Sternocleidomastoid

The sternocleidomastoid muscle, often shortened to SCM, is the long muscle you can see on each side of your neck, running from behind the ear down to the top of the breastbone. It is a prominent muscle that is easy to see and feel, and in people with neck pain it is often one of the busiest.

Where is the sternocleidomastoid?

The name lists its attachments: sterno for the breastbone (sternum), cleido for the collarbone (clavicle) and mastoid for the mastoid process, the bony bump behind the ear. The muscle starts from the top of the breastbone and the inner end of the collarbone, and attaches to the mastoid process and a ridge at the back of the skull (Bordoni et al., 2026).

Doctors use it as a map. It divides each side of the neck into a front and a back triangle, and it is a key landmark in examining the neck and in surgery such as operations on the carotid artery (Bordoni et al., 2026).

Its movement nerve is the accessory nerve, the same one that powers the trapezius (Bordoni et al., 2026; Ourieff et al., 2023).

What does the sternocleidomastoid muscle do?

• Turns the head to the opposite side, so the left SCM turns your face to the right.
• Tilts the head towards the same shoulder.
• Bends the neck forward when both sides work together, or helps extend it, depending on how the rest of the cervical spine is held.
• Helps you breathe in.

It also takes part in controlling posture and in jaw movement, working together with other muscles of the face and neck (Bordoni et al., 2026).

If your main screen sits off to one side, your head spends the working day turned, which is the SCM’s job. The guide to monitor setup covers where to put it.

Why can the SCM feel sore in people with neck pain?

The SCM is a superficial muscle. Underneath it, closer to the spine, lies a group of smaller muscles, the deep neck flexors. Studies of people with neck pain keep finding the same pattern: the deep muscles do less and the superficial ones, including the SCM, do more.

• In a small study of 10 people with chronic neck pain and 10 without, the deep neck flexors were less active during a clinical test of head control, with a trend, not statistically significant, towards more SCM activity (Falla et al., 2004a).
• Female office workers with neck pain showed more activity in their superficial neck flexors, the SCM and anterior scalene, during the same test (Johnston et al., 2008a).
• During a repetitive arm task, people with neck pain used the SCM and other accessory neck muscles more than people without pain (Falla et al., 2004b).
• During typing tasks, office workers with and without neck pain both had higher SCM activity than women who were not in paid work (Johnston et al., 2008b).

The researchers read this as a change in strategy, with the larger muscles stepping in to stabilise the head and neck. None of these studies could say whether the change causes pain or follows it. A muscle that works overtime can feel tender, and the SCM is often discussed alongside forward head posture. At the back of the neck, the small suboccipital muscles are the ones most often linked with head position and headaches.

Which conditions involve the sternocleidomastoid?

• Torticollis and cervical dystonia. In cervical dystonia, one of the most common forms of dystonia in adults, muscles contract involuntarily and pull the head into a turned or tilted position. The SCM is the muscle most often involved (Cunha et al., 2023).
• Neck pain and stiffness. Problems with the SCM can contribute to postural changes, limited neck movement and altered jaw function (Bordoni et al., 2026). See neck pain from desk work and stiff neck after sleeping.

When should you see a doctor?

Call 999 or go to A&E if a headache comes with a very high temperature, a stiff neck, sensitivity to bright light, or a rash that doesn’t fade when a glass is rolled over it (NHS). See a GP if neck pain hasn’t gone after a few weeks, if painkillers haven’t helped, or if you have pins and needles or a cold arm (NHS). See a GP too if your neck keeps twisting to one side by itself: the NHS advises getting possible dystonia checked (NHS).

Stretches for the front and sides of the neck, where the SCM sits, are part of Bavelu’s neck sessions, designed and filmed by physiotherapists.