Dominic Stone
The pectoralis major is the largest muscle in the chest, lying underneath the breast. It is thick and fan-shaped, with two heads: the clavicular and the sternocostal. The pectoralis minor is situated beneath the pectoralis major and is an important chest muscle that assists in key functions. It is integral to shoulder movement, especially shoulder rotation. The serratus anterior is also known as the boxer muscle because it enables a long and strong reach. The subclavius is a small, triangle-shaped muscle that elevates the clavicle bone when we breathe in. These muscles are prone to painful tightness and injury, especially with overuse.
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Pectoralis Major
The pectoralis major is the 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 lies underneath the pectoralis major.
The pectoralis major has two heads: the clavicular and the sternocostal. The sternocostal head is the larger of the two and originates from the anterior surface of the manubrium and body of the sternum, the anterior surface of the superior six costal cartilages, and the superior part of the aponeurosis of the external oblique muscle. The clavicular head originates from the anterior surface of the medial half of the clavicle. The upper and lower fibres of the pectoralis major insert into the crest of the greater tubercle of the humerus. The two heads of the pectoralis major have different nerve supplies. The clavicular head derives its nerve supply from the lateral pectoral nerve, while the sternocostal head receives innervation from the C7, C8, and T1 nerve roots via the lower trunk of the brachial plexus and the medial pectoral nerve.
The pectoralis major is responsible for flexion, adduction, and internal rotation of the humerus, as well as keeping the arm attached to the trunk of the body. The clavicular part contributes to flexion, horizontal adduction, and inward rotation of the humerus, while the sternocostal part contributes to downward and forward movement of the arm and inward rotation when accompanied by adduction. The pectoralis major also assists in breathing when the diaphragm and intercostal muscles are insufficient, and it has been used in flap repairs in neck surgery due to its high vascularisation.
Poland syndrome is a rare congenital condition in which the pectoralis major muscle is completely missing, usually on one side of the body. This condition may be accompanied by the absence of the breast in females.
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Pectoralis Minor
The pectoralis minor is a triangularly-shaped muscle located in the pectoral region of the body. It lies underneath the pectoralis major, a larger muscle in the same region. Together, the pectoralis minor and pectoralis major form the anterior wall of the axilla. The pectoralis minor originates from the margins of the third to fifth ribs adjacent to the costochondral junction. The fibres of the pectoralis minor pass upward and laterally to insert into the medial border and superior surface of the coracoid process of the scapula.
The pectoralis minor is clinically important and is used as a surgical landmark. This is due to the structures that lie beneath the muscle and its tendon. The nerves and blood supply to the upper limb run deep into the pectoralis minor muscle. These include the posterior, lateral, and medial cords of the brachial plexus. The pectoralis minor also divides the axillary artery into three parts. The primary nerve supply to the pectoralis minor comes via the medial pectoral nerve, one of the minor branches of the brachial plexus that arises from the cervical portion of the spinal cord. The pectoralis minor also receives innervation from the lateral pectoral nerve, via a communicating branch known as the 'ansa pectoralis'.
The pectoralis minor is crucial in the stabilization of the scapula by pulling it downward and anteriorly against the thoracic wall. It also acts as an accessory muscle of inspiration. When the ribs are immobilized, the pectoralis minor brings the scapula forward. The pectoralis minor 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 pain and restriction in neck movements, stiffness and pain in the upper back, and an increased risk of rotator cuff pathologies. It can also cause thoracic outlet syndrome, where the muscle impinges on the brachial plexus and axillary blood vessels, leading to shoulder impingement.
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Serratus Anterior
The serratus anterior is 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 and inserts along the superior angle, medial border, and inferior angle of the scapula. Its main part lies deep under the scapula and the pectoral muscles. The serratus anterior is located more laterally in the chest wall and forms the medial border of the axilla region.
The serratus anterior is divided into three parts according to the points of insertion: serratus anterior superior (insertion near the superior angle), serratus anterior intermediate (insertion along the medial border), and serratus anterior inferior (insertion near the inferior angle). The serratus anterior is also known as the "boxer's muscle" because it is largely responsible for the protraction of the scapula, a movement that occurs when throwing a punch.
The serratus anterior acts in concert with the upper and lower fibres of the trapezius muscle to sustain upward rotation of the scapula, which allows for overhead lifting. It is a key scapular stabiliser, keeping the shoulder blades against the ribcage when at rest and during movement. The muscle is easily palpated between the pectoralis major and latissimus dorsi muscles.
The serratus anterior is a prime mover in both scapular protraction and scapular upward rotation. When the shoulder blade is in a fixed position, the serratus anterior lifts the rib cage and supports breathing. The most common causes of serratus muscle pain include tension, stress, and overuse. This pain may also result from serratus anterior myofascial pain syndrome (SAMPS), a rare myofascial pain syndrome.
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Subclavius
The pectoralis region of the upper limb contains three muscles: the pectoralis major, pectoralis minor, and the serratus anterior. The subclavius is a small, triangular muscle of the pectoral region that depresses the shoulder. It is located directly underneath the clavicle, running horizontally.
The subclavius originates from the junction of the first rib and its costal cartilage. It inserts onto the inferior surface of the middle third of the clavicle. The muscle's primary function is to actively stabilize the clavicle at the sternoclavicular joint during movements of the shoulder and arm. When the muscle contracts, it depresses the clavicle at its sternal end, elevating the first rib. This movement helps prevent dislocation of the clavicle at the sternoclavicular joint during certain activities, such as forced breathing during an asthma attack.
The subclavius also plays a role in protecting the underlying neurovascular structures. In the event of a clavicular fracture or other trauma, it shields the superior trunk of the brachial plexus and the subclavian blood vessels from potential damage. This protective function is particularly important for preventing injuries to these delicate structures.
The subclavius lies deep to the pectoralis major muscle, with the clavipectoral fascia surrounding it. This fascia is the connective tissue between the clavicle and the pectoralis minor muscle. The subclavius is a small muscle with a significant role in maintaining the stability of the clavicle and protecting the adjacent neurovascular structures during shoulder movements.
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Shoulder Stability
The pectoralis region contains three muscles: the pectoralis major, pectoralis minor, and the serratus anterior. These muscles form part of the anterior wall of the axilla region and help to stabilise the scapula.
- Stand facing a wall with your arms at right angles out in front. Your thumbs should be facing away from the wall, and your forearms should be aligned with it. Keep your shoulder blades back and gently squeeze the muscles in this area. Then, slowly raise both arms upwards, keeping your shoulder blades back and together. Slowly lower your arms to return to the starting position.
- Stand tall with your arms extended in front of you at chest height, holding a band in both hands with a bit of slack, and palms facing down. Keep your arms straight as you open your arms out to the sides, maintaining them at chest height and avoiding lifting your shoulders towards your ears.
- Stand in a doorway with your elbows bent and each forearm resting on one side of the door frame at about chest height. Step one foot through the doorway to move your hips and chest forward while your forearms remain in place. Focus on creating length through your chest and arms, then move your hips and chest back to relax out of the stretch.
It is important to note that you should not feel pain during these exercises. If you experience pain, stop the exercise and consult a healthcare professional.
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Frequently asked questions
The pectoral muscles are a group of skeletal muscles that connect the upper extremities to the anterior and lateral thoracic walls. They are responsible for a wide range of movements, including flexion, adduction, and internal rotation of the humerus, as well as stabilizing the scapula.
There are three types of pectoral muscles: pectoralis major, pectoralis minor, and serratus anterior.
The pectoralis major is the largest and most superior muscle of the anterior chest wall. It is thick and fan-shaped, lying underneath the breast tissue. It has two heads: the clavicular and the sternocostal.
The pectoralis minor is situated beneath the pectoralis major and assists in key functions, including shoulder movement and rotation. It is an important but problematic muscle due to its sensitive location near the rib cage, making it prone to painful tightness.
There are many exercises that focus on the pectoralis muscle, including pressing exercises such as the flat barbell bench press and flat dumbbell bench press.
