The pectoralis major muscle adducts and medially rotates the humerus.  The upper fibers flex and horizontally adduct the shoulder.  The lower fibers depress the shoulder girdle through their attachments on the humerus.  A common clinical finding is that the fibers forming the sternal portion of the pec major test short, while the fibers forming the clavicular portion test long. 

Actions

Muscle as a whole:

• Adducts the shoulder (glenohumeral joint)
• Medially rotates the shoulder (g/h joint)
• May assist in elevating the thorax in forced inhalation (if arm is fixed)

Upper Fibers:

• Flex the shoulder (g/h joint)
• Horizontally adduct the shoulder (g/h joint)

Lower Fibers:

• Extend the shoulder (g/h joint)

Origin

• Medial half of the clavicle, sternum, and cartilage of ribs 1-6

Insertion

• Crest of greater tubercle of humerus

Nerve Innervation

• Medial pectoral nerve and the lateral pectoral nerve (also known as the lateral anterior thoracic nerve).