The sternocleidomastoid is the thick, rope-like muscle that runs diagonally along each side of your neck, from behind the ear down to the collarbone and breastbone. It is one of the most visible muscles in the human body and one of the most functionally versatile, responsible for turning your head, tilting it sideways, helping you breathe hard, and feeding your brain information about where your body is in space. It is also one of the most common sources of neck pain, headaches, and even dizziness when it malfunctions. That range of jobs, from the mundane to the surprising, makes it worth understanding in more detail than most people ever consider.
Where It Sits and How It Got That Name
The name is a mouthful, but it is just a description of where the muscle attaches. “Sterno” refers to the sternum (breastbone), “cleido” to the clavicle (collarbone), and “mastoid” to the mastoid process, the bony bump you can feel just behind your ear. The muscle has two lower heads: one anchored to the top of the sternum and the other to the inner end of the collarbone. These merge as they climb the neck and attach behind the ear. You can see and feel your own SCM by turning your head sharply to one side and looking in a mirror. The taut cord that pops out on the opposite side of your neck is it.
The nerve supply to the SCM comes primarily from the accessory nerve, the eleventh cranial nerve, along with contributions from upper cervical spinal nerves. Dissection studies have shown that the exact arrangement of these nerve fibers is highly variable from person to person. One study of 100 dissections found the “classic” pattern of innervation in only about half of cases, and having matching nerve patterns on the left and right sides was an unusual finding.1SpringerLink / Anat Clin. A contribution to the study of the innervation of the sternocleidomastoid muscle This variability matters to surgeons working in the neck, because cutting or stretching the wrong nerve branch during an operation can leave the muscle partially paralyzed.
What the SCM Does Beyond Turning Your Head
The most obvious job of the SCM is rotating the head. Contract the SCM on your left side and your chin swings to the right, and vice versa. Fire both sides together and your head tilts forward, bringing your chin toward your chest. These movements are so basic that people rarely think about them, but the SCM is also deeply involved in two less obvious systems: breathing and balance.
During quiet, relaxed breathing, the SCM stays mostly silent. Your diaphragm handles the work. But when breathing demand increases, whether from exercise, respiratory illness, or anxiety, the SCM kicks in as an “accessory” respiratory muscle, lifting the sternum and upper ribs to expand the chest cavity. Electromyographic studies have confirmed that the SCM becomes prominently active during deep or forceful inhalation, particularly in people who tend to breathe using their upper chest rather than their diaphragm.2PubMed. Participation of the sternocleidomastoid muscle on deep inspiration in man. An electromyographic study Research measuring muscle activity during increased breathing effort in healthy adults found the SCM reaching roughly half of its maximum voluntary contraction capacity during intensified inspiration.3PubMed. Electromyographic activity and coordination of accessory respiratory muscles during increased breathing effort in healthy adults Clinicians actually look at the SCM as a visual sign of respiratory distress: if you can see the neck muscles straining with every breath, the patient is working hard.
The SCM also plays a role in proprioception, your body’s internal sense of position and movement. The muscle is packed with sensory receptors that tell your brain about neck position. When researchers applied vibration to the SCM and other neck muscles during whole-body rotation experiments, subjects’ perception of how much they were spinning changed significantly.4PubMed. Self-motion perception and vestibulo-ocular reflex during whole body yaw rotation in standing subjects: the role of head position and neck proprioception In other words, what the SCM feels affects what you believe about your own motion through space. This has real consequences for balance and will come up again when we get to dizziness.
Why a Neck Muscle Can Cause Headaches, Dizziness, and Even a Runny Nose
One of the most underappreciated clinical features of the SCM is the sheer range of symptoms it can produce when it goes wrong. A case study documenting what has been called “sternocleidomastoid syndrome” described a patient whose SCM dysfunction caused head and face pain, nausea, dizziness, a runny nose, and tearing of the eyes.5PubMed Central. Sternocleidomastoid syndrome: a case study The muscle had developed tightness and weakness with multiple trigger points within both heads. That symptom list sounds bizarre for a neck muscle, but it makes more sense when you remember the SCM’s nerve connections and its role in proprioception.
Trigger points in the SCM can refer pain to the forehead, around the eye, into the cheek, and behind the ear. Dizziness from SCM problems is thought to be related to the scrambled proprioceptive signals the muscle sends when it is tight, knotted, or in spasm. Your brain relies on agreement between what your inner ear senses, what your eyes see, and what your neck muscles report. When the SCM sends garbled information, the result can be a sensation of unsteadiness or vertigo. The runny nose and tearing are likely autonomic nervous system effects triggered by the same irritated nerve pathways.
These symptoms can easily be mistaken for migraine, sinusitis, inner ear disorders, or anxiety. The fact that a physical problem in one muscle can mimic all of those conditions means that clinicians sometimes overlook the SCM as a source, leaving patients in a cycle of misdiagnosis.
Neck Pain and SCM Fatigue
Chronic neck pain is extraordinarily common, and the SCM is frequently involved. Electromyographic research comparing neck pain patients to healthy controls found that both the SCM and the anterior scalene muscles fatigued faster in people with chronic neck pain, even during relatively low-level sustained contractions at a quarter of their maximum effort.6PubMed. Myoelectric manifestations of sternocleidomastoid and anterior scalene muscle fatigue in chronic neck pain patients The researchers suggested this might reflect a shift in the muscle fibers toward types that tire more quickly, or simply that the muscles of people with chronic pain are more vulnerable to fatigue.
Interestingly, a follow-up study by the same research group found no correlation between how long someone had been experiencing neck pain and the degree of SCM fatigue.7PubMed. Lack of correlation between sternocleidomastoid and scalene muscle fatigability and duration of symptoms in chronic neck pain patients The fatigability may increase early in the course of the problem and then plateau, rather than progressively worsening over years. For someone who has lived with neck pain for a decade, this is mildly reassuring: the muscle dysfunction likely is not spiraling downward indefinitely.
Forward Head Posture and Screen Time
The modern epidemic of staring at screens has brought the SCM into focus in occupational health. When your head drifts forward of your shoulders, as it does when hunching over a laptop or phone, the SCM and other anterior neck muscles shorten to hold the head in its new position. A study of IT professionals measured this relationship directly and found a significant positive correlation between SCM shortening and the degree of forward head posture: the more the muscle had shortened, the more the head jutted forward.8International Journal of Advanced Research in Science, Communication and Technology. Correlation of Shortening of Sternocleidomastoid Muscle with Forward Head Posture in IT Professionals
This is relevant because forward head posture is associated with increased neck pain, reduced range of motion, and headaches. The SCM is both a contributor to and a victim of the posture: it shortens to accommodate the forward position, and that chronic shortening then creates trigger points and fatigue. Breaking the cycle usually requires both postural correction and direct work on the muscle itself.
Congenital Muscular Torticollis
Sometimes the SCM causes problems from birth. Congenital muscular torticollis is a condition in which one SCM is abnormally tight or fibrotic, pulling the infant’s head to one side. The baby’s head tilts toward the affected side while the chin rotates toward the opposite shoulder. It results from structural changes in the SCM that occur either during fetal development or due to perinatal trauma, and ultrasound typically reveals tissue fibrosis or post-traumatic changes within the muscle.9PubMed Central. Congenital Muscular Torticollis-Current Understanding and Perinatal Risk Factors: A Retrospective Analysis
Histological examination of affected muscle tissue shows clear interstitial fibrosis surrounding the remaining muscle fibers, with significant differences in the amount of fibrosis even within the same specimen.10Journal of Craniofacial Surgery. Basement Membrane Changes of Myofiber and Fibrosis in Sternocleidomastoid Muscle of Congenital Muscular Torticollis The condition is usually treated with physical therapy involving gentle stretching, and most cases resolve within the first year of life if addressed early. Left untreated, it can lead to facial asymmetry and developmental delays in head control.
Cervical Dystonia
In adults, the SCM is one of the major muscles responsible for the abnormal head positioning seen in cervical dystonia, a neurological movement disorder in which involuntary muscle contractions cause the head to twist or tilt uncontrollably. The standard treatment is botulinum toxin injections into the overactive muscles, including the SCM, to weaken them and reduce the abnormal pulling. Researchers have also investigated more permanent approaches: one study tested direct injection of the chemotherapy drug doxorubicin into the SCM to permanently destroy muscle fibers, essentially a non-surgical way of reducing the muscle’s ability to contract.11Muscle & Nerve. Doxorubicin chemomyectomy as a treatment for cervical dystonia: Histological assessment after direct injection into the sternocleidomastoid muscle The approach is drastic, reflecting how debilitating severe cervical dystonia can be when standard therapies fall short.
The SCM in Vestibular Testing
Your SCM plays an unexpected role in diagnosing inner ear problems. A test called the cervical vestibular evoked myogenic potential, or cVEMP, uses the SCM as a recording surface to evaluate part of the inner ear’s balance system. The patient tenses their SCM (usually by turning the head to one side) while loud clicks or tone bursts are played into the ear. This stimulates the saccule, a gravity-sensing organ in the inner ear, which produces a reflex response that can be measured as a brief electrical signal in the contracted SCM.12PubMed Central. Vestibular evoked myogenic potentials: an overview It is the only clinical tool available for assessing saccule function and the inferior vestibular nerve specifically.13Ear and Hearing. The Effect of Age on the Vestibular Evoked Myogenic Potential and Sternocleidomastoid Muscle Tonic Electromyogram Level
The practical upshot is that if you ever undergo testing for vertigo or balance disorders, you may be asked to hold your head in a specific position to activate your SCM while sounds play in your ear. The quality of the test result depends on how well the SCM is contracted, which means muscle weakness or an inability to hold the contraction can affect the reliability of the reading.
Anatomical Variations
The “standard” SCM has two heads at the bottom merging into one belly, but nature does not always follow the textbook. Anatomists have documented a wide range of variations, including SCMs with three or even four distinct heads. One case report described an SCM with three heads plus an accessory head, a configuration that could complicate surgical approaches to the upper neck.14PubMed. Variation of the sternocleidomastoid muscle: a case report of three heads and an accessory head Another report documented bilateral four-headed SCMs in a cadaver, with different configurations on each side.15PubMed. Bilateral four heads of the sternocleidomastoid muscle
These variations are not just anatomical curiosities. Extra muscle bellies can obscure or displace the landmarks that surgeons and anesthesiologists rely on during neck procedures. The gap between the two standard heads of the SCM, called Sedillot’s triangle, is routinely used to locate the internal jugular vein for central venous catheter placement. If that gap does not exist because of an unusual muscle configuration, the entire approach may need to change.
The SCM as a Landmark for Venous Access
Speaking of central lines, the SCM’s role as a surgical landmark deserves its own discussion because of how many patients it affects. When a patient in intensive care or surgery needs a central venous catheter placed into the internal jugular vein, clinicians often use the triangle between the sternal and clavicular heads of the SCM to guide needle insertion. A cadaveric study found that this triangle was fully formed on about 92% of sides examined, but was absent on 8% of left sides. Even when the triangle was present, a needle placed at its apex hit the jugular vein on the right side about 83% of the time but only about 61% on the left.16PubMed Central. Reliability of Sedillot’s triangle as a potential anatomical landmark for central venous catheter insertion: insights from cadaveric study
Another study using ultrasound simulation in living subjects found that the traditional central landmark missed the middle portion of the vein in about a third of subjects and crossed the carotid artery in roughly a quarter. Both problems were more common in obese patients.17Anesthesia & Analgesia. The Accuracy of the Central Landmark Used for Central Venous Catheterization of the Internal Jugular Vein An alternative landmark technique using visible venous swelling during respiration reduced the median distance to the true vein position from about 17.5 mm down to 3.5 mm.18PLOS ONE. The Accuracy of the New Landmark Using Respiratory Jugular Venodilation and Direct Palpation in Right Internal Jugular Vein Access These findings are a large part of why ultrasound-guided catheter placement has become standard practice in many hospitals. The old approach of relying purely on muscle landmarks worked most of the time but carried real risk.
The SCM in Reconstructive Surgery
The SCM has a second surgical identity: it can be harvested as a flap for reconstructive surgery of the head and neck. After tumor removal in the mouth or throat, surgeons sometimes need tissue to fill the resulting defect. A flap based on the SCM, carrying its own blood supply, can be swung into place to close wounds in the oral cavity or pharynx. This approach works best for smaller defects, generally not exceeding about six centimeters in diameter, and only when removing the SCM does not compromise cancer treatment goals.19JAMA Otolaryngology–Head & Neck Surgery. The Sternocleidomastoid Myocutaneous Flap
The blood supply to the SCM has been the subject of renewed investigation. Traditionally classified based on its dominant blood vessels, recent work has proposed reclassifying the muscle’s vascular pattern because a branch of the superior thyroid artery may supply the entire muscle, not just part of it. This alternative blood supply could explain why some SCM-based flaps survive better than expected.20PubMed Central. Sternocleidomastoid flap for pedicled reconstruction in head & neck surgery- revisiting the anatomy and technique For patients, what matters is that the SCM flap offers a relatively simple, single-stage repair option that avoids taking tissue from a more distant part of the body.
Physical Therapy Targeting the SCM
Given the SCM’s involvement in headaches, neck pain, and postural problems, it is a frequent target of physical therapy. Several lines of research support this. A pilot randomized trial found that manual trigger point therapy directed at active trigger points in the SCM produced large reductions in headache and neck pain intensity in patients with cervicogenic headache, outperforming a simulated treatment with large effect sizes.21PubMed. Manual treatment for cervicogenic headache and active trigger point in the sternocleidomastoid muscle: a pilot randomized clinical trial A separate randomized trial testing a combination of SCM stretching and massage in people with chronic neck pain found greater improvements in pain, disability, range of motion, and endurance compared to a control group.22PubMed. The effects of combined sternocleidomastoid muscle stretching and massage on pain, disability, endurance, kinesiophobia, and range of motion in individuals with chronic neck pain: A randomized, single-blind study And a trial focused specifically on pressure-point release of the SCM in women with chronic cervicogenic headache found significant pain reduction and improved neck function beyond what a standard cervical pain management program achieved alone.23Physiotherapy Practice and Research. The effectiveness of sternocleidomastoid muscle pressure points in patients with cervicogenic headache: A randomized controlled trial
These are generally small studies, and larger trials would strengthen the evidence. But the consistent direction of the findings suggests that directly treating the SCM, whether through stretching, massage, or trigger point release, can produce real improvements for people dealing with neck pain and related headaches. The SCM is easy to access, sitting right on the surface of the neck, which makes self-massage and stretching practical even without a clinician’s hands. A gentle pinch-and-roll technique along the muscle’s length is one common method physiotherapists teach patients to perform at home, though anyone with unexplained neck lumps or vascular concerns should have those checked before poking around their own neck.