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Sternocleidomastoid
The prominent strap on the side of the neck that turns the head and divides the neck into surgical triangles.
Latin name: Musculus sternocleidomastoideus. In plain English, the rope of muscle down the side of the neck. It is filed here under neck.
Reference information, not a diagnosis. Where a symptom is described below, it is there so you can describe your own more precisely to a clinician — not so you can name what you have.
Where it is
Sternocleidomastoid is the muscle that stands out when you turn your head hard to one side. It runs diagonally from the top of the breastbone and the inner end of the collarbone up to the bony bump behind the ear. Its name is simply a list of its attachments in Latin, which makes it one of the easiest muscle names to decode.
Anatomically it is the great landmark of the neck, dividing it into the anterior and posterior triangles. The carotid artery, internal jugular vein and vagus nerve run in the carotid sheath beneath its front border, and the accessory nerve crosses the posterior triangle behind it, where it is unusually superficial and vulnerable.
| Feature | Detail |
|---|---|
| Origin | Sternal head from the manubrium of the sternum; clavicular head from the medial third of the clavicle |
| Insertion | Mastoid process of the temporal bone and the lateral part of the superior nuchal line |
| Nerve | Accessory nerve (cranial nerve XI) for movement, with C2-C3 for sensation |
| Main actions | Turns the head to the opposite side and side-bends it to the same side; both together flex the neck; accessory muscle of breathing |
What it does
One side working alone turns the face to the opposite side and tilts the head toward the same shoulder — the position of someone listening on a phone held in the shoulder. Both sides together flex the neck forward, while also extending the head at the joint with the skull, which is why lifting the head off a pillow uses them so specifically.
They are also accessory muscles of breathing. In quiet breathing they do nothing; in laboured breathing they help lift the rib cage, and visible sternocleidomastoid use during breathing is a recognised clinical sign of respiratory effort.
Exercises that load it
| Exercise | Category | Muscles the exercise library lists |
|---|---|---|
| Swimming, freestyle | Cardio | Whole body, heart, lungs |
| Sit-ups | Core | Abdominals, hip flexors |
| Yoga (hatha) | Mobility | Whole body, balance, breath |
| Plank | Core | Deep core, shoulders, glutes |
It is rarely trained deliberately outside of contact sport. Swimming loads it through repeated head rotation for breathing; sit-ups load it if the head is allowed to lead the movement. Where neck strength genuinely matters — combat and collision sports — it is trained under supervision with progressive isometric and controlled range work, because the neck is not a place for improvised loading.
How it is lengthened
Turn the head away from the side you want to stretch, then tilt it back and away slightly, keeping the shoulder down. Hold gently and briefly. The neck contains major arteries and nerves, so this is one of the few stretches where forcing the end range is genuinely inadvisable — and any stretch producing dizziness, visual change or nausea should be stopped and reported to a clinician.
When it goes wrong
| Problem | What it typically feels like | Usual setting |
|---|---|---|
| Neck strain | A stiff, sore side of the neck with painful turning, often waking with it | An awkward sleeping position, a sudden movement, or prolonged screen work |
| Congenital muscular torticollis | A baby who consistently holds the head tilted to one side and turned the other way | A shortened sternocleidomastoid present from birth; managed by paediatric physiotherapy |
| Cervicogenic headache | A one-sided headache spreading from the neck to the back of the head, with neck stiffness | Referred from upper cervical structures; the neck muscles are involved but rarely the whole story |
Neck pain after significant trauma, neck stiffness with fever and headache, or neck pain with weakness, numbness, slurred speech, visual change or dizziness needs immediate medical assessment. A new lump in the neck that persists should also be examined.
Nothing on this page is a treatment plan. If a problem is limiting what you do, is not improving, or is getting worse, the right next step is an assessment by a doctor or a physiotherapist who can examine you.
Sources: descriptive anatomy from Gray's Anatomy of the Human Body, 20th US edition (1918), which is in the public domain; Latin names follow Terminologia Anatomica, published by the Federative International Programme for Anatomical Terminology. Nerve root levels are the conventional ranges taught from those sources and vary between individuals and between textbooks. Exercise cross-links use the TempoLife exercise library. Checked 2026-09-06.
Frequently asked questions
Why does my neck hurt on one side after sleeping?
A night in a sustained end-range position loads one side of the neck continuously. Most acute wry necks settle within days with gentle movement and normal activity, which is why guidance for neck pain has moved away from collars and rest.
Is text neck a real diagnosis?
The posture is real; the diagnosis is not established. Studies have not shown a clear relationship between neck posture on a phone and long-term neck pain. Time spent in any single position, and total load, matter more than the angle itself.
Can neck muscles cause headaches?
Upper neck structures can refer pain into the head, and that pattern is recognised as cervicogenic headache. It is a clinical diagnosis, and headaches have many other causes that need ruling out first.
Nearby in this reference
Levator scapulae
The strap from the neck to the top corner of the shoulder blade
Deltoid
The cap of the shoulder
Rotator cuff
The four small muscles that hold the shoulder in its socket
Trapezius
The big diamond across the upper back and neck
Rhomboids
The straps that pull the shoulder blades together
Serratus anterior
The finger-like muscle on the ribs that pins the shoulder blade down
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