Serratus Anterior Muscle Anatomy

The serratus anterior muscle, often referred to as the “boxer’s muscle,” plays a crucial role in shoulder biomechanics and scapular stability. This comprehensive examination explores the detailed anatomy, function, and clinical significance of the serratus anterior, providing medical professionals with essential information for diagnosing and treating conditions related to this important muscle.

Serratus Anterior Muscle Anatomy

The serratus anterior muscle is a broad, fan-shaped muscle located on the lateral aspect of the thorax, between the ribs and the scapula. This muscle has a distinctive appearance that resembles a serrated knife edge, which contributes to its nomenclature.

Origin & Insertion

The serratus anterior muscle originates from the superolateral surfaces of the upper eight or nine ribs at the side of the chest, with each digitation forming a muscular slip. These digitations interdigitate with the external oblique muscle of the abdomen in the lower portion. The muscle fibers course posteriorly around the thoracic wall and insert along the entire anterior surface of the vertebral border of the scapula. Specifically, the upper digitations insert on the superior angle, while the lower digitations attach to the inferior angle of the scapula.

See Also: Scapula Anatomy

Neurovascular Supply

The serratus anterior muscle receives its innervation exclusively from the long thoracic nerve, which originates from the anterior rami of the fifth, sixth, and seventh cervical spinal nerves (C5, C6, C7). This nerve courses inferiorly along the lateral thoracic wall, making it vulnerable to injury during certain surgical procedures or traumatic events. The arterial supply to the serratus anterior muscle primarily comes from the circumflex scapular artery, a branch of the subscapular artery, which itself branches from the axillary artery. Additional vascular contributions may come from the lateral thoracic artery and thoracodorsal artery.

Functional Significance

The serratus anterior muscle serves multiple essential functions in shoulder biomechanics. Its primary actions include drawing the scapula forward (protraction) and upward, while simultaneously rotating the inferior angle laterally and upward. This rotation positions the glenoid cavity optimally for overhead arm movements. The muscle plays a critical role in scapular stabilization during arm elevation, preventing scapular winging and maintaining proper scapulothoracic rhythm. During pushing movements, the serratus anterior contracts to stabilize the vertebral border of the scapula against the thoracic wall, enabling effective force transfer from the trunk to the upper extremity.

In overhead activities such as throwing or reaching, the serratus anterior muscle works synergistically with the trapezius muscle to upwardly rotate the scapula. This coordinated action is essential for full arm elevation and for maintaining the subacromial space, thereby preventing impingement of subacromial structures. The serratus anterior is also active during forced inspiration, where it can elevate the ribs and assist in expanding the thoracic cavity.

Clinical Considerations

Dysfunction of the serratus anterior muscle, often due to injury or compression of the long thoracic nerve, leads to a condition known as scapular winging (winged scapula). This pathology is characterized by prominence of the vertebral border of the scapula away from the thoracic wall, particularly evident when the patient pushes against resistance. Scapular winging can significantly impair shoulder function and lead to secondary pain syndromes. The long thoracic nerve’s superficial course makes it susceptible to injury during surgical procedures involving the axilla, such as mastectomy or axillary lymph node dissection.

See Also: Serratus Anterior Muscle Pain Causes, Symptoms, & Treatment

Medical professionals should be aware that serratus anterior weakness can contribute to shoulder impingement syndrome by disrupting normal scapulohumeral rhythm. The resultant altered biomechanics can lead to excessive stress on rotator cuff tendons and the subacromial bursa. Physical examination of patients with suspected serratus anterior muscle dysfunction should include the wall push-up test, where a patient is observed pushing against a wall while the examiner evaluates for scapular winging.

Rehabilitation of serratus anterior muscle weakness typically involves progressive strengthening exercises, beginning with scapular protraction activities in supported positions and advancing to more functional movements. Neuromuscular electrical stimulation may be employed as an adjunct to facilitate muscle activation in cases of significant weakness. Persistent scapular winging due to long thoracic nerve palsy that does not respond to conservative management may require surgical intervention, such as muscle transfers or scapulothoracic fusion in severe cases.

Anatomical Variations and Developmental Aspects

Anatomical variations of the serratus anterior muscle are relatively uncommon but can include differences in the number of digitations, variations in attachment sites, or accessory muscle slips. The muscle develops from the hypaxial portions of the myotomes corresponding to its segmental innervation. Understanding these developmental aspects can provide insight into unusual presentations or anomalies encountered in clinical practice.

Imaging Considerations

On imaging studies, the serratus anterior muscle is best visualized on axial and sagittal MRI or CT scans of the thorax. The muscle appears as a thin, fan-shaped structure wrapping around the lateral thoracic wall. In cases of denervation, MRI may demonstrate muscle atrophy and fatty infiltration. Electromyography (EMG) can be valuable in confirming long thoracic nerve palsy and distinguishing it from other causes of scapular dyskinesis.

See Also: Scapular Dyskinesis

Conclusion

The serratus anterior muscle is an anatomically and functionally significant muscle that plays a vital role in shoulder biomechanics and stability. Its proper function is essential for normal scapulothoracic rhythm and overhead activities. Medical professionals should maintain a thorough understanding of serratus anterior anatomy, function, and pathology to effectively diagnose and manage conditions affecting this muscle. Recognizing the importance of this muscle in the broader context of shoulder function can guide more effective treatment approaches and improve patient outcomes.

Resources

1. Gray’s Anatomy: The Anatomical Basis of Clinical Practice – https://www.elsevier.com/books/grays-anatomy/standring/978-0-7020-5230-9
2. Journal of Shoulder and Elbow Surgery – https://www.jshoulderelbow.org/
3. Lung K, St Lucia K, Lui F. Anatomy, Thorax, Serratus Anterior Muscles. [Updated 2024 Sep 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK531457/