Deltoid Muscle: Origin, Insertion, Nerve, Action & Clinical Testing

The Deltoid is the large, thick, triangular muscle that covers the shoulder joint, giving the shoulder its rounded contour. Named after the Greek letter "Delta" (Δ) due to its inverted triangle shape, it is the primary mover of the arm and consists of three distinct heads with opposing functions.

[Image of Deltoid muscle anatomy]

Quick Anatomy Snapshot

Head	Origin (Proximal)	Primary Action
Anterior (Clavicular)	Lateral third of the Clavicle.	Flexion, Internal Rotation, Horizontal Adduction.
Middle (Acromial)	Acromion Process of the Scapula.	Abduction (after the first 15°).
Posterior (Spinal)	Spine of the Scapula.	Extension, External Rotation, Horizontal Abduction.
Insertion (All Heads)	Deltoid Tuberosity on the lateral shaft of the Humerus.
Nerve Supply	Axillary Nerve (C5, C6).

Deep Dive: Three Muscles in One

The Deltoid is a classic example of a multipennate muscle (especially the middle head), allowing it to generate significant force without becoming excessively bulky.

1. The Abduction Mechanism

The Middle Deltoid is the most powerful abductor of the humerus. However, it cannot initiate abduction effectively. The Supraspinatus initiates the first 0–15 degrees of movement. Once the arm is slightly lifted, the Deltoid takes over as the prime mover up to 90 degrees.

2. The Antagonistic Heads

The Anterior and Posterior heads act as antagonists to each other.
• Anterior: Pulls the arm forward (Flexion) and inward (Internal Rotation).
• Posterior: Pulls the arm backward (Extension) and outward (External Rotation).
However, they work together with the middle head to abduct the arm.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Deltoid is the most common site for intramuscular (IM) injections in adults (specifically the middle head, 2-3 finger widths below the acromion) due to its thickness and vascularity.

Palpation

The muscle is superficial and easily palpable.
• Anterior: Palpate below the lateral clavicle while the patient flexes the shoulder.
• Middle: Palpate below the acromion while the patient abducts.
• Posterior: Palpate below the scapular spine while the patient extends horizontally.

⚠️ Clinical Pathology: Axillary Nerve Injury
The Axillary nerve winds around the surgical neck of the humerus, deep to the deltoid. A fracture of the surgical neck or an anterior shoulder dislocation can damage this nerve.
Signs: Paralysis of the Deltoid (inability to abduct arm) and loss of sensation over the "Regimental Badge" area (a patch of skin on the lateral shoulder).

Manual Muscle Testing (MMT)

Each head can be tested individually by altering the arm's position.

Testing Tip: For the Middle Deltoid (General Abduction), ensure the patient does not "hike" the shoulder using the Upper Trapezius to compensate for weakness.

Step-by-Step Procedure (Oxford Scale)

Test	Patient Action & Resistance
Middle Deltoid (Abduction)	Position: Sitting. Arm at side. Action: Patient abducts arm to 90°. Resistance: Applied at distal humerus (above elbow), pushing down.
Anterior Deltoid (Flexion)	Position: Sitting. Arm at side, slightly abducted. Action: Patient flexes shoulder to 90°. Resistance: Applied at distal humerus, pushing down and back.
Posterior Deltoid (Extension)	Position: Prone (face down). Arm abducted to 90° hanging off table. Action: Patient horizontally abducts (lifts elbow to ceiling). Resistance: Applied at distal humerus, pushing down.

Frequently Asked Questions

Why is the Supraspinatus needed if the Deltoid is so strong?

When the arm is at the side, the line of pull of the Deltoid is vertical (parallel to the bone), which pulls the humerus up into the socket rather than out. The Supraspinatus provides the initial leverage to lift the arm away from the body so the Deltoid can work effectively.

Can you isolate the Posterior Deltoid?

Yes. Exercises like "Reverse Flyes" or "Face Pulls" target the posterior head specifically by utilizing horizontal abduction and external rotation.

What is the "Regimental Badge" sign?

It refers to the area of skin over the lower half of the deltoid muscle. Numbness here is a hallmark sign of Axillary nerve damage.

Test Your Knowledge: Deltoid Quiz

1. Which nerve supplies the Deltoid muscle?

2. Where does the Deltoid insert?

3. Which head of the Deltoid is responsible for shoulder Flexion?

4. Which muscle initiates abduction (0-15 degrees)?

5. The Axillary nerve winds around which part of the humerus?

6. Where does the Posterior Head originate?

7. Atrophy of the Deltoid results in:

8. The Anterior Deltoid assists with which rotation?

9. The Deltoid muscle shape is described as:

10. Which artery accompanies the Axillary nerve deep to the Deltoid?

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Magee, D. J. (2014). Orthopedic Physical Assessment. 6th ed. Elsevier.