Scalenes

Function

Three muscles from neck to ribs

There are three scalene muscles on each side of the neck¹. The anterior scalene sits in front, the middle scalene between, and the posterior scalene behind¹. The middle one is the longest and the posterior one the smallest¹. The scalenes begin as one shared muscle in the eighth week of development, which the growing brachial plexus splits into anterior and middle parts³.

The anterior and middle scalenes start on the transverse processes (small bony spurs on each side of a vertebra) of the neck vertebrae^5,12:

Muscle	Where it starts	Where it ends
Anterior	The front knobs of the spurs on the third to sixth neck vertebrae5,12	The scalene tubercle, a point on the upper surface of the first rib5,12
Middle	The back knobs of the spurs on the second to seventh; another account gives the first to seventh5,12	The first rib, behind the groove made by the subclavian artery, the large artery beneath the collarbone5,7,12
Posterior	In 10 cadavers (bodies donated for study), the fourth to sixth in half and the fifth to sixth in half13	The second rib3

In the same 10 cadavers, the levels varied: the anterior scalene reached the seventh vertebra in 30%, and the middle scalene in 60%¹³.

Where they meet the first rib

On top of the first rib, the scalene tubercle lies between two grooves⁵. The subclavian artery makes the outer groove, and the subclavian vein, the large vein beneath the collarbone, makes the inner one^5,7. In a study of 42 Japanese cadavers, no prominent raised bump was seen where the anterior scalene met the rib in any of them⁴. In that study, the anterior scalene attached to the rib’s upper, back and lower surfaces, and the middle scalene to its upper surface only⁴. Both attached by fibrous tissue, with no fibrocartilage (a tough, fibrous kind of cartilage) at either site⁴.

What they move

Working together, the scalenes bend the head and neck forward, bend them to the side and turn them^1,2. They also help keep the head and neck steady². One review describes the anterior and middle scalenes lifting the first rib and resisting forces that pull it down¹⁴.

How they help breathing

The scalenes are described as accessory (helper) breathing muscles, which raise the upper rib cage³. In humans, they are among the muscles that widen the rib cage on a breath in by lifting the ribs¹⁵. Studies in healthy adults found^16,17,18:

• In needle recordings from five people, the scalenes were always active in quiet breathing, usually switching on as a breath in began¹⁶.
• In the same five, breathing in with the diaphragm (the main breathing muscle) alone cut scalene activity, and the upper ribs moved inward¹⁶.
• With skin sensors in 10 people, averaging many breaths showed a low burst of activity with each breath in, in eight¹⁷.
• In 13 people breathing against a rising load, activity rose with the load, with no electrical signs of fatigue¹⁸.

Their nerves and fibres

Nerves reach the scalenes from two networks: the brachial plexus and the cervical plexus (a network of nerves in the neck)². In 26 cadavers, the anterior scalene took inner front branches from the front roots of the neck nerves, and the middle scalene outer back branches¹⁹. In four elderly male cadavers, 71.9% of anterior scalene fibres were slow (type 1) fibres, the most of the neck muscles studied²⁰.

Breakdown

The gap the nerves pass through

The scalene triangle is the space bounded by the anterior scalene, the middle scalene and the first rib^4,5. The brachial plexus runs through it, behind the subclavian artery and in front of the middle scalene³. The anterior scalene separates the subclavian vein, in front of it, from the artery behind it³. The phrenic nerve, which supplies the diaphragm, runs down the front of the anterior scalene^2,3. In 42 Japanese cadavers, the triangle’s base averaged 8.2 mm wide, against 10.7 mm reported in Western groups⁴.

This triangle, above the collarbone, is one of the thoracic outlet’s three areas³. A 2022 consensus calls the nerve form of thoracic outlet syndrome the most common⁷. One study says it typically comes from nerves and vessels pinched between the anterior and middle scalene⁸.

Causes one review lists

A case report with a literature review lists causes of one subtype of the nerve form, involving the upper brachial plexus⁵:

• an enlarged scalene⁵;
• the brachial plexus passing through the anterior scalene⁵;
• pressure on the middle scalene from a long spur on the seventh neck vertebra⁵;
• a cervical rib (an extra rib in the neck)⁵.

Here, an enlarged scalene as a cause of this subtype rests on that list⁵. Separately, a systematic review (a summary of the research on a question) covers the nerve to the rhomboids²¹. It reports an enlarged middle scalene, particularly in bodybuilders, may press on that nerve^21,22.

How often the textbook layout holds

In 51 adult cadavers, the textbook path of the brachial plexus between the anterior and middle scalene was seen in only 60% of cases¹¹. Other cadaver studies also found variations^8,23:

In 51 adult cadavers, the textbook path of the brachial plexus between the anterior and middle scalene was seen in only 60% of cases.

Study	What it found
51 adult cadavers	The fifth and sixth neck nerve roots pierced the anterior scalene: fused first in 15%, the fifth alone in 13%, separately in 6%11
65 cadavers	31 variations (47.7%); in 27, part or all of the plexus’s upper trunk (its top nerve bundle) pierced the anterior scalene8
93 cadavers (186 sides)	The plexus’s upper trunk took an unusual course on 12.9% of sides, most often through the muscle23
Meta-analysis of 40 studies (3,055 arms)	The usual relationship in 86% of cases, with a 95% confidence interval (a range showing how precise the result is) of 66% to 98%6

The authors of the 65-cadaver study suggest that people whose plexus pierces the muscle would not show positive signs on the traditional arm-position tests⁸. Their reason is that those tests rely on the artery being squeezed⁸.

An extra scalene

The scalenus minimus is the commonest extra scalene muscle³. It passes between the subclavian artery and the brachial plexus, and may irritate the plexus’s lower trunk (its lowest main nerve bundle)³. It was present in 46% of 51 adult cadavers, but in three of 73 Greek cadavers (4.11%)^10,11. The Greek study’s authors found published rates from 7.8% to 71.7%¹⁰. In 200 operations for compression at the thoracic outlet, surgeons found 20 extra scalene muscles²⁴.

Injury, whiplash and neck pain

One review describes neck hyperextension injuries (bending the neck back past its normal limit) as tearing some anterior and middle scalene fibres³. In 45 scalenes removed from people with thoracic outlet syndrome after an injury, connective tissue averaged 36%, against under 15% in control muscles²⁵. Slow fibres made up 78% of fibres, against 53% in controls²⁵.

One systematic review gathered what is known about the scalenes after whiplash²⁶:

• It says strain on the scalene triangle during whiplash would in theory put the thoracic outlet at risk, but needs more study²⁶.
• The same review found five studies reporting thoracic outlet syndrome in 31% to over 74% of people with chronic whiplash, from very low quality evidence²⁶.
• It found trigger points (tender muscle knots) in the scalenes and nearby ones in 48% to 65%, on low or very low quality evidence²⁶.

A study following 110 people after injury to the upper body classed 39 as having the definite nerve form of thoracic outlet syndrome²⁷. Its authors suggest that cases after injury relate to direct damage to the scalenes²⁷.

In 10 people with chronic neck pain and 10 matched controls, the neck was held bent forward at 25% and 50% of maximum effort²⁸. The anterior scalene showed more electrical signs of fatigue in the neck pain group²⁸. In another study, people with neck pain used accessory neck muscles, including the anterior scalene, more than people without symptoms during a repeated arm task²⁹.

Recovery

Healing after a tear

One review describes torn scalene fibres healing with scar tissue³. That scarred muscle is less pliable and can press on the brachial plexus, the review says³. Separately, in anterior scalene samples from people with thoracic outlet syndrome, 85.1% of fibres were slow fibres, and those fibres were enlarged³⁰. After the anterior scalene’s tendon was cut in people with thoracic outlet syndrome, the share of slow fibres fell, they shrank, and fast fibres increased³⁰.

How training changes them

In a randomised trial of 58 women with chronic neck pain, six weeks of endurance-strength training of the neck flexors raised neck flexion strength³¹. It also reduced electrical signs of fatigue in the anterior scalene and the sternocleidomastoid³¹. In a trial of 46 people with chronic neck pain, six weeks of practising a small nod lowered anterior scalene activity in a nod test³². In a trial of 25 healthy men, five weeks of breathing-muscle training or a sham (a dummy version) left scalene activity in exercise unchanged³³.

A trial injected botulinum toxin (a drug that relaxes the muscle it is injected into) into the scalenes of 38 people with thoracic outlet syndrome³⁴. It brought no clinically or statistically significant gain in pain, tingling or function over a placebo (a dummy injection)³⁴.

Connections

Nerves and vessels beside them

The brachial plexus and the subclavian artery pass between the anterior and middle scalene and the first rib, according to a 2022 consensus⁷. In 18 cadaver sides, the long thoracic nerve lay between the middle and posterior scalene in 56%³⁵. The dorsal scapular nerve, which supplies the rhomboids, pierces the middle scalene²¹. A systematic review names the middle scalene as that nerve’s most common site of entrapment²².

Bones they pull on

The scalenes start on the neck vertebrae and end on the ribs³. One review describes a hypothesis that opposing pulls from the scalenes (upward) and the serratus anterior (downward) cause most first-rib stress fractures in baseball pitchers¹⁴.

Muscles they work with

In 12 control subjects (people studied for comparison) moving one arm fast, the anterior scalene switched on less than 50 ms after the deltoid³⁶. So did the sternocleidomastoid and the deep neck flexors³⁶. In 10 healthy volunteers, a conventional neck flexion hold used the anterior scalene and sternocleidomastoid significantly more than a small nod, at each effort tested³⁷. Related exercise pages: neck isometric hold³⁷.

Sources

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Every source the library cites

Changes

• October 28, 2026 First published.