Pectoralis Major Muscle Anatomy

The pectoralis major stands as one of the most prominent muscles in the anterior thoracic region, playing a crucial role in upper extremity movement and thoracic wall stability. Medical professionals must possess a thorough understanding of this muscle’s complex anatomy to effectively diagnose and treat related pathologies.

Pectoralis Major Muscle Anatomy

The pectoralis major is a large, fan-shaped muscle that covers the superior part of the thorax. It has clavicular and sternocostal heads. The sternocostal head is much larger, and its lateral border forms the muscular mass that makes up most of the anterior wall of the axilla. Its inferior border forms the anterior axillary fold.

The pectoralis major and adjacent deltoid muscles form the narrow deltopectoral groove, in which the cephalic vein runs; however, the muscles diverge slightly from each other superiorly and, along with the clavicle, form the clavipectoral (deltopectoral) triangle.

Origin and Insertion

The pectoralis major demonstrates a distinctive fan-shaped structure with multiple points of origin. The muscle originates from three primary regions: the clavicular portion arises from the medial half of the clavicle, the sternocostal portion from the anterior surface of the sternum and the first six costal cartilages, and the aponeurosis of external oblique muscle. These distinct origins contribute to the muscle’s complex functional capabilities.

The insertion of the pectoralis major occurs at the lateral lip of the bicipital groove of the humerus. The muscle’s insertion demonstrates a unique twisted configuration, where the clavicular fibers insert anteriorly and inferiorly, while the sternocostal and abdominal fibers insert posteriorly and superiorly. This anatomical arrangement creates a distinctive bilaminar tendon, crucial for understanding injury patterns and surgical repair techniques.

Neurovascular Supply

The pectoralis major receives its primary innervation from the lateral and medial pectoral nerves, derived from the brachial plexus (C5-T1). The lateral pectoral nerve (C5-C7) primarily supplies the clavicular head, while the medial pectoral nerve (C8-T1) innervates the sternocostal portion. This dual innervation pattern becomes particularly relevant in cases of neurological injury or surgical planning.

See Also: Brachial Plexus Anatomy

The muscle’s blood supply primarily comes from branches of the thoracoacromial artery, with additional contributions from the lateral thoracic artery and anterior intercostal arteries. Understanding this vascular network is essential for surgical approaches and maintaining tissue viability during reconstructive procedures.

Functional Anatomy and Biomechanics

The pectoralis major’s complex fiber arrangement enables it to perform multiple movements at the glenohumeral joint. The clavicular head primarily functions in forward flexion and internal rotation of the humerus, while the sternocostal and abdominal portions are powerful adductors and internal rotators. When acting together, all portions contribute to horizontal adduction of the arm, a movement crucial in many daily activities and athletic performances.

See Also: Shoulder Range of Motion

In the context of respiration, the pectoralis major serves as an accessory muscle of inspiration when the upper extremities are fixed. This function becomes particularly relevant in patients with respiratory distress or neuromuscular conditions affecting primary respiratory muscles.

Clinical Significance

Understanding the pectoralis major’s anatomy is crucial for accurate diagnosis and treatment of various pathologies. Ruptures commonly occur at the musculotendinous junction or the humeral insertion, with characteristic patterns based on the anatomical arrangement of muscle fibers. The most frequent mechanism involves eccentric loading during chest press exercises or contact sports.

Surgical repair techniques must account for the muscle’s complex fiber arrangement and the twisted nature of its insertion. The success of repair procedures largely depends on accurate anatomical restoration of the bilaminar tendon configuration.

The muscle’s relationship with surrounding structures, including the axillary region and neurovascular bundles, makes it an important surgical landmark. Knowledge of these relationships is essential for procedures ranging from mastectomies to thoracic outlet decompression.

To test the clavicular head of pectoralis major, the arm is abducted 90°; the individual then moves the arm anteriorly against resistance. If acting normally, the clavicular head can be seen and palpated. To test the sternocostal head of pectoralis major, the arm is abducted 60° and then adducted against resistance. If acting normally, the sternocostal head can be seen and palpated.

References & More

1. Gray’s Anatomy, 42nd Edition – https://www.elsevier.com/books/grays-anatomy/standring/978-0-7020-7710-5
2. Clinically Oriented Anatomy, 8th Edition – https://www.lww.com/moore-clinically-oriented-anatomy
3. Solari F, Burns B. Anatomy, Thorax, Pectoralis Major Major. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK525991/