The Pectoralis Major Muscle
The pectoralis major muscle is a large, broad, superficial muscle on the front of our chest. This muscle is actually a pair. We have a right and left pec major. Unlike some other muscles we’ve covered recently, you’ve probably heard of this muscle and have a general sense of where it’s located. That’s because these are muscles we train for many sports and athletic activities. Like most structures in our body, though, the pectoralis major muscle is more complex than we might imagine at first glance. Read on for a closer look at the attachments and actions of the pec major muscle.
Location
Most generally, our pectoralis major muscles are located on the upper part of the front of our chest. They are part of the shoulder complex and thoracic wall. More specifically, we can think of each side of the pectoralis major (right and left sides) as having three distinct parts based on the attachments and fiber directions.
Pectoralis major muscle attachments
One section of pec major originates on the medial half of the clavicle (collar bone). This is called the clavicular head of the muscle. A second part of this muscle originates on the sternum and costal cartilages 1-6. Remember that the costal cartilages are the cartilagenous area of the medial ribs. That portion is called the sternal head of the pec major muscle. A third portion of the pectoralis major originates on the anterior part of the connective tissue that forms part of the abdominal wall.
From its broad area of what we consider the origin, all the fibers come together. The myofascial bands of the pectoralis major are arranged like the overlapping blades of a fan. If our arm is at our side, all the sections of the muscle twist before inserting at the edge of the bicipital groove on the upper part of the humerus. This means that the fibers on the inferior section of the origin attach at the superior part of the insertion, and those fibers that originate at the superior part of the origin attach at the inferior part of the insertion.
Pectoralis major muscle actions
Because of their multiple attachments, the pec major muscles can affect multiple joints and directions of movement. The specific movements that this muscle contributes to depend on the starting position of the arm, shoulder, and torso. Actions of pectoralis major also change based on what parts of the body are fixed or held in some way, and which are available to move. As with many muscles, ideas of origin and insertion can easily be reversed here, depending on what attachments of pectoralis major are held in place and which are freely movable.
All fibers working together
Generally speaking, when we’re standing in anatomical position, all fiber sections of pectoralis major work together to do adduction, horizontal adduction (reaching your arm across your body), and medial (internal) rotation. Also, when we’re moving from anatomical position with our arms at our sides, the clavicular fibers assist with flexion of the arm at the shoulder.
Other actions
Because of the broad area of origin, this muscle can do two opposite actions. As I said, the clavicular section can flex the shoulder joint. However, when the shoulder and arm are fixed, pectoralis major contributes to movements in a different way. When the other end of the attachments, what we think of as the insertion, is held in place, the available movements are different. If our shoulder is already in a flexed position and has a resistance, and pectoralis major contracts, the inferior fibers of pec major (the sternal, costal, and abdominal fibers) assist with extension of the flexed arm at the shoulder joint. They assist with pulling it downward.
Assists with breathing
Additionally, because of its specific attachments, when the humerus is in a fixed position, pectoralis major assists with breathing as well. When the shoulder is fixed, pec major can assist with inspiration. So, interestingly, we can see that, like several other muscles we’ve explored with multiple fiber sections, pectoralis major can act as an antagonist to itself.
Conclusion
The pectoralis major muscle is an important muscle for many athletic activities, from weightlifting to swimming to throwing. Many people have a general idea where it’s located. However, its specific structure of multiple myofascial regions arranged in overlapping bands is less well-known. This arrangement contributes to the many different types of actions that this muscle assists with.
References
Gilroy, A.M., B.R. MacPherson, and J.C. Wikenheiser (Eds.) 2025. Atlas of Anatomy. (5th ed.) Thieme.
Travell, J.G. and D.G. Simons. 1999. Chapter 42: Pectoralis Major Muscle and the Subclavius. In Myofascial Pain and Dysfunction - The Trigger Point Manual. Volume 1 (2nd Edition). New York, NY: Lippincott, Williams, and Wilkins Publishing.