The SCM, or sternocleidomastoid, is a large paired muscle that runs along each side of your neck. It’s one of the most visible and prominent neck muscles, forming the diagonal band you can see and feel when you turn your head to one side. The SCM plays a central role in head movement, neck stability, and even breathing under stress.
Where the SCM Attaches
The name “sternocleidomastoid” is a mouthful, but it’s essentially a map of the muscle’s attachment points: sternum, clavicle, and mastoid. At the bottom, the muscle has two heads. One originates from the upper edge of the breastbone (the manubrium of the sternum), and the other from the inner quarter of the collarbone. These two heads merge into a single muscle belly that angles upward and to the side. At the top, it attaches to the mastoid process, the bony bump you can feel just behind your ear, and continues along the base of the skull.
Because it crosses so much of the neck diagonally, the SCM creates the characteristic V-shape visible at the front of the throat when both muscles contract at once.
What the SCM Does
The SCM’s job depends on whether one side contracts alone or both sides work together. When only the right SCM fires, it rotates your head to the left and tilts your ear toward your right shoulder. The left side does the mirror image. When both sides contract simultaneously, they flex the neck forward, bringing your chin toward your chest.
These movements matter constantly throughout the day. Checking a blind spot while driving, looking down at a phone, nodding in conversation, and turning to talk to someone beside you all rely heavily on the SCM. It also helps stabilize the head during sudden movements, keeping your skull steady on top of your spine.
Role in Breathing
The SCM doubles as an accessory breathing muscle. During quiet, relaxed breathing it stays electrically silent, meaning it doesn’t contribute at all. But when your body demands more air, during intense exercise or respiratory distress, the SCM kicks in to help lift the ribcage and increase the volume of each breath. Its activation threshold is high, so it only recruits when normal breathing muscles can’t keep up.
This becomes clinically significant in conditions like COPD, where the diaphragm flattens and loses efficiency. When that happens, the SCM and other neck muscles compensate by taking on a larger share of the breathing workload. Visible contraction of the SCM during breathing is actually a sign doctors look for when assessing respiratory distress.
Nerve Supply
The SCM is controlled primarily by the spinal accessory nerve, the eleventh cranial nerve. This nerve travels from the brainstem, passes through the skull, and courses deep to the SCM, either penetrating the muscle directly or sending a branch to its inner surface. Additional nerve fibers from the upper cervical spine (C1 through C4) also feed into the muscle, providing extra motor input and likely contributing to the muscle’s proprioceptive function, your brain’s ability to sense where your head is positioned in space.
SCM Trigger Points and Referred Pain
The SCM is one of the most common sources of trigger points in the body, and the symptoms they produce can be surprisingly far-reaching. Active trigger points in the SCM don’t just cause neck stiffness. They refer pain and other sensations to areas that seem completely unrelated to the neck.
The sternal head (the portion originating from the breastbone) typically contains about four trigger points. These can produce pain at the back of the skull, around the eye, in the cheek, across the eyebrow, and even in the throat. They’re also associated with drooping eyelids, blurred vision, and sinus-type headaches that mimic an actual sinus infection.
The clavicular head (the portion from the collarbone) typically holds three trigger points. These refer pain to the forehead, the ear, and the area behind the ear. More strikingly, they can cause nausea, dizziness, vertigo, and a sense of unsteadiness. Because the clavicular division plays a role in your sense of equilibrium, tightness or trigger points here can disrupt the proprioceptive signals your brain uses to maintain balance.
This combination of face pain, dizziness, watery eyes, and eyelid twitching from a single neck muscle frequently goes undiagnosed. Patients often cycle through multiple specialists before anyone considers the SCM as the source.
SCM Problems Linked to Posture
Forward head posture, sometimes called “tech neck,” directly affects the SCM. When your head drifts forward of your shoulders for prolonged periods (at a desk, looking at a phone, or driving), the SCM shortens along with the cervical extensor muscles in the back of the neck. A shortened muscle generates less force than one at its natural resting length, so the SCM becomes both tight and weak at the same time.
Research using electromyography (EMG) confirms this. People with forward head posture show significantly reduced SCM muscle activity during neck movements compared to people with normal posture. The muscle’s shortened position limits how effectively it can contract, which compromises neck stability and can contribute to chronic neck pain and headaches. Strengthening exercises that restore the SCM to its proper length, particularly chin tucks and neck retraction movements, help reverse this pattern.
Congenital Muscular Torticollis
The most well-known SCM condition in infants is congenital muscular torticollis, where a baby is born with a shortened or tightened SCM on one side. This causes the head to tilt toward the affected side while the chin rotates away from it. The global incidence ranges from 0.3% to 1.9% of newborns, roughly 1 in 250 births, making it the third most common congenital orthopedic condition after hip dysplasia and a foot deformity called calcaneovalgus. The rate is higher in traumatic deliveries (about 2%) compared to nontraumatic ones (0.3%).
Most cases respond well to physical therapy involving gentle stretching and positioning techniques, especially when started early. Left untreated, torticollis can lead to asymmetrical skull development and restricted neck mobility that persists into childhood.
How to Locate Your SCM
Finding your own SCM is simple. Turn your head fully to the left and look at the right side of your neck in a mirror. The thick band running diagonally from behind your ear to the inner end of your collarbone is the right SCM. You can pinch it gently between your fingers and feel both the sternal and clavicular heads near the base of the neck where they separate into a V shape. Pressing along its length and noticing tender spots can help you identify whether SCM tension might be contributing to headaches, neck stiffness, or dizziness you’re experiencing.