Sternocleidomastoid
The prominent strap down the side of the neck, whose name is simply a list of its attachments.
- Type
- Muscle
- Region
- Neck
- Standard name
- m. sternocleidomastoideus
- Origin
- Manubrium of the sternum (sternal head) and the medial third of the clavicle (clavicular head)
- Insertion
- Mastoid process of the temporal bone and the superior nuchal line
- Actions
- neck rotationneck sidebendingneck flexion
- Verify
- TA2 2156 TA98 A04.2.01.008
What it is
The most visible muscle of the neck: a thick strap running diagonally from the top of the breastbone and the inner collarbone up to the bony bump behind the ear.
Its name is a route description — sterno (sternum), cleido (clavicle), mastoid (mastoid process) — which makes it one of the more learnable names in anatomy once you notice.
It divides the neck into anterior and posterior triangles, the standard anatomical map of the region. Like trapezius, it is supplied by the accessory nerve rather than by ordinary spinal nerves, and the two are the only skeletal muscles that are.
Turn your head to one side and the muscle on the opposite side stands out.
What it does
Acting on one side, it rotates the head to the opposite side and side-bends it towards its own. That crossed action is why it becomes prominent on the far side when you turn your head.
Acting on both sides together, it flexes the neck — nods the head forward — while also extending the joint between the skull and the first vertebra, which tips the chin up. The combination is what makes lifting the head off a pillow work.
It is an accessory muscle of breathing, lifting the sternum and clavicle during laboured breathing.
What loads it
Resisted neck flexion and rotation. It also works whenever the head is held forward of the body against gravity.
What is commonly got wrong about it
"Text neck" as an established condition. The claim spread from a single much-shared 2014 paper, which was a biomechanical calculation of the load on the neck at increasing angles of head tilt — not a study of whether phone use causes neck pain, which is the claim it is almost always cited for.1 The calculation is not the issue; the leap from it to structural harm is, and the paper does not make that leap on the reader's behalf.
"Forward head posture" as a defect requiring correction. Head position varies widely between people without pain.
That neck tension is this muscle being tight. Neck symptoms have many sources, several not muscular. Persistent neck pain, and any neck pain with arm symptoms, needs assessment.
Sources
- Hansraj KK. Assessment of stresses in the cervical spine caused by posture and position of the head. Surgical Technology International 2014; 25:277–279. PMID 25393825 The source of the widely shared 'text neck' figures. A single-author biomechanical calculation of loads at increasing head tilt — not a study of whether phone use causes neck pain, which is the claim it is usually cited for. Carries no DOI.
Every source here was checked against PubMed rather than recalled. Where this site says something is disputed or unsupported and no source appears, that is deliberate: it means the claim is being conceded rather than cited.