Anatomical Atlas of the Upper Limb Lesson Preview: The Pectoralis Major Muscle

By Hannah Friedman | July 8, 2025

A spotlight from the Anatomical Atlas of the Upper Limb Related to Neuromodulator Injections and Management of Spasticity

The Pectoralis Major muscle is a critical component of the anterior chest wall and plays an important role in upper limb mobility and function. As the superior and largest muscle of the anterior thorax, it is often considered in neuromodulator injection planning for patients with spasticity-related impairments affecting the shoulder and arm.

This lesson, featured in the Anatomical Atlas of the Upper Limb Related to Neuromodulator Injections and Management of Spasticity course on the Myoguide Academy, provides a detailed overview of the muscle’s anatomical structure, function, and clinical injection considerations.

Overview of the Pectoralis Major

The Pectoralis Major is a thick, fan-shaped muscle situated beneath the breast tissue. It contributes to the formation of the anterior axillary fold and defines the anterior wall of the axilla. This muscle is unique in both its structure and function.

Origin

The muscle originates from several anatomical structures:

  • Anterior surface of the medial half of the clavicle
  • Anterior surface of the sternum
  • First seven costal cartilages
  • Sternal end of the sixth rib
  • Aponeurosis of the External Oblique muscle of the anterior abdominal wall

Insertion

The muscle inserts on the lateral lip of the intertubercular sulcus of the humerus.

Structural Segmentation and Function

The Pectoralis Major has two distinct heads:

  • Clavicular head
  • Sternocostal head (comprised of 2 to 7 segments)

The function of the muscle varies depending on which head is engaged. Together, they act on the glenohumeral joint to allow:

  • Arm flexion (clavicular head, when the arm is extended)
  • Arm extension (sternocostal head, when the arm is flexed)
  • Adduction
  • Medial rotation

Due to its non-uniform fiber length, the muscle is capable of variable shortening velocities, contributing to increased power production during movement.

Injection Planning: Anatomical and Procedural Considerations

This lesson also covers the specifications relevant to therapeutic neuromodulator injection.

Patient Position

Sitting or supine, with the arm abducted to 45–90 degrees.

Muscle Activation

With the patient lying on their back, ask them to push up against resistance with the elbows bent at 90 degrees. This activates the Pectoralis Major for accurate localization.

Finding the Muscle

The Pectoralis Major forms the anterior axillary fold.

  • The sternocostal fibers define the anterior border of the axilla.
  • The clavicular fibers define the border of the infraclavicular fossa.
  • The muscle is easily palpable lateral and superior to the nipple and can be grasped for injection in the area of the anterior axillary line.

Needle Length

40 mm

Needle Insertion

Injection is typically performed in the anterior axillary fold, with needle orientation medial and in line with the direction of the muscle fibers. The number of injection points ranges from 1 to 3, depending on clinical need and muscle presentation.

Cautions

  • Excessive depth of injection may result in pneumothorax.
  • Deep or misplaced injections can puncture the Coracobrachialis muscle, Brachial Plexus, or surrounding vasculature.
  • Injections placed too medially risk entering the Biceps Brachii muscle.

Clinical Relevance

Understanding the structural complexity and functional roles of the Pectoralis Major is essential for effective treatment of upper limb spasticity. Injection of this muscle can:

  • Reduce muscle tone
  • Improve functional mobility of the shoulder and arm
  • Relieve discomfort associated with spastic muscle activity
  • Support rehabilitation efforts by promoting better positioning and limb control

This lesson provides essential clinical guidance for safe and effective injection of the Pectoralis Major, including positioning, palpation techniques, and safety considerations.

Explore the Full Course

This is one of many comprehensive lessons featured in the Anatomical Atlas of the Upper Limb Related to Neuromodulator Injections and Management of Spasticity—part of the Anatomical Atlas course series available through the Myoguide Academy.

Each course in the series offers:

  • Individualized, muscle-specific injection specifications
  • Anatomical visuals and references
  • Safety notes relevant to therapeutic neuromodulator use

Courses are $225 CAD for three months of access and can be found in the Myoguide Academy Course Catalog.

References

Delp, S. L., et al. (1990). An interactive graphics-based model of the lower extremity to study orthopedic surgical procedures. IEEE Transactions on Biomedical Engineering, 37(8), 757–767.

Duchen, L. W. (1995). Motor neuron disease and related disorders. Pathobiology of the Nervous and Muscular Systems. Springer.

Gracies, J. M. (2005). Pathophysiology of spastic paresis. I: Paresis and soft tissue changes. Muscle & Nerve, 31(5), 535-551.

Kendall, F. P., McCreary, E. K., Provance, P. G., Rodgers, M. M., & Romani, W. A. (2005). Muscles: Testing and Function, with Posture and Pain. Lippincott Williams & Wilkins.

Wissel, J., Ward, A. B., Ertzgaard, P., et al. (2009). European consensus table on the use of botulinum toxin type A in adult spasticity. Journal of Rehabilitation Medicine, 41(1), 13–25.