Serratus anterior

Key points

The serratus anterior is a fan-shaped muscle that runs from the upper eight or nine ribs, on the side of the chest, to the inner edge of the shoulder blade¹.
Its main actions are protraction (drawing the shoulder blade forward) and turning the shoulder blade upward on the chest wall².
One review says its only nerve is the long thoracic nerve (a nerve on the side of the chest that supplies the serratus anterior)³.
One review names paralysis of this muscle, from injury to that nerve, as the commonest cause of scapular winging (the shoulder blade sticking out from the back)³.
A systematic review of six studies found that, without surgery, winging from this paralysis recovered on its own in 21% to 78%⁴. It concluded that a substantial share of people with nerve-related winging managed without surgery seem to have lasting complaints⁴.

Function

Where it sits and what it attaches to

The muscle runs from the side wall of the chest to the scapula (shoulder blade)¹. It starts on the sides of the first nine ribs². These rib attachments look like the teeth of a saw, which gives the muscle its name². On the fifth to ninth ribs, it interlocks with the start of the external oblique, the outermost flat muscle of the abdomen⁵.

From the ribs, it passes backward in front of the shoulder blade². It attaches along the whole inner edge of the bone’s front surface². There it lies close to the subscapularis (a muscle on the front of the shoulder blade, next to the ribs), and often links with it².

Two panels of the shoulder girdle. A front-side view of the left chest shows the serratus anterior fanning from the side of the upper ribs back toward the shoulder blade, with the pectoralis minor and subclavius above it and the pectoralis major cut away. A back view shows the trapezius over the upper back and, beneath it on the other side, the rhomboid major and minor running from the spine to the shoulder blade. — Muscles that position the shoulder bladeBetts JG, Young KA, Wise JA, et al. Anatomy and Physiology, Figure 11.22. OpenStax, 2013. Access for free at openstax.org. (source, CC BY 4.0). Resized to 1400 pixels wide.

Most of it lies beneath the shoulder blade and chest muscles¹. It can be felt between the pectoralis major (the large, fan-shaped muscle at the front of the chest) and the latissimus dorsi (the lats)¹.

Three parts

Sources usually split the muscle into three parts, by where each one lands on the shoulder blade¹. The bone’s inner border ends in a superior angle at the top and an inferior angle at the bottom⁶. One review gives each part’s rib levels³:

Part	Starts on	Attaches to
Upper	Ribs one and two	The upper inner corner³
Middle	Ribs three to five	The inner border³
Lower	Ribs six to nine	The lower corner³

What it does to the shoulder blade

The muscle’s primary actions are protraction and turning the shoulder blade upward on the chest wall². Protraction happens when reaching forward or pushing things away². It protracts strongly because its line of pull is close to level and it has a long moment arm, meaning good leverage². One review of scapular winging adds that it keeps the shoulder blade close against the chest wall³. That review says this places the shoulder socket for arm movement³.

Sources give each part a different job, and their wording differs^3,7,8:

One review describes the upper part as the anchor that lets the shoulder blade turn as the arm goes overhead³.
From 70 dissections of cadavers (bodies donated for study), one team suggests the middle part moves the shoulder blade away from the body’s midline⁷.
The same team suggests the lower part helps turn the shoulder blade upward, move it away from the midline and tilt it backward⁷.
A review of shoulder muscle studies says the middle and lower parts turn the shoulder blade upward, tilt it back and turn it outward⁸.

That review describes these two parts as the only muscles between shoulder blade and chest able to both turn it upward and tilt it back⁸.

EMG (electromyography) measures a muscle’s activity by picking up its electrical signals. In an EMG study of 29 healthy people, the lower part was more active than the upper part⁹. This held in tests that mainly turned the shoulder blade upward⁹. The two parts did not differ significantly in tests of full protraction⁹.

When the hands are fixed, as in a push-up, the muscle works in reverse². It lifts the ribs and trunk toward the shoulder blades². In a full push-up, one review says its forward pull drives the late phase, lifting the chest highest from the floor².

Its nerve

One review says the long thoracic nerve is the muscle’s only nerve, and that it averages 24 cm long³. It runs down the chest wall on the muscle’s outer surface¹⁰. Another source says it comes from the brachial plexus (the network of nerves, starting at the neck, that serves the arm)¹⁰. That source says it typically gets fibres from the C5, C6 and C7 nerve roots¹⁰.

A study of 70 dissections of 35 cadavers looked at the supply to each part⁷. There, the C5 nerve root gave most of the upper part’s supply⁷. The long thoracic nerve, formed from the C6 and C7 roots, supplied the middle and lower parts⁷.

Breakdown

When the muscle is weak: winging

Scapular winging is rare³. When the serratus anterior is paralysed, the shoulder blade’s inner edge sticks out³. The trapezius is the large muscle across the upper back and neck that moves the shoulder blade. When it or the rhomboids (muscles between the shoulder blades and the spine) are paralysed, the outer edge sticks out instead³.

When a weak muscle works against resistance to draw the shoulder blade forward, the inner border lifts away from the ribs². The worse the weakness, the worse the winging tends to be².

With the muscle weak, the arm-lifting muscles can turn the shoulder blade downward during attempts to raise the arm². One review says winging can affect lifting, pushing and pulling heavy objects and daily tasks³. It can reduce power, limit raising the arm, and cause considerable pain³.

How often it happens

Paralysis of the serratus anterior is uncommon³. One review gives two figures. One EMG laboratory reported 15 cases in 7,000 patients, and the Mayo Clinic found one case in 38,500 patients³. It has been recorded in athletes in many sports, such as weightlifting, tennis, baseball and gymnastics³. It has also been seen in manual workers, such as mechanics, welders and carpenters³.

What damages the nerve

The nerve’s long, winding path is thought to make it easy to injure³. Most injuries from trauma follow blunt trauma, such as the shoulder being forced suddenly downward³. Injury can also follow repeated movements in sport, or in tasks such as digging and hedge-clipping³.

How trauma injures the nerve is not settled³. In one cadaver study, tilting and turning the head away while raising the arm doubled the nerve’s length³. That supports the idea that stretch does the damage, though at least one pair of authors was not convinced³.

One study of 92 people with this paralysis recorded what came first¹¹:

Share of cases (%). Bars start at zero; the table below gives each value with its source.

What came before it	Share of cases
Exertion	40%¹¹
Trauma	24%¹¹
Infection	15%¹¹
Surgery or anaesthesia	11%¹¹
No clear cause	10%¹¹

Fatigue and shoulder pain

In one study of 30 people, tiring the serratus anterior changed how the shoulder blade moved as the arm rose¹³. It tilted back less and turned inward more, but its upward turn did not change¹³.

A systematic review looked at 12 EMG studies that compared people with shoulder problems against control groups¹⁴. Shoulder impingement syndrome is a pinching of tissue in the space beneath the bony point at the top of the shoulder. In 3 of 5 studies of it, serratus activity was lower than in the controls¹⁴. Results in unstable shoulder joints were contradictory¹⁴.

Recovery

How paralysis tends to recover

A systematic review looked at six studies of 234 shoulders with this paralysis managed without surgery⁴. Across these studies, winging recovered on its own in 21% to 78%, with a median follow-up of 72 months⁴. It concluded that a substantial share of people with nerve-related winging managed without surgery seem to have lasting complaints⁴.

A different review, of scapular winging, states that most cases of serratus anterior paralysis clear up on their own within 24 months³. One long-term study of 37 people with this paralysis found a mixed picture¹⁵:

Over 2 to 31 years of observation alone, winging lasted 16 months on average and went away in 78%¹⁵.
Raising the arm forward returned to normal in 82%, and raising it to the side in 89%¹⁵.
Symptoms mostly recovered within 2 years, but at least a quarter had lasting symptoms, mainly pain¹⁵.
30% of that group were free of pain¹⁵.

What the cause changes

In 92 people with this paralysis, followed for 18 years on average, recovery differed by cause¹¹. Paralysis after infection recovered better, and paralysis after surgery or anaesthesia recovered worse¹¹. Cases after trauma and after overuse or exertion followed the same course¹¹.

A Mayo Clinic study followed 106 people for about four years¹⁶. Of these, 50 had damage to this nerve and 56 had damage to the spinal accessory nerve, which works the trapezius^16,17. Across the whole group, good recovery of function was generally seen, whatever their nerve tests showed¹⁶. Injuries from trauma generally did worse than those from other causes¹⁶. This result is about function, while the systematic review above looked at winging and lasting complaints^4,16.

Connections

The trapezius

The serratus anterior and the trapezius are the primary force couple that turns the shoulder blade upward as the arm rises². A force couple is two muscles pulling in different directions that together turn a bone. The serratus anterior’s leading role comes from its good leverage and its many fibres meeting at the lower corner².

The middle trapezius holds back the serratus anterior’s strong forward pull². Without enough of it, the shoulder blade usually drifts far forward as the arm rises².

Other shoulder-blade muscles

The levator scapulae runs from the neck to the top of the shoulder blade. The pectoralis minor is a smaller chest muscle beneath the pectoralis major. When the trunk rises in a push-up, both help the trapezius and rhomboids steady the shoulder blade². As the arm rises, the rhomboids and levator scapulae are less active, and are thought to help resist the serratus anterior’s sideways pull⁸.

The deltoid and rotator cuff

Muscles that position the shoulder girdle, including this one, give the deltoid a steady base to move the arm from⁶. The shoulder girdle is the shoulder blades and collarbones, and the deltoid is the rounded muscle that caps the shoulder. As the arm rises, the middle deltoid and the supraspinatus, a muscle along the top of the shoulder blade, pull it toward a downward turn². The upward turning force from the serratus anterior and trapezius must be greater².

The rotator cuff is a group of four muscles that cross the shoulder joint and help hold it steady. A shoulder blade out of place can change their line of pull².

Exercises that load it

Related exercise pages: plank, shrug, overhead press and band external rotation^19,20,21,22.

Exercise or study	What was found
10 exercises, 30 healthy people	Activity was highest in exercises that turned the shoulder blade far upward²³
Push-up plus, 30 people, healthy or with mild shoulder problems	The standard version drew the most activity of the versions tested, up to 123% of maximum²⁴
Floor and suspended planks, 21 people	Activity peaked in the normal floor plank and the feet-suspended plank¹⁹
Shrug, 20 people whose shoulder blades turn downward	Activity was greatest with the arms raised 150 degrees to the side²⁰
Overhead press, 12 men lifting half their maximum	An unsteady bar with kettlebells hung from bands raised activity above a steady barbell²¹
Band external rotation, 13 healthy men	Activity rose with the arm held 90 degrees out to the side²²