The underarm area, or axilla, is a complex underarm area. This space, between the arm and chest wall, serves as a passageway for nerves, vessels, and lymphatic structures, supported by muscles. Understanding these muscles is fundamental to upper body movement and well-being. They contribute to shoulder joint stability and mobility, as it is the most mobile joint.
Primary Muscles of the Underarm
Several muscles contribute to the underarm’s structure and function. The latissimus dorsi, a broad, flat muscle, originates from the spine and pelvis, inserting on the humerus in the axilla. Adjacent, the teres major is a thick, rounded muscle connecting the shoulder blade to the humerus, often called the “little lat” due to its similar actions and close relationship. These two muscles form a posterior boundary of the armpit.
On the anterior side, the lower fibers of the pectoralis major, a large chest muscle, contribute to the underarm’s structure extending towards the humerus. Deeper within the axilla, the coracobrachialis is a small muscle originating from the shoulder blade’s coracoid process, inserting on the middle of the humerus. The short head of the biceps brachii, one of two heads forming the muscle associated with arm flexion, originates from the coracoid process, passing through the armpit.
The long head of the triceps brachii, a large muscle on the back of the upper arm, traverses the underarm, originating from the shoulder blade below the shoulder joint. The subscapularis, part of the rotator cuff, lies on the anterior surface of the shoulder blade, passing beneath the shoulder joint towards its attachment on the humerus. The serratus anterior, wrapping around the rib cage, has a saw-toothed appearance, originating from the upper ribs and inserting onto the medial border of the shoulder blade, playing a role in scapular movement.
How These Muscles Function
Underarm muscles work in concert to facilitate a wide range of movements involving the arm, shoulder, and upper torso. The latissimus dorsi and teres major are primary movers for adduction (bringing the arm closer) and extension (moving the arm backward). They are powerful internal rotators of the humerus, important for actions like pulling a rope or performing a swimming stroke. These muscles provide power for downward and backward arm movements.
The coracobrachialis primarily assists in flexing and adducting the arm. Its position allows it to stabilize the shoulder joint, acting as a synergist to larger muscles during arm movements. The short head of the biceps brachii, known for elbow flexion, contributes to shoulder flexion and adduction, particularly when the arm is rotated externally. This dual function highlights the interconnectedness of upper limb musculature.
The long head of the triceps brachii, extending through the axilla, assists in extending the elbow and adducting the shoulder joint. This makes it active in pushing movements, such as pushing a door or performing a triceps dip. The subscapularis, a rotator cuff muscle, is a powerful internal rotator of the humerus, important for movements like reaching behind the back or serving in sports. It helps stabilize the humeral head within the shoulder socket, preventing dislocation.
The serratus anterior protracts the scapula, moving the shoulder blade forward from the spine. This action is important for pushing movements, such as a punch or pushing heavy objects. It helps upwardly rotate the scapula, allowing for overhead reaching and lifting, ensuring the shoulder blade moves smoothly with the arm.
Maintaining Underarm Muscle Health
Maintaining underarm muscle health is important for upper body function and preventing discomfort. Proper posture plays a role; slouching or rounded shoulders can stress these muscles and compromise their function. Being mindful of how you sit, stand, and carry objects can help distribute forces evenly across the shoulder girdle. This awareness supports the natural alignment of muscles and joints.
Regular, gentle stretching can help maintain flexibility and range of motion of these muscles. Simple stretches involving reaching overhead, across the body, or behind the back can promote suppleness and prevent stiffness. Incorporating basic strengthening exercises can build resilience in these muscles, supporting the shoulder joint and reducing injury risk. Activities that engage back, chest, and shoulder muscles in a balanced way are beneficial.
Muscle imbalances or strains can arise from repetitive movements or prolonged positions that overwork or weaken certain muscles. For example, prolonged desk work can lead to tightness in chest muscles and weakness in back muscles, affecting the underarm area. Paying attention to persistent aches or limitations in movement and adjusting daily habits can help prevent minor issues from becoming significant. Listening to your body’s signals is important for early intervention and maintaining muscle well-being.