The chest contains several layers of muscle, but two dominate: the pectoralis major and pectoralis minor. Together with a few supporting muscles, they control arm movement, stabilize your shoulder blades, and even help you breathe. Here’s how each one works and why it matters.
Pectoralis Major: The Largest Chest Muscle
The pectoralis major is the thick, fan-shaped muscle that forms the visible bulk of your chest. It has two distinct sections, called heads, that work together but can fire independently depending on the movement.
The clavicular head attaches along the inner half of your collarbone. It’s the upper portion of the chest and is primarily responsible for raising your arm forward from a lowered position. The sternocostal head is the larger lower section, anchoring to your breastbone, the cartilage of your first seven ribs, and connective tissue from the abdominal wall. This head pulls the arm downward when it’s already raised, like during the downward motion of a pull-up.
Both heads converge into a single tendon that attaches to the upper arm bone near the shoulder. Working together, they pull the arm across the body, rotate it inward, and drive pushing movements. Any time you push a door open, throw a ball, or press something overhead, your pectoralis major is doing most of the heavy lifting.
Pectoralis Minor: The Hidden Stabilizer
Sitting directly beneath the pectoralis major is a thinner, rectangular muscle called the pectoralis minor. It runs from the third through fifth ribs up to a bony hook on your shoulder blade called the coracoid process. Its main job is pulling the shoulder blade forward and downward against the rib cage, which stabilizes your shoulder during pushing and reaching movements.
The pectoralis minor plays an outsized role in posture. When this muscle gets chronically tight, often from prolonged sitting or hunching over a screen, it tilts the shoulder blade forward and pulls the shoulders into a rounded position. This pattern, known as rounded shoulder posture, is one of the most common postural issues. Over time it can shorten the pectoralis major and upper trapezius as well, while weakening the muscles between the shoulder blades. The result is a cascade of tension and pain that can affect the neck, jaw, upper back, and arms.
Serratus Anterior: The Boxer’s Muscle
The serratus anterior wraps around the side of your rib cage like fingers gripping a wall. It originates on the outer surfaces of the first through eighth or ninth ribs and inserts along the inner border of the shoulder blade. Its primary job is pulling the shoulder blade forward around the rib cage, a movement called protraction. This is the motion that extends your reach at the end of a punch, which earned it the nickname “boxer’s muscle.”
Beyond punching, the serratus anterior is essential for any overhead movement. It rotates the shoulder blade upward so you can raise your arm above your head without impingement. When it’s weak, the shoulder blade “wings” outward from the rib cage, limiting overhead strength and increasing injury risk.
Subclavius: A Small but Protective Muscle
The subclavius is a small muscle tucked beneath the collarbone, running from the first rib to a groove on the underside of the clavicle. For a long time it was thought to simply stabilize the joint where the collarbone meets the breastbone. More recent research suggests it does something more dynamic: it helps regulate how force transfers through the collarbone and into the shoulder during arm movements. It also works with the pectoralis minor and other nearby muscles to control the position of the shoulder blade relative to the rib cage. Despite its small size, it acts as a cushion that protects the blood vessels and nerves running beneath the collarbone during sudden impacts.
Intercostal Muscles: The Breathing Engine
Between each pair of ribs sit two thin sheets of muscle: the external intercostals and the internal intercostals. These aren’t muscles you’ll ever see in a mirror, but they power every breath you take.
The external intercostals are primarily inspiratory. They lift and expand the rib cage when you inhale, with the greatest effect in the upper ribs toward the back. The internal intercostals are more complex. The portion closest to the breastbone, called the parasternal intercostals, actually assists with inhalation by lifting the ribs. The rest of the internal intercostals, particularly in the lower rib spaces, compress the rib cage during forceful exhalation, like when you cough, sneeze, or blow out candles.
Their function shifts depending on location: upper and rear intercostals tend to favor inhalation, while lower and more forward-facing fibers lean toward exhalation. This gradient means the rib cage doesn’t move as a single unit but expands and contracts in a coordinated wave.
How Chest Muscles Get Injured
The pectoralis major is the chest muscle most vulnerable to tears, and the mechanism is remarkably specific. Injuries typically happen when the muscle is fully stretched and contracting hard at the same time. The classic scenario is the bottom of a bench press: the arm is extended back and rotated outward while pushing against heavy weight. The tendon where the muscle attaches to the upper arm bears the greatest load, and that’s where most tears occur.
Injuries are graded by severity. A contusion or minor muscle tear causes pain and bruising but the muscle still functions. A partial tear involves noticeable weakness and a visible change in the muscle’s shape. A complete tear, where the tendon fully detaches, produces a dramatic shift in the muscle’s appearance, often described as the chest “rolling up” toward the center. Complete tears at the tendon almost always require surgical repair to restore full strength.
An Extra Muscle Some People Have
About 6% of the adult population has a small accessory muscle called the sternalis, running vertically along the breastbone on top of the pectoralis major. Its prevalence varies by ethnicity: roughly 9% in East Asian populations and about 4% in those of European descent. The sternalis has no clear function and causes no symptoms. It’s most often discovered incidentally on a CT scan or mammogram, where it can occasionally be mistaken for a mass. If you’ve been told you have one, it’s a harmless anatomical variant and nothing to worry about.