The sternocleidomastoid muscle controls several essential movements of your head and neck, including turning your head side to side, tilting it toward your shoulder, and nodding forward. It also plays a supporting role in breathing during physical exertion and contributes to your sense of balance and spatial orientation. This thick, rope-like muscle runs along each side of your neck, making it one of the most visible and easily felt muscles in the body.
Where the Muscle Attaches
The sternocleidomastoid (often shortened to SCM) has two distinct starting points, or “heads,” at the base of the neck. The sternal head originates from the top of the breastbone, while the clavicular head starts at the inner third of the collarbone. These two heads merge as they travel upward across the neck, attaching behind the ear at a bony bump on the skull called the mastoid process. This diagonal path across the neck is what gives the muscle its distinctive appearance, especially when you turn your head sharply to one side.
Head and Neck Movements
What the SCM does depends on whether one side contracts alone or both sides fire together. When only the muscle on one side contracts, it produces two simultaneous movements: it tilts your head toward the same shoulder (lateral flexion) and rotates your chin toward the opposite side. This is why when you turn to look over your left shoulder, you can feel the right SCM tighten and become prominent under the skin.
When both SCMs contract at the same time, the effect changes. The upper portion of the neck extends slightly while the lower portion flexes, producing an overall forward bending of the neck toward the chest. This bilateral action is what lets you lift your head off a pillow when lying on your back, or nod your head forward with controlled force.
Role in Breathing
The SCM doubles as an accessory breathing muscle. During normal, relaxed breathing, your diaphragm does most of the work. But during heavy exercise, respiratory illness, or any situation where you need to pull in more air, the SCM kicks in to help. Because it attaches to the breastbone and collarbone below and the skull above, contracting the SCM can lift the ribcage slightly, expanding the chest cavity and allowing the lungs to take in a larger volume of air.
This is why you can often see the SCMs visibly straining in someone who is breathing hard after a sprint, or in a person experiencing an asthma attack. Healthcare providers actually look for visible SCM engagement as a clinical sign that someone is working harder than normal to breathe.
Balance and Spatial Awareness
Beyond movement and breathing, the SCM plays a surprisingly important role in balance and body orientation. Your neck muscles are packed with proprioceptors, sensory receptors that tell your brain where your head is positioned relative to your body. The SCM works alongside your inner ear (vestibular system) and your eyes to keep you upright and oriented in space.
The neck muscles, including the SCM, also coordinate with the small muscles that control eye movement during large gaze shifts that involve both your eyes and head turning together. This integration of neck position, vision, and inner ear signals is what allows you to track a moving object while walking, or maintain your balance when you quickly turn your head. When the SCM is tight, injured, or weakened, this sensory integration can be disrupted, sometimes contributing to dizziness or unsteadiness.
Trigger Points and Referred Pain
The SCM is one of the most common sources of referred pain in the head and face. Tight, irritable spots in the muscle (known as trigger points) can send pain to areas far from the neck itself, which often leads to misdiagnosis.
The two heads of the muscle each produce their own pain patterns. Trigger points in the sternal division can refer pain to the top of the head, the back of the skull, the cheek, above the eye, and even down to the throat and breastbone. Trigger points in the clavicular division tend to send pain across the forehead (sometimes spreading to both sides when severe), deep into the ear, behind the ear, and occasionally into the upper jaw and molars. These patterns frequently get mistaken for tension headaches, atypical facial pain, or even ear infections.
Nerve Supply
The SCM is powered by the spinal accessory nerve, a cranial nerve that originates from the upper segments of the spinal cord (C1 through C4), travels upward through the base of the skull, then loops back down through an opening called the jugular foramen to reach the muscle. This nerve is a pure motor nerve, meaning it only carries movement signals, not sensation. It also supplies the trapezius muscle in the upper back and shoulder. Damage to this nerve, whether from surgery, trauma, or a tumor near the jugular foramen, can weaken or paralyze the SCM and make it difficult to turn the head against resistance.
Congenital Muscular Torticollis
One of the most well-known conditions involving the SCM is congenital muscular torticollis, a postural deformity that typically becomes visible in infants by two to four weeks of age. The SCM on one side becomes shortened or tightened, causing the baby’s head to tilt toward the affected side while the chin points toward the opposite shoulder. This mirrors exactly what a contracted SCM does mechanically: lateral flexion to the same side, rotation to the opposite side.
The causes can be muscular (such as injury during birth or positioning in the womb) or nonmuscular. Early detection matters because untreated torticollis can affect skull shape and facial symmetry as the infant grows. Stretching programs and physical therapy started early tend to resolve most cases.
How SCM Strength Is Tested
Clinicians assess SCM function using manual muscle testing, typically with a “break test.” You’re positioned so the SCM is the primary muscle working, then asked to hold your head in a specific position while the examiner applies gentle pressure to see if the muscle can maintain it. For example, you might be asked to rotate your head to one side while the examiner pushes against your chin to test the opposite SCM. The key is isolating the SCM from the many other neck muscles that assist with similar movements, which requires precise positioning and stabilization.
Weakness on one side compared to the other can point to nerve damage, muscle injury, or underlying conditions affecting the neck. Persistent tightness or tenderness in the SCM is also commonly assessed in people with chronic neck pain, headaches, or dizziness.