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The pectoralis muscle, commonly referred to as pecs, are a vital component of the upper body musculature. There are two types of pectoralis muscles: the pectoralis major and the pectoralis minor. The pectoralis major is the superior-most and largest muscle of the anterior chest wall. It is a thick, fan-shaped muscle that lies underneath the breast tissue and forms the anterior wall of the axilla. The pectoralis minor is a small, triangular muscle that lies beneath the pectoralis major. The pectoralis muscles play a crucial role in various movements, including pushing, lifting, and stabilizing the shoulders and arms.
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What You'll Learn
Pectoralis major
The pectoralis major (from Latin pectus 'breast') is a thick, fan-shaped or triangular convergent muscle of the human chest. It is the superior most and largest muscle of the anterior chest wall. It makes up the bulk of the chest muscles and lies under the breast. The pectoralis major arises from parts of the clavicle (collarbone) and sternum, costal cartilages of the true ribs, and the aponeurosis of the abdominal external oblique muscle; it inserts onto the lateral lip of the bicipital groove.
The pectoralis major has two heads: the clavicular and the sternocostal, which reference their area of origin. The sternocostal head is the larger of the two heads. The sternal head accounts for 80 per cent of the pec major's total size. The muscle receives dual motor innervation by the medial pectoral nerve and the lateral pectoral nerve, also known as the lateral anterior thoracic nerve. The sensory feedback from the pectoralis major follows the reverse path, returning via first-order neurons to the spinal nerves at C5, C6, C8, and T1 through the posterior rami.
The pectoralis major has four actions which are primarily responsible for movement of the shoulder joint. The first action is flexion of the humerus, as in throwing a ball underhand, and in lifting a child. The second action is adduction, or pulling the arms towards the midline of the body. The third action is internal rotation of the humerus. The fourth action is keeping the arm attached to the trunk of the body. The two parts of the muscle, the clavicular and sternocostal, are responsible for different actions. The clavicular part contributes to flexion, horizontal adduction, and inward rotation of the humerus. The sternocostal part contributes to downward and forward movement of the arm and inward rotation when accompanied by adduction.
The pectoralis major is active in deep or forced inspiration, but not expiration. When the diaphragm and intercostal muscles are insufficient for proper breathing, the pectoralis major can assist in inspiration. Pectoralis major tendon rupture is a rare shoulder injury, most commonly seen in weight lifters.
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Pectoralis minor
The pectoralis minor is a small, triangular muscle that lies beneath the pectoralis major. It originates from the margins of the third to fifth ribs and inserts into the scapula (shoulder blade). The pectoralis minor is important clinically and as a surgical landmark due to the structures that lie below or deep to the muscle and its tendon. The muscle is also used as a reference point for the three divisions of the axillary artery.
The pectoralis minor has several functions, mostly related to the movement of the scapula. It aids in drawing the shoulder forward and downward, and it is crucial in the stabilization of the scapula by pulling it downward and anteriorly against the thoracic wall. When the scapula is stabilized, the pectoralis minor acts as an accessory muscle of inspiration. The pectoralis minor also helps with breathing and can assist in pulling the trunk up when climbing.
The pectoralis minor is covered anteriorly by the clavipectoral fascia, and the medial pectoral nerve pierces the muscle and the fascia. The pectoralis minor attaches to the coracoid process, forming a "bridge" for structures passing into the upper limb from the thorax. The muscle creates a passage between the ribs for the transit of the brachial plexus and the subclavian artery and vein.
A shortened pectoralis minor can cause issues such as thoracic outlet syndrome and shoulder impingement. It can also entrap nerves in the armpit region, causing pain, numbness, and tingling down the arm and into the hand. Exercises that target the serratus anterior and minimize the work of the pectoralis minor can help address these issues.
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The subclavius muscle
The pectoralis major and the pectoralis minor are the two muscles on either side of the chest, commonly referred to as the pecs. The pectoralis major is the larger and more superficial of the two, spanning half of the upper chest and having attachment points on the sternum, ribs, clavicle, and humerus.
The pectoralis minor lies, for the most part, beneath the pectoralis major, arising from the middle ribs and inserting into the scapula or shoulder blade.
The main function of the subclavius is to stabilize the clavicle during movements of the shoulder and arm, as well as to prevent injuries to the adjacent neurovascular structures in the event of a fractured clavicle. The subclavius also helps to prevent dislocation of the clavicle at the sternoclavicular joint. Myofascial pain or muscle strain of the subclavius may result in pain along the clavicle, sternoclavicular joint, shoulder, or radiate into the arm.
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Serratus anterior
The pectoralis major and the pectoralis minor are the two muscles that make up the chest, or the "pecs". The pectoralis major is the larger and more superficial of the two, spanning half of the upper chest on either side of the body. It is a thick, fan-shaped or triangular convergent muscle, originating at the clavicle (collarbone), the sternum, the ribs, and a tendinous extension of the external oblique abdominal muscle. The pectoralis minor lies beneath the pectoralis major, arising from the middle ribs and inserting into the scapula (shoulder blade).
The serratus anterior is a muscle that is not part of the pectoralis group, but it does lie deep under the scapula and the pectoral muscles. It is also a fan-shaped muscle that originates on the superolateral surfaces of the first to eighth or ninth ribs at the lateral wall of the thorax. Its main actions are protraction and upward rotation of the scapulothoracic joint. It is also a key scapular stabiliser, keeping the shoulder blades against the ribcage when at rest and during movement. It is occasionally called the "big swing muscle" or "boxer's muscle" because it is largely responsible for the protraction of the scapula, or the pulling of the scapula forward and around the rib cage, which occurs when someone throws a punch.
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Pectoral fascia
The pectoral fascia is a thin layer of connective tissue, covering the surface of the pectoralis major muscle. It is attached to the clavicle and sternum, and continuous with the fascia of the shoulder, axilla, and thorax. The pectoral fascia is rich in elastic fibres and is made up of collagen fibres.
The fascia is continuous inferiorly with the fascia of the abdominal wall and extends laterally to be continuous with the fascia of the back. It is very thin over the upper part of the pectoralis major, but thicker in the interval between it and the latissimus dorsi. Here, it closes in the axillary space and forms the axillary fascia. The axillary fascia, or the suspensory ligament of the axilla, is the hollow of the armpit, seen when the arm is abducted.
The pectoral fascia is made up of two laminae that envelope the pectoralis major muscle. The superficial lamina is continuous with the superficial investing cervical fascia, while the deep lamina arises from the periosteum of the clavicle. The deep lamina attaches to the sternum, while the superficial lamina blends with the pectoral fascia from the contralateral side.
The pectoral fascia is continuous with the deltoid fascia and sends some fibrous expansions to the brachial fascia. It blends with the fibres of the rectus sheath. The fascia forms the roof of the infraclavicular fossa, the surface indentation immediately inferior to the clavicle, between the deltoid and the pectoralis major.
The fascia allows for the transmission of myofascial forces from the trunk to the limbs. It also allows the pectoralis major muscles on both sides of the body to modulate their contractions and generate balance in the trunk.
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Frequently asked questions
The pectoralis major (from Latin pectus 'breast') is a thick, fan-shaped or triangular convergent muscle of the human chest. It makes up the bulk of the chest muscles and lies under the breast.
The pectoralis major's primary functions are flexion, adduction, and internal rotation of the humerus.
The pectoralis major consists of two heads: the clavicular and the sternocostal, which reference their area of origin.
