• Pectoralis major adducts and medially rotates the humerus.
• Composed of two segments: upper fibers (clavicular portion) and lower fibers (sternocostal portion). Each section can be tested separately.
• Important muscle to examine because its function is often maintained in even very severe brachial plexus injuries where it can act as a donor nerve for nerve transfer procedures.
Origin Fixed: Adducts and medially rotates the humerus. The upper fibers (clavicular portion) are responsible for horizontal adduction towards the opposite shoulder while maintaining slight medial rotation of the humerus. The lower fibers (sternal portion) are responsible for oblique adduction towards the opposite hip while medially rotating the humerus.
Insertion Fixed: May assist in elevating the thorax, as in forced inspiration.
PECTORALIS MAJOR (CLAVICULAR PORTION)
Palpation of the Upper Fibers: Inferior to the medial ⅓ of the clavicle.
Strength Testing of Upper Fibers: Position – the subject in supine with shoulder in 90º flexion, slight medial rotation, and elbow flexed. Stabilize – against the opposite shoulder. Resist – against horizontal adduction of the humerus towards the opposite shoulder (in the direction of horizontal abduction).
Possible Substitutions of Upper Fibers: Sternocostal fibers of pectoralis major, coracobrachialis, anterior deltoid, biceps brachii, or general trunk movement.
PECTORALIS MAJOR (STERNOCOSTAL PORTION)
Palpation of the Lower Fibers: In the fold of the anterior axilla or lateral to the midsternum.
Strength Testing of Lower Fibers: Position – the subject in supine with shoulder in approximately 45º flexion, slight medial rotation, and elbow flexed. Stabilize – against the opposite hip. Resist – against adduction of the arm obliquely toward the opposite iliac crest in a lateral and cranial direction. If the subject’s abdominal muscles are weak, the trunk may need to be stabilized.
Possible Substitutions of Lower Fibers: Clavicular portion of the pectoralis major, pectoralis minor, or latissimus dorsi.
• Clavicular Head: C5, C6, C7.
• Sternocostal Head: C6, C7, C8, T1.
• Clavicular Head: Lateral pectoral nerve.
• Sternocostal Head: Lateral and medial pectoral nerve.
• Innervation Route:
• Clavicular Head: C5, C6, C7 → anterior pectoral nerve → lateral pectoral nerve → clavicular head of pectoralis major branch.
• Sternocostal Head: C6, C7, C8, T1 → anterior pectoral nerve → medial pectoral nerve → sternocostal head of pectoralis major branch.
Note that pectoralis major heads could be cross innervated by the lateral and medial pectoral nerve, hence the variation and range of root origins for these nerves.
Origin of Upper Fibers (Clavicular): Anterior surface of sternal ½ of clavicle.
Insertion of Upper Fibers (Clavicular): Crest of greater tubercle of humerus. Insertions of upper fibers are more anterior and caudal on the crest than the insertion of lower fibers.
Origin of Lower Fibers (Sternocostal): Anterior surface of sternum, cartilages of superior six or seven ribs, and aponeurosis of external oblique.
Insertion of Lower Fibers (Sternocostal): Crest of greater tubercle of humerus. Insertions of lower fibers are more posterior and cranial than the insertion of lower fibers.