The armpit, or axilla, is an anatomical space situated beneath the glenohumeral (shoulder) joint. This region serves as a critical junction, creating a pathway that connects the neck and torso to the upper limb. The boundaries of the axilla are formed by a network of bone, muscle, vessels, and nerves. Understanding the arrangement of these muscles is central to comprehending upper body movement and stability.
The Musculature of the Anterior Axilla
The front border of the axilla is primarily defined by the two pectoral muscles, the Pectoralis Major and the Pectoralis Minor. The Pectoralis Major is a large, fan-shaped muscle that spans the upper chest, originating from the clavicle, sternum, and upper ribs. This muscle inserts onto the humerus, the upper arm bone. Its main actions are powerful movements of the shoulder joint, specifically adduction (bringing the arm toward the body) and internal rotation.
Lying beneath the Major is the Pectoralis Minor, a smaller, triangular muscle that does not attach to the arm bone itself. This muscle originates from the third through fifth ribs and inserts onto the coracoid process of the scapula (shoulder blade). The Pectoralis Minor assists in stabilizing the shoulder blade and contributes to protraction, which is the movement of pulling the shoulder forward across the rib cage.
The Musculature of the Posterior Axilla
The rear boundary of the armpit is formed by a robust group of muscles, including the Latissimus Dorsi and the Teres Major. The Latissimus Dorsi is the broadest muscle of the back, spanning a large area from the lower spine, pelvis, and lower ribs. Its fibers sweep up and around the side of the torso, inserting into the front of the humerus. This extensive attachment makes it a powerful mover, primarily responsible for extension, adduction, and internal rotation of the arm.
The Teres Major muscle is often described as the “Lat’s little helper” due to its similar function. It originates from the lower angle of the scapula and travels alongside the Latissimus Dorsi to insert onto the humerus. Both the Latissimus Dorsi and Teres Major are involved in pulling movements, such as those performed during a chin-up or rowing exercise. Their combined mass defines the posterior wall of the axilla.
Stabilizing Muscles of the Thoracic Wall
The muscles that stabilize the shoulder blade against the rib cage are distinct from the large movers, most notably the Serratus Anterior. This fan-shaped muscle originates from the upper eight or nine ribs and inserts along the inner edge of the scapula. The Serratus Anterior is sometimes called the “boxer’s muscle” because its primary action is scapular protraction, the forward-reaching motion of a punch.
The function of the Serratus Anterior is necessary for the smooth, coordinated movement of the arm when lifting overhead. It holds the scapula firmly against the chest wall, preventing the shoulder blade from “winging” or lifting away from the ribs during movement. Surrounding the ribs are the intercostal muscles, which are involved in breathing mechanics. The Serratus Anterior forms the most prominent muscular component of the medial wall of the armpit.
Understanding Common Sources of Pain and Discomfort
Pain originating in the armpit, known as axillary pain, frequently stems from the muscular and neural structures that pass through this confined space. Muscle strain is a common source of discomfort, particularly involving the Pectoralis Major or the Latissimus Dorsi after intense upper body exercises. These strains involve overstretching or tearing of the muscle fibers, causing localized pain that worsens with movement.
The armpit is also a major conduit for nerves, including the brachial plexus, which supplies the entire arm and hand. Compression or irritation of these nerves, sometimes associated with conditions like Thoracic Outlet Syndrome, can lead to referred pain, tingling, or numbness that is felt in the axilla and down the arm.
Lymph nodes are a different, non-muscular source of armpit pain. Swollen lymph nodes, often a sign of infection or inflammation elsewhere in the body, can become tender and cause discomfort. Persistent or severe pain in this region warrants consultation with a medical professional to accurately determine the underlying cause.