Sternocleidomastoid

Function

Where it starts and ends

The sternocleidomastoid is a prominent muscle near the surface of the neck^1,2. Seen from the side, it splits the neck into a front triangle and a back triangle^1,2. It forms a muscular ridge that runs upward and across the neck, thick in the middle and thinner at each end³.

It starts on the manubrium (the upper part of the breastbone) and the inner part of the clavicle (collarbone)^1,2. These are usually two separate heads³:

Head	Where it starts	How it starts
Sternal	The front surface of the manubrium³	A distinct, rounded tendon³
Clavicular	The inner third of the collarbone, on an upper area of its front surface^3,9,10	A band-like tendon mixed with muscle fibres, varying in width³

The two heads rise up the neck, with the clavicular head passing under the sternal head³. About halfway up, they join into one thick belly³.

Engraving of the left side of the neck, from the jaw and ear at the top down to the collarbone. The sternocleidomastoid runs as a broad strap diagonally across the side of the neck, from two lower heads labelled sternal origin and clavicular origin at the breastbone and collarbone up to the mastoid process behind the ear. In front of it lie the thin strap muscles of the throat below the jaw; behind it lie the splenius, levator scapulae and scalene muscles, with the trapezius at the back of the neck. — The muscles of the neck from the sideHenry Vandyke Carter, in Henry Gray, Anatomy of the Human Body (1918), Fig. 385. Public domain, via Wikimedia Commons. (source, Public domain)

At the top, the muscle usually ends in two places on the skull^1,3. One is the outer surface of the mastoid process, behind the ear^1,3. The other is the outer part of the superior nuchal line, on the back of the skull^1,3. These lie on the temporal bone and the occipital bone (the bone at the back and base of the skull)².

What it moves

An anatomy textbook calls it the major muscle that bends the head to the side and turns it². Acting on one side, it turns the head toward the opposite side². When both sides work together, they can bend or straighten the neck, depending on how stiffly the neck spine is held¹. In a computer model of the upright neck, it had the largest moment arm (leverage) for bending forward and sideways¹².

It also works during chewing¹³. One reference adds roles in posture, in jaw function and in coordinating the muscles of the neck and face¹.

Its nerve, blood supply and fibres

The accessory nerve (cranial nerve XI) is its main nerve supply¹. Fibres for proprioception, the body’s sense of where its joints are, come from the cervical plexus, a network of nerves in the neck¹. Its blood comes from branches of the external carotid artery¹.

In autopsy samples from young men presumed healthy, about 31.5% of fibres were the slow type¹³. About 26.8% were hybrids, carrying two fast forms of myosin, the protein used to sort fibres into types¹³. Such a high share of hybrid fibres differs from adult limb muscles, where hybrids are rare¹³. The authors suggest this may be linked to its two nerve supplies and its many jobs¹³. In an MRI study of 13 men and 14 women, the women’s muscle was smaller for their body mass¹⁴.

A helper for hard breathing

The muscle helps with breathing in, especially when breathing takes more effort^1,13. Three studies of healthy adults breathing in against loads measured it^4,15,16:

In 62 young adults, its activity rose with load up to the top load, while diaphragm thickening peaked midway and then fell⁴. Focusing on breathing with the diaphragm went with less sternocleidomastoid activity⁴.
In 13 adults, its activity rose in a straight line with load and was higher in lower-oxygen air¹⁵. It showed no electrical signs of fatigue¹⁵.
In 12 adults, it switched on earlier in each breath as they reached the point of stopping¹⁶. Earlier onset went with feeling more out of breath¹⁶.

In heading and tackling

In 15 college women soccer players, the sternocleidomastoid switched on before the trapezius when heading a ball¹⁷. It was more active before the ball arrived, and the trapezius more active after¹⁷. Jumping headers gave higher right-side activity than standing ones, though impact forces did not differ¹⁷. In 28 high-school rugby players, its activity was significantly higher when tackling with the head up than head down¹⁸.

Breakdown

In rear-end impacts

A 2018 narrative review says whiplash symptoms likely come from sprained ligaments or strained muscles in the neck¹⁹. The whiplash page covers the condition itself. In 42 volunteers in low-speed rear-end collisions, this muscle switched on before the back-of-neck muscles⁵. The head’s first backward movement then stretched the already active muscle⁵. The authors describe this as consistent with an injury caused by a muscle contracting while it lengthens⁵.

In that study, neck muscles switched on earlier in women and in the faster collisions⁵. A computer model drawn from human rear-impact data put the sternocleidomastoid’s peak fibre strain (stretch) at 4% to 7%²⁰. The model put it higher, 21%, in the splenius capitis at the back of the neck²⁰. In 20 volunteers on a test sled, a loud warning tone cut this muscle’s activity by 16%²¹. The authors propose that the startle reaction is an overreaction that may raise forces in neck tissues²¹.

After whiplash and in neck pain

Two MRI studies of chronic (long-lasting) whiplash-associated disorders, the range of symptoms that can follow a whiplash injury, found different results^22,23:

In 109 women, the whiplash group had significantly more fat in this muscle and a larger cross-sectional area²².
In 31 people and matched controls, only those with severe, mainly right-sided symptoms had more fat in the right muscle²³. No other differences were found²³.

In 75 volunteers, people with neck pain, whether or not it followed whiplash, showed more sternocleidomastoid activity than controls in a craniocervical flexion test²⁴. The neck pain page covers that condition.

Neck strength, head motion and concussion

Greater neck strength and bracing are thought to lower concussion risk by limiting head motion, but earlier research was mixed²⁵. In 46 contact sport athletes aged 8 to 30, greater isometric neck strength went with smaller head speed changes under a test force²⁵. So did bracing the neck in advance²⁵. In 40 active volunteers, women had 50% greater head acceleration under a test force, despite switching this muscle on earlier²⁶. They also had 49% less neck strength and 43% less head mass²⁶.

In 100 soccer players heading a ball, neck strength explained about 13% to 17% of the spread in head acceleration²⁷. Head and neck size explained about 22% to 23%, and muscle activity did not significantly predict it²⁷. A systematic review of 60 heading studies found some evidence that a stronger neck lowered head acceleration, but not in all cases²⁸. Head acceleration is often used to stand in for concussion risk²⁸.

The studies of concussion itself measured neck strength, and they disagree^6,7,8:

Who was studied	What was found
6,704 high school soccer, basketball and lacrosse athletes	Weaker overall neck strength was associated with concussion⁶
43 high-school rugby players aged 15 to 18	The 11 concussed during the season had weaker preseason neck extension strength; other neck measures did not differ²⁹
136 male professional rugby players	Concussed players had neck strength similar to the others⁷. Flexion strength and neck endurance were not associated with concussion⁷
8 studies, 7,625 team-sport athletes (meta-analysis)	Very low-certainty evidence of a small, non-significant link between stronger necks and lower concussion risk³⁰
5 studies of athletes (2026 meta-analysis)	No significant differences in neck strength in five directions; evidence judged inconclusive⁸

Among 136 male professional rugby players, a past concussion was the strongest predictor of another⁷. In that group, 40 players had 51 concussions in one season, and 65% reported a past concussion⁷. Stronger neck extension went with slightly higher odds of concussion, an effect the authors judged likely too small to matter in practice⁷. In 27 club rugby players making simulated tackles, those concussed in the past year had the highest head acceleration³¹. The concussion page covers the condition.

Recovery

How it changes with training

In a cross-sectional study, which looks at a group once, 129 sub-elite male rugby players had thicker sternocleidomastoid muscles than non-players of the same age³². They also had larger necks and less neck movement³². In a randomised trial of 60 people with chronic neck pain, ten weeks of neck flexion exercise made this muscle thicker³³. Craniocervical flexion exercise (a small nod of the head) increased the size of the longus colli more³³.

Training has also changed how much the muscle is used in people with neck pain^34,35. In 46 people with chronic neck pain, six weeks of nodding practice lowered its activity during the nodding test³⁴. A review of trials in people with chronic neck pain found lower activity after neck exercise than after control programmes³⁵.

Does neck training change head motion?

A meta-analysis of 26 articles found a large effect of resistance training on isometric neck strength³⁶. Studies of head motion after neck training found mixed results^36,37,38:

In 36 college soccer players, eight weeks of training raised neck flexor strength by 15%³⁷. It did not change head motion or sternocleidomastoid activity under a test force³⁷.
In a pilot study of 17 young athletes, neck training raised neck girth and strength more than a control programme did³⁸. Changes in head speed were mixed³⁸.
One meta-analysis found no significant effect of neck training on head acceleration, from low to very low quality evidence³⁶.
A review of 60 heading studies concluded that standard neck strengthening did not lower head acceleration, but adding neuromuscular exercises appeared to²⁸.

Neuromuscular training is exercise aimed at how nerves and muscles work together to control movement. A review of 10 studies found two in which neck strengthening in male rugby players went with fewer concussions³⁹. No study in that review showed this in soccer or women’s rugby³⁹.

Connections

Muscles it works with

The spinal accessory nerve is the main motor nerve for both this muscle and the trapezius⁴⁰. In a study of three men, the sternocleidomastoid and the sternohyoid, another front neck muscle, worked in similar directions⁴¹. In 10 healthy volunteers, a standard neck flexion hold worked it and the anterior scalene harder than a gentle nod⁴². The anterior scalene is one of the scalenes, at the side of the neck. In the 46-person trial, nodding practice raised deep neck flexor activity as this muscle’s activity fell³⁴.

In a model of rear-end impacts, this muscle lengthened as the head moved back, and the neck extensors lengthened on the rebound²⁰.

Exercises that load it

Related exercise pages: neck isometric hold. The study of 10 volunteers compared the two holds at full, half and one-fifth effort, and the flexion hold gave more activity each time⁴².