and posterior branches of the axillary nerve innervate the deltoid muscle, and these smaller branches work in coordination with other muscles of the shoulder girdle, including the pectoralis major and supraspinatus.
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Frequently asked questions
The deltoid muscles are located in the shoulder.
The deltoid muscles help move your arms in different directions and protect and stabilise your shoulder joint.
The deltoid muscle is anatomically divided into three parts: the anterior, or clavicular; lateral, or acromial; and posterior, or spinal.
Common deltoid muscle injuries include adhesive capsulitis (frozen shoulder), axillary nerve palsy, and bursitis.
