What Muscles Are Under Your Armpit?

The armpit, known in anatomy as the axilla, is a pyramid-shaped space formed by muscles on all sides, packed with nerves, blood vessels, and lymph nodes. At least six named muscles contribute to its walls or pass directly through it, including the pectoralis major and minor in front, the latissimus dorsi and teres major behind, the serratus anterior along the ribcage, and the coracobrachialis running through the middle. Understanding which muscles live here and what they do helps make sense of everything from shoulder pain to post-surgical complications, and the anatomy turns out to be more variable from person to person than most people expect.

The Muscles That Form the Armpit’s Walls

Think of the armpit as a small room with walls, a floor, and a ceiling. Each wall is built from muscle and connective tissue, and different muscles make up different surfaces. The front wall is dominated by the pectoralis major, the large fan-shaped chest muscle you can feel when you press your palm against a doorframe and push. Beneath it sits the pectoralis minor, a smaller triangular muscle that runs from your third through fifth ribs up to a bony hook on your shoulder blade called the coracoid process. The pec minor is a key landmark because major nerves and blood vessels pass directly underneath it on their way into the arm.

The back wall is formed by the subscapularis (a rotator cuff muscle plastered to the front surface of your shoulder blade), the latissimus dorsi, and the teres major. The latissimus dorsi is the broadest muscle in the body, sweeping from your lower back and pelvis up to a narrow tendon that attaches to the upper arm bone. The teres major runs alongside it and assists with the same movements, pulling the arm downward and rotating it inward.¹ Together, these two muscles form a thick fold of tissue you can grab at the back of your armpit.

The inner (medial) wall is the serratus anterior, a muscle that hugs the ribcage and attaches along the inner border of the shoulder blade. Its middle and lower portions are the main drivers of upward rotation when you raise your arm overhead, and they also tilt the shoulder blade backward and hold it flat against the ribs.² The outer (lateral) wall is less muscular and is formed mainly by the upper arm bone itself and the coracobrachialis and biceps muscles that cling to it.

Muscles Passing Through the Space

Two muscles don’t just form walls of the armpit but actually travel through it. The coracobrachialis is a slender muscle originating from the coracoid process of the shoulder blade and inserting about halfway down the inner surface of the upper arm bone. It shares its starting point with the short head of the biceps brachii; in fact, cadaver studies show that in about half of people, these two muscles arise from a single shared tendon before separating.³ The coracobrachialis helps bring your arm forward and toward your body, and it serves as an important anatomical landmark because the musculocutaneous nerve pierces directly through it.

The short head of the biceps runs a similar path, descending from the coracoid process through the armpit and down the front of the arm to join the long head of the biceps. You can sometimes feel its tendon as a firm cord in the inner part of your armpit when your arm is outstretched. Injuries to both the coracobrachialis and short head of the biceps at once are rare but do happen, typically from sudden forceful extension of the arm.⁴

What These Muscles Do Together

The armpit muscles don’t work in isolation. They form overlapping teams that control shoulder movement and stability. When you pull something toward your body, the pectoralis major, latissimus dorsi, and teres major all fire together. The latissimus dorsi extends, adducts, and medially rotates the humerus, and the teres major works alongside it in those same movements.¹ Raising your arm overhead requires the serratus anterior to rotate the shoulder blade upward while the pec minor must lengthen to get out of the way. When the pec minor is too tight or overactive, it can pull the shoulder blade into positions that oppose normal overhead reaching and contribute to impingement symptoms.²

The subscapularis, though often discussed as a rotator cuff muscle, also plays a role in the armpit space. It fills the entire front surface of the shoulder blade and its tendon crosses through the deep part of the axilla to reach the upper arm bone. It is the most powerful internal rotator of the shoulder and acts as a dynamic stabilizer, keeping the ball of the humerus centered in its socket during arm movements.

The Axillary Arch, a Muscle Some People Have and Most Don’t

About 9% of the Central European population has a bonus muscle strip in the armpit called the axillary arch, also known as Langer’s muscle. It is a thin band of tissue, usually muscular, that bridges from the latissimus dorsi across the armpit to insert into the pectoralis major or nearby structures.⁵ In cadaver studies, these arches averaged about 80 mm long and roughly 7 mm wide, though their thickness varied considerably.

The axillary arch matters clinically because it crosses right over the neurovascular bundle, the package of nerves and blood vessels running through the armpit. When researchers passively moved the arm into abduction and external rotation (the position you’d assume when throwing a ball or reaching behind your head), roughly two-thirds of these arches made direct contact with the underlying neurovascular bundle.⁵ This means a person with a thick axillary arch could experience nerve tingling, numbness, or vascular symptoms in specific arm positions without any underlying disease. About half of people who have one have it on both sides.

Surgeons operating in the armpit, whether for breast cancer lymph node removal, shoulder repair, or cosmetic procedures, occasionally encounter the axillary arch unexpectedly. If not recognized, it can be confused with a displaced nerve or abnormal tissue. In one case report, the variant muscle was found crossing the axilla superficial to the median nerve, spanning about 90 mm from the latissimus dorsi to the pectoralis major.⁶

Nerves and Blood Vessels Sharing the Space

The muscles under your armpit don’t just move your arm. They also serve as a protective housing and passageway for the brachial plexus, a network of nerves that powers nearly all movement and sensation in the arm and hand. The axillary artery and vein run alongside these nerves, and the whole bundle threads its way between the muscles of the armpit on its journey from the neck to the fingertips.

The long thoracic nerve is especially vulnerable in this region. It runs along the surface of the serratus anterior, and damage to it causes the serratus to weaken or shut down entirely. The result is a “winged scapula,” where the shoulder blade sticks out from the back like a small wing. This is the most common cause of scapular winging, and it can happen from something as straightforward as carrying a heavy load pressed into the armpit.⁷ One or more branches of this nerve consistently supply the upper serratus anterior fibers, making it a single-nerve muscle with no backup supply.⁸

Variations in nerve layout are more common than most anatomy diagrams suggest. Every anatomical structure observed in the axilla shows some variation in at least one feature: its entry point, exit point, path, or branching pattern. Cadaver studies have found that the musculocutaneous nerve sometimes doesn’t pierce the coracobrachialis at all, instead running alongside it, and the median nerve can sit lateral to the axillary artery rather than in its more typical position.⁹ Lymph nodes are also concentrated here, which is why the armpit is such an important area in breast cancer staging and treatment.

When Armpit Muscles Cause Compression Problems

The tight quarters of the axilla mean that any muscular thickening, swelling, or postural change can squeeze the nerves and vessels passing through. Pectoralis minor syndrome is one of the better-known examples. It occurs when the brachial plexus nerves, axillary artery, and axillary vein get compressed beneath the pec minor muscle.¹⁰ This is classified as a subtype of thoracic outlet syndrome, where neurovascular compression happens at one of three spots: the space behind the pec minor, the gap between the collarbone and first rib, or the triangle formed by the scalene muscles in the neck.¹¹ Pectoralis minor syndrome and the related quadrilateral space syndrome are most often described in overhead-motion athletes, such as swimmers, volleyball players, and baseball pitchers.¹²

Symptoms can include pain radiating down the arm, numbness or tingling in the fingers, coldness in the hand, and sometimes visible color changes in the limb. These problems are frequently misdiagnosed because the armpit isn’t the first place clinicians think to look when a patient reports hand numbness or arm fatigue.

Axillary Web Syndrome After Surgery

People who undergo lymph node removal in the armpit for breast cancer sometimes develop a condition called axillary web syndrome, or AWS. It shows up as one or more tight, cord-like bands of tissue running from the armpit down the inner surface of the arm, sometimes extending past the elbow to the forearm or even the base of the thumb.¹³ These cords restrict arm movement and can be quite painful, especially when trying to raise the arm overhead.

The cause has been debated, but biopsy studies point toward a lymphatic origin. In patients biopsied four to five weeks after axillary node surgery, researchers found dilated lymphatic vessels with thickened walls, confirmed by specialized staining. In at least one case, the vessel’s lumen was completely blocked by an organized blood clot that was already beginning to recanalize, or form new channels through itself. This fits with the observation that AWS tends to resolve over time as lymphatic flow gradually restores.¹⁴ Other researchers, using high-frequency ultrasound, were unable to identify a specific structure responsible, which led them to suggest that the pathology might involve microlymphatic stasis or protein binding in the interstitial space, too fine to see on imaging.¹⁵ Either way, understanding that this condition involves the soft tissue and lymphatic channels running alongside the armpit muscles, rather than the muscles themselves, helps guide treatment, which typically includes gentle stretching and physical therapy.

Sports Injuries to Armpit Muscles

The pectoralis major is the armpit muscle most commonly torn during athletics. The typical mechanism is an eccentric contraction under heavy load, where the muscle is lengthening while trying to generate force. The bench press is the classic culprit: as the bar descends toward the chest, the pec major stretches under enormous weight, and if it fails, the tendon or muscle belly can rupture.¹⁶ A person with a pec major tear typically reports a sudden pop, immediate pain in the chest and armpit, bruising that spreads down the arm, and an obvious change in the shape of the chest muscle.

The latissimus dorsi and teres major are less frequently injured but are significant when they do tear, especially in overhead throwing athletes. These tears are most often seen in professional baseball pitchers, who generate extreme forces during the throwing motion.¹⁷ A systematic review of professional baseball pitchers with latissimus dorsi or teres major injuries identified 159 players, with an average age at injury of about 26 years. Most of these athletes, roughly 136 of 159, were managed nonoperatively, while 23 required surgery.¹⁸ Pain from these injuries is typically felt deep in the posterior armpit during the acceleration phase of the throw.

Winged Scapula and the Serratus Anterior

The serratus anterior deserves special attention because it’s both easy to injure and essential for overhead arm use. As mentioned earlier, its nerve supply runs exposed along the chest wall within the armpit, making it susceptible to stretch, compression, or direct trauma. Carrying a heavy bag with a strap pressing into the armpit is enough to damage the long thoracic nerve, and cases of scapular winging from exactly this mechanism have been documented with ultrasound showing changes in both the nerve and the muscle itself.⁷

When the serratus anterior stops working, the shoulder blade loses its ability to rotate upward and stay pressed flat against the rib cage. The result is that you can’t fully raise your arm, and pushing movements, like a wall push-up, cause the inner border of the shoulder blade to lift off the back. Recovery depends on the severity of nerve damage. Mild stretch injuries often resolve over months, but more severe cases can take a year or longer, and some never fully recover.

How Armpit Anatomy Varies Across Primates

The configuration of muscles around the armpit reflects evolutionary pressures related to how a species uses its arms. Comparative studies of humans, common chimpanzees, and bonobos reveal meaningful differences in the pectoral muscles specifically. In all six chimpanzees examined in one study, the pectoralis minor inserted at the supraspinatus tendon rather than the coracoid process, while bonobos and humans share the coracoid attachment.¹⁹ Some chimpanzees also retain a separate pectoralis abdominis muscle, a distinct belly at the lower margin of the pec major that is absent in most humans.

The clavicular portion of the pec major varies strikingly across primates. A significant clavicular attachment exists only in howler monkeys, woolly monkeys, gibbons, chimpanzees, gorillas, and humans. Interestingly, spider monkeys and orangutans, both highly arboreal species, lack this clavicular origin, which complicates any simple story about the attachment being an adaptation for climbing.²⁰ In humans, the clavicular head of the pec major is the part you use most during forward arm movements and overhead reaching. Its evolutionary origin may have more to do with manipulative versatility than with swinging through trees.