Dominic Stone
The deltoid muscle is a large, thick, triangular shoulder muscle, named for its resemblance to the Greek letter delta (Δ). It is responsible for elevating the arm in the scapular plane and is a common site for intramuscular injections and vaccinations. The deltoid overlies several other muscular structures and is divided into three distinct portions: the anterior or clavicular, middle or acromial, and posterior or spinal. The deltoid muscle has a very broad origin and a narrow base, with three areas of origin and two or three insertion areas. The muscle fibres converge into a strong tendon, inserting into the deltoid tuberosity on the humerus. Locating the correct site for an injection is important to avoid injury to the axillary nerve.
| Characteristics | Values |
|---|---|
| Shape | Thick, triangular |
| Name origin | Named after the Greek letter 'delta' due to its shape |
| Location | Shoulder |
| Function | Abduction of the arm, stabilisation of the shoulder joint, flexion, internal rotation, external rotation, horizontal abduction, horizontal adduction |
| Parts | Anterior or clavicular, middle or acromial, posterior or spinal |
| Insertion | Divided into two or three areas, with an arch-like structure |
| Blood supply | Thoracoacromial artery, circumflex humeral arteries, profunda brachii artery |
| Innervation | Axillary nerve |
| Common abnormalities | Tears, fatty atrophy, enthesopathy |
| Injection site | Lower half of the deltoid to avoid injury to the axillary nerve |
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What You'll Learn
The deltoid muscle's shape and name origin
The deltoid muscle is a large, thick, triangular shoulder muscle. It gets its name from its shape, which resembles the Greek letter 'delta' (Δ). The deltoid has a very broad origin and a narrow base, which creates its triangular shape. The three parts of the deltoid—the clavicular, acromial, and scapular spinal parts—each have a different origin. The clavicular (anterior) part originates from the superior surface and the anterior border of the lateral third of the clavicle. The acromial (middle) part arises from the lateral margin and superior surface of the acromion of the scapula. The scapular spinal (posterior) part originates from the lateral third of the spine of the scapula, on the crest.
The deltoid muscle is the principal abductor of the arm at the glenohumeral joint. It is responsible for elevating the arm in the scapular plane, and its contraction also elevates the humeral head. The deltoid is formed of acromial, clavicular, and scapular spinal parts, which serve different functions. The 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. The clavicular part (anterior fibres) can act as a flexor and internal rotator of the arm, while the scapular spinal part (posterior fibres) can extend and externally rotate the arm.
The deltoid overlies a number of other muscular structures, including the rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis), the pectoralis major, and the tendon of pectoralis minor. It also covers the coracoacromial ligament, subacromial bursa, and neurovascular structures of the shoulder region. The deltoid is innervated by the axillary nerve, which originates from the anterior rami of the cervical nerves C5 and C6.
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Its location and insertion
The deltoid muscle is a thick, triangular shoulder muscle. It gets its name from its shape, which resembles the Greek letter 'delta' (Δ). The deltoid has a wide origin and a narrow base, giving it this distinctive triangular form. The muscle has a broad origin, spanning the clavicle, acromion and spine of the scapula. It passes inferiorly, surrounding the glenohumeral joint on all sides, and inserts onto the humerus.
The deltoid is composed of three functionally and anatomically distinct parts, each with a different origin: the clavicular (anterior) part, the acromial (middle) part, and the scapular spinal (posterior) part. The clavicular part originates from the superior surface and the anterior border of the lateral third of the clavicle. The acromial part arises from the lateral margin and superior surface of the acromion of the scapula. The scapular spinal part originates from the lateral third of the spine of the scapula, on its crest.
The muscle fibres then converge towards the humeral shaft, forming a narrow, strong tendon. This tendon inserts into the deltoid tuberosity, located approximately halfway down the lateral aspect of the shaft of the humerus.
The deltoid is a superficial muscle, lying deep only to its overlying fascia, the platysma muscle, and skin. This makes it easily observable and palpable. The deltoid overlies several other muscular structures, including the rotator cuff muscles, the pectoralis major, and the tendon of pectoralis minor. It also covers the coracoacromial ligament, subacromial bursa, and neurovascular structures such as the axillary nerve and circumflex humeral vessels.
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Its function and assistance to other muscles
The deltoid muscle is a thick, triangular shoulder muscle that gets its name from its shape, which resembles the Greek letter delta (Δ). It has three distinct sets of muscle fibres: 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's primary function is to abduct the arm at the glenohumeral joint, guiding it through the abduction motion and stabilising the joint. It also assists in forward elevation and acts as a flexor and internal rotator of the arm. The anterior deltoid works with the subscapularis, pecs, and lats to internally (medially) rotate the humerus. The posterior deltoid assists the latissimus dorsi in extending the shoulder, with other transverse extensors such as the infraspinatus and teres minor working in tandem as external (lateral) rotators.
The deltoid is a powerful muscle used in many everyday activities, such as putting on clothes, carrying shopping bags, or washing hair, as well as athletic activities like netball, swimming, and water polo. It helps move the arm in different directions, including up, down, and out to the side of the body. The deltoid also helps prevent the dislocation of the humeral head when carrying heavy loads.
The deltoid muscle overlies several other muscular structures, including the rotator cuff muscles, the pectoralis major and minor, the coracobrachialis, the biceps brachii, and the triceps brachii muscle. It also covers the coracoacromial ligament, subacromial bursa, and bony structures of the shoulder region.
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Common abnormalities and injuries
The deltoid muscle is a large, thick, triangular muscle that gives the shoulder its range of motion. It is a common "problem area" and is susceptible to several injuries and abnormalities.
Deltoid strains are common injuries, often caused by overuse, heavy lifting, or accidents. They are graded in severity from I to III, with grade I being mild tightness in the muscle and minimal swelling, and grade III being severe tears in the muscle with severe pain, swelling, and deformation of the muscle. Strains can be treated conservatively, with the P.R.I.C.E. principle (Protection, Rest, Icing, Compression, Elevation) being effective for grade I strains.
Deltoid tendonitis is another common injury, caused by inflammation of the rotator cuff and surrounding muscles and tendons. It is a repetitive stress injury, often seen in athletes and those with physically demanding jobs. Shoulder impingement, or swimmer's shoulder, is another possible cause of deltoid pain, occurring when tendons in the shoulder are pinched by the bones.
Deltoid tears are more severe strains that can cause partial or complete deformation of the muscle. They are characterised by severe pain, swelling, and bruising, and may result in the inability to move the arm.
Other possible injuries to the deltoid muscle include shoulder dislocations, fractures, and soft tissue contusions. These injuries may require surgical intervention and should be evaluated by a sports medicine physician.
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How to test for the deltoid muscle
The deltoid muscle is a large triangular-shaped muscle that sits over the glenohumeral joint, giving the shoulder its rounded contour. It is made up of three distinct sections: anterior or clavicular, middle or acromial, and posterior or spinal. The deltoid is a powerful muscle that is used in many everyday activities, such as putting on clothes, and athletic activities, such as swimming and water polo.
To test for the deltoid muscle, a therapist will typically perform a Manual Muscle Test (MMT) to assess the muscle's strength. This involves specific positioning and movements to target the deltoid. Here is a step-by-step guide on how to test for the deltoid muscle:
- The patient should be lying on their back with their knees bent and feet flat on the ground. The therapist stands behind the patient, holding the testing shoulder.
- The therapist will then instruct the patient to perform specific movements to target the deltoid muscle. This may include:
- Flexion: Moving the arm forward and upward, as if reaching for something.
- Internal Rotation: Moving the arm across the body, as if reaching for the opposite shoulder.
- Horizontal Adduction: Bringing the arm across the body, as if giving a hug.
- During these movements, the therapist will apply gradual pressure to the patient's shoulder, attempting to "break" the patient's position.
- The patient should resist the therapist's force, trying to maintain their position.
- The therapist will assess the patient's strength by evaluating their ability to hold the position and resist pressure. This can be graded on a scale from "Poor" to "Normal", depending on the patient's performance.
- For a more comprehensive assessment, the therapist may also test the patient in a standing position, instructing them to perform similar movements as in step 2.
By performing these tests, the therapist can evaluate the strength and function of the deltoid muscle, identifying any weaknesses or pathologies that may be present.
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Frequently asked questions
The deltoid muscle is a large, thick, triangular shoulder muscle. It gets its name from its shape, which resembles the Greek letter delta (Δ).
The deltoid is a superficial muscle of the shoulder, lying over the glenohumeral joint. It is comprised of three distinct portions: the anterior or clavicular, middle or acromial, and posterior or spinal.
The deltoid is responsible for elevating the arm in the scapular plane, as well as flexion, internal rotation, and horizontal adduction. It also plays a crucial role in stabilizing the shoulder joint, preventing subluxation or dislocation of the head of the humerus.
The deltoid muscle can be easily observed and palpated. To locate it, first identify the acromion process, the bony prominence where the head of the humerus articulates with the shoulder joint. Then, place your finger in the dip formed at the joint and ask the person to relax their arm—your finger will now be in the correct place to palpate the deltoid muscle.
