Sternocleidomastoid: Origin, Insertion, Nerve, Action & Torticollis

The Sternocleidomastoid (SCM) is the most prominent superficial muscle of the neck. It is a strap-like muscle that spirals obliquely across the side of the neck, dividing it into anterior and posterior triangles. Its long name is a perfect roadmap of its attachments: Sterno (Sternum) - Cleido (Clavicle) - Mastoid (Process).

[Image of External Oblique muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Sternal Head: Anterior surface of the Manubrium Sterni. Clavicular Head: Medial third of the Clavicle (superior surface).
Insertion (Distal)	Lateral surface of the Mastoid Process (Temporal Bone) and the lateral half of the Superior Nuchal Line (Occipital Bone).
Nerve Supply	Motor: Spinal Accessory Nerve (CN XI). Sensory/Proprioception: Ventral rami of C2 and C3.
Primary Actions	Unilateral: Lateral flexion (ear to shoulder) to the same side; Rotation to the Opposite side. Bilateral: Flexion of the neck; Accessory muscle of Inspiration (elevates sternum/clavicle).

Deep Dive: The Two Heads

The SCM has two distinct heads of origin, often creating a visible gap between them known as the Lesser Supraclavicular Fossa. This gap is a key landmark for accessing the internal jugular vein in medical procedures.

1. Sternal Head

This is the medial, rounded, tendon-like head. It is very prominent when you turn your head.

2. Clavicular Head

This is the lateral, wider, and flatter fleshy head. It lies deep to the platysma muscle.

Physio Corner: Clinical Relevance

💪 Functional Fact: The SCM creates the "Look away" motion. If you hear a noise to your left, your Right SCM contracts to rotate your head to the left.

Palpation

Ask the patient to turn their head to the opposite side and look slightly up. The SCM will bulge diagonally across the neck. You can pinch the muscle belly between your thumb and fingers. Be gentle, as the Carotid Artery lies just deep to its midpoint.

⚠️ Clinical Pathology: Torticollis (Wry Neck)
Congenital or spasmodic shortening of one SCM results in Torticollis. The head is characteristically stuck in a position of Side-Bending toward the tight muscle and Rotation away from it.
Example: Tight Right SCM = Head tilted Right + Face turned Left.

Manual Muscle Testing (MMT)

We test the combined action of flexion and rotation.

Testing Tip: To test the Right SCM, ask the patient to turn their head to the Left and lift their head off the table.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Supine (face up). Action: Patient rotates head to the left (to test Right SCM) and flexes the neck (lifts head off pillow). Resistance: Applied to the temporal region (side of forehead) pushing down and back toward neutral. Grade 3: Lifts head through full range. Grade 4/5: Holds against resistance.
Grade 0, 1 (Palpation)	Action: Palpate the muscle belly on the anterolateral neck. Cue: "Try to lift your head and turn left." Grade 1: Tendon tension felt at the sternum. Grade 0: No activity.

Frequently Asked Questions

Does SCM extend the head?

It's complex. It primarily flexes the lower cervical spine. However, because it inserts behind the axis of the AO joint (ears), it can slightly extend the upper neck (Chin Poke) if the deep neck flexors are weak.

Why does SCM cause dizziness?

The SCM (clavicular head specifically) has a very high density of proprioceptors helping with balance. Trigger points here can refer pain to the ear and eye, and cause "cervicogenic dizziness" or vertigo-like sensations.

What anatomical triangles does it define?

The anterior and posterior borders of the SCM divide the neck into the Anterior Triangle (front) and the Posterior Triangle (side/back).

Test Your Knowledge: Sternocleidomastoid Quiz

1. Which cranial nerve supplies motor function to the SCM?

2. Contraction of the Right SCM causes rotation to the:

3. Where does the SCM insert?

4. What deformity is caused by a tight/shortened SCM?

5. Which blood vessel runs deep to the SCM?

6. Unilateral contraction causes lateral flexion to the:

7. Which muscle lies deep to the SCM?

8. The SCM is an accessory muscle for which function?

9. How many heads of origin does SCM have?

10. To stretch the Right SCM, you should:

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.
• Travell, J. G., & Simons, D. G. (1999). Myofascial Pain and Dysfunction: The Trigger Point Manual. 2nd ed. Williams & Wilkins.