Pectoralis major

Function

A fan built from several parts

The pectoralis major is a large muscle shaped like a fan¹. One source describes it as typically having three parts: clavicular, sternocostal and abdominal¹. Another describes two heads, clavicular and sternocostal, named for where they start⁷. The clavicular part is usually set apart from the sternocostal part by a slight cleft¹.

It starts from the inner clavicle (collarbone), the sternum (breastbone) and other points⁷. These include the first seven costal cartilages, the strips of cartilage that join ribs to the breastbone⁷.

The fibres end on the humerus (upper arm bone), at the outer lip of a groove called the bicipital groove⁷. In one study of 80 muscles from 40 cadavers (bodies donated for study), this attachment was 43.9 to 83.2 mm wide¹. The average was 65.3 mm¹.

Engraving of the right chest and arm from the front. The pectoralis major fans from the clavicle and sternum and converges into a flat tendon that passes under the deltoid to attach to the upper humerus, beside the biceps brachii and coracobrachialis. — Pectoralis majorHenry Vandyke Carter, in Henry Gray, Anatomy of the Human Body (1918), Fig. 410. Public domain, via Wikimedia Commons. (source, Public domain)

In the same 80 muscles, the attachment had a front layer and a back layer¹. The clavicular part joined the front layer in every case, and the sternocostal and abdominal parts joined the back layer¹. Studies disagree here, though: one study of 14 donated arms found a single-layer attachment to bone, with no true tendon layers there⁸.

What it moves

The muscle is important for moving the arm, especially for adduction and for turning the arm inward¹. A study of 8 donated arms measured each muscle’s moment arm, meaning its leverage⁹. The upper fibres were among the best at flexion, which at the shoulder means raising the arm forward⁹. The middle and lower fibres were among the best adductors⁹.

One anatomist, cited by a study’s authors, describes the clavicular part as a recent addition in higher primates (anthropoids)¹. It is said to help steady the arm against the chest¹.

Different regions, different jobs

One study used electromyography (EMG, measuring a muscle’s activity from its electrical signals) in 29 healthy young women holding still efforts¹⁰. In some tasks, the middle sternocostal region was 12% to 108% more active than the clavicular and upper sternocostal regions¹⁰. These tasks were moving the arm backward, adduction with the arm turned outward, and turning the arm inward with it raised high¹⁰.

When the women raised the arm forward or swung it across the body, the three regions activated similarly at every force level¹⁰. The one exception was forward raising at 25% of greatest force, where the clavicular region was 21% more active than the upper sternocostal region¹⁰. The authors conclude that the regions may be specialised for different actions¹⁰.

Its nerves and blood supply

In the 80-muscle study, the lateral pectoral nerve was the main nerve to the muscle¹. In specimens stained to show nerve branches, it alone supplied the clavicular part, and its branches reached about the upper half of the sternocostal part¹. The medial pectoral nerve supplied the lower half and the abdominal part, when present¹. Intercostal nerves, which run between two ribs, also sent branches in every stained specimen¹.

A review and meta-analysis of pectoral nerve anatomy reports the split this way: upper parts from the lateral nerve, lower parts from the medial nerve¹¹. The medial nerve also supplies the pectoralis minor, a smaller chest muscle beneath the pectoralis major¹¹. It runs along the pectoralis minor’s lower border¹². In 50% to 100% of cases, at least some of its branches pass through that muscle¹¹.

In 11 healthy people, the zones where nerves enter the muscle lay in its inner part¹³. They sat about 20% to 40% of the way along the muscle-tendon unit, and moved by up to 13.3% as arm position changed¹³. A surgical case report describes its blood supply as mainly from one artery branch, with more from branches of two other arteries¹⁴.

Breakdown

Where injuries happen

An analysis of 365 published injury cases, from 1822 to 2010, found that 75% came from the last 20 years¹⁵. The authors of another study state that how often injuries occur differs between the clavicular, sternal and abdominal regions¹⁶. A system for sorting injuries, proposed from those 365 cases, groups them by location¹⁵:

at the muscle’s starting point or in the muscle belly, the thick middle part¹⁵
anywhere from the musculotendinous junction, where muscle fibres join their tendon, to the tendon’s attachment¹⁵
a bony avulsion, where the attachment pulls away from the bone¹⁵

In a pooled analysis of 112 reported cases, all in men, most tears were complete and at the attachment on the humerus¹⁷. A later systematic review of surgical repairs agrees, with complete tears at the attachment most common (75%)¹⁸. One centre, studying consecutive patients assessed for a tear, found otherwise¹⁹. There, 60.4% of tears were at the musculotendinous junction, 34.9% at the attachment and 4.7% in the belly¹⁹.

Who tears it, and how

In a systematic review of 330 surgically repaired tears, every patient was male, with an average age of 30.4 years¹⁸. Across the published repairs in another review, the bench press was the most common cause, in 49% of cases²⁰. In the 112 pooled cases, tears happened most often in sport, with the arm raised out to the side and turned outward¹⁷.

A review found few studies on injuries linked to the deadlift, and a pectoralis major tear was among those reported²¹.

When its nerves are hurt

A review reports that the pectoral nerves can be injured on their own in several ways¹¹:

a direct blow¹¹
squeezing by an enlarged muscle¹¹
surgery, such as breast surgery or removing lymph nodes from the armpit¹¹

How its shape varies

The muscle’s shape varies between people¹. In the 80-muscle study, the most common variation was a separate clavicular part¹. The clavicular part was underdeveloped in 2.5% of muscles and fused with the deltoid in 5%¹.

A rare condition present from birth

Poland’s syndrome is a rare condition, present from birth, in which the pectoralis major is missing²². Other differences of the chest wall and arm on the same side come with it²². A literature review reports that functional limits are minimal, so surgical correction is done mainly for appearance²².

Recovery

How it grows with training

Several trials measured muscle thickness, meaning how thick a muscle is from its surface to the bone beneath²³. Most found the muscle grew, but one short trial did not^3,6:

People studied	What they did	What changed
7 untrained young men²³	Bench press at 75% of their one-repetition maximum (1RM, the heaviest load they could lift once), 3 days a week, 24 weeks²³	Thickness rose significantly from week 1, strength from week 3²³
10 young men²⁴	Light bench press (30% of 1RM) twice a day for 2 weeks, with or without cuffs limiting blood flow to the arms²⁴	Thickness rose 16% with cuffs, and did not change without them²⁴
18 young men³	Bench press at 40% of 1RM, or push-ups set to the same load, twice a week, 8 weeks³	Thickness rose 18.3% with push-ups and 19.4% with the bench press³
15 untrained young men⁴	Pec deck, one set or three sets, 12 weeks⁴	Clavicular part 17–18% thicker, sternocostal part 21%, with no difference between one and three sets⁴
81 active people⁵	Stretching 15 minutes a day, 4 days a week, or weight training 3 days a week, 8 weeks⁵	Both groups grew thicker than controls, with no difference between them⁵
23 moderately trained men⁶	Push-ups or bench press, 3 days a week, 4 weeks⁶	Both gained 1RM strength, but neither showed a significant change in thickness⁶

How growth changes over months

In the 7 men who trained for 24 weeks, thickness and strength rose significantly in each of three training phases²³. The triceps grew thicker only in the first two phases²³. In another 24-week study of young men, cross-sectional area (the size of a slice across the muscle) rose gradually for 6 weeks²⁵. After that, the rate of increase gradually fell²⁵.

That study also had a second group among its 14 men²⁵. This group trained in three 6-week blocks, with 3-week breaks between them²⁵. Overall gains in size and strength were similar to those of the men who trained without breaks²⁵.

Where it grows

In 47 untrained men, flat, incline or mixed bench press once a week for 8 weeks changed thickness differently at only one of three sites²⁶. That was the upper site, which grew most in the incline group²⁶.

Connections

Its tendon and the deltoid

The muscle inserts on the humerus through the pectoralis major tendon^7,27. In an ultrasound study of 46 volunteers, the authors concluded that the muscle ends in two separate tendons²⁷. One came from the sternal and abdominal parts²⁷. The other emerged between the deltoid and the clavicular fibres, and merged with a tendon inside the deltoid²⁷.

The clavicular part may reach along the collarbone as far as the deltoid, the rounded muscle that caps the shoulder, and may fuse with it¹. The authors of one study describe the two clavicular parts as working together¹.

The latissimus dorsi and triceps

For pulling the arm in toward the body, this muscle and the latissimus dorsi (the lats) matter more than the triceps²⁸.

Exercises that load it

Related exercise pages are bench press, overhead press, band external rotation and dead hang^29,30,31,32.

Exercise	What studies found
Bench press	Counted as a main mover in the traditional model, with the front deltoid and triceps²⁹
Bench press	In a review of 14 EMG studies, it showed similar activity to the triceps, and both were more active than the front deltoid²⁹
Bench press	A review reports that its activity rises with heavier loads, and with fatigue²⁹
Bench press	In 30 trained adults, upper chest activity peaked at a 30° bench, and middle and lower activity was higher on a flat bench³³
Bench press	In 13 men, the clavicular part was more active than the sternocostal part on the incline, and the reverse on the flat bench²
Bench press	In 28 men, its activity was similar across three grip widths³⁴
Bench press and fly	In 12 college-age men and women, its activation did not differ significantly between barbell press, dumbbell press and dumbbell fly³⁵
Overhead press	In 14 trained men pressing seated, pressing in front of the head gave higher activity on the way up than pressing behind the neck³⁰
Band external rotation	In a 30-person study, the standing version with a towel roll drew its highest activity of five exercises³¹
Dead hang	In 11 male climbers, the authors classed it as a postural muscle³²