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The armpit, also known as the axilla, is an area on the human body directly under the shoulder joint. Several muscles run under the armpit, including the pectoralis minor, serratus anterior, latissimus dorsi, teres major, and subscapularis. These muscles are important for the function of the shoulders and arms, and their proper utilisation can help prevent overuse and injury to the neck and front of the shoulders.
| Characteristics | Values |
|---|---|
| Area | Directly under the shoulder joint |
| Includes | Axillary space, an anatomical space within the shoulder girdle between the arm and the thoracic cage |
| Bounded superiorly | Plane between the superior borders of the first rib, clavicle and scapula |
| Medially bounded | Serratus anterior muscle and thoracolumbar fascia |
| Anteriorly bounded | Pectoral muscles |
| Posteriorly bounded | Subscapularis, teres major and latissimus dorsi muscle |
| Contains | Axillary vein and artery, brachial plexus, lymph nodes and fat |
| Muscle assisting shoulder and arm function | Serratus anterior |
| Pectoralis muscle | Comprised of pectoralis major and pectoralis minor muscles |
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What You'll Learn
The pectoralis major and minor muscles
The pectoralis major and pectoralis minor are two of the four muscles in the pectoral region, the others being the serratus anterior and the subclavius. The pectoralis major is the largest and most superficial muscle in the pectoral region, lying underneath the breast tissue. It is thick and fan-shaped, with two heads: the clavicular and the sternocostal. The clavicular head originates from the anterior surface of the medial clavicle, while the sternocostal head originates from the anterior surface of the sternum, the superior six costal cartilages, and the aponeurosis of the external oblique muscle. The pectoralis major is active during deep or forced inspiration, assisting in breathing. It can be tested by transversely adducting the arm at the glenohumeral joint against resistance.
The pectoralis minor lies underneath the pectoralis major and is much thinner and triangular in shape. It originates from the margins of the third to fifth ribs, adjacent to the costochondral junction, and occasionally from the second rib. The fibres pass upward and laterally to insert into the medial border and superior surface of the coracoid process. The pectoralis minor is crucial in stabilising the scapula by pulling it downward and anteriorly against the thoracic wall.
The serratus anterior is located more laterally in the chest wall and forms the medial border of the axilla. It originates from the outer surface of the first to eighth ribs and inserts into the costal aspect of the medial border of the scapula. It rotates the scapula, allowing the arm to be raised over 90 degrees. The subclavius is a small muscle located directly underneath the clavicle, running horizontally. It affords minor protection to the underlying neurovascular structures.
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Serratus anterior muscle
The serratus anterior muscle is a fan-shaped muscle that originates on the superolateral surfaces of the first to eighth or ninth ribs at the lateral wall of the thorax. Its main part lies deep under the scapula and the pectoral muscles. It acts on the scapula and is the prime mover in both scapular protraction and scapular upward rotation. It is a key scapular stabiliser, keeping the shoulder blades against the ribcage when at rest and during movement.
The serratus anterior is also referred to as the "big swing muscle" or the "'boxer's muscle'" because it is largely responsible for the protraction of the scapula, or the pulling of the scapula forward and around the rib cage that occurs when someone throws a punch. The muscle is divided into three parts according to the points of insertion: the serratus anterior superior is inserted near the superior angle; the serratus anterior intermediate is inserted along the medial border; and the serratus anterior inferior is inserted near the inferior angle.
The serratus anterior is innervated by the long thoracic nerve, a branch of the brachial plexus. The long thoracic nerve travels inferiorly on the surface of the serratus anterior muscle. The serratus anterior lies deep to the subscapularis, from which it is separated by the subscapularis (supraserratus) bursa. It is separated from the rib by the scapulothoracic (infraserratus) bursa. All three parts of the serratus anterior pull the scapula forward around the thorax, which is essential for the anteversion of the arm. As such, the muscle is an antagonist to the rhomboids.
The serratus anterior is easily palpable between the pectoralis major and latissimus dorsi muscles. In athletic bodies, the muscle may be visible to the naked eye along the ribs underneath the axilla.
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Teres major and minor muscles
The teres major and teres minor are muscles of the shoulder joint. They are located in the axilla, or armpit, and are important for the function of the shoulders and arms.
The teres major is a thick muscle that runs along the lateral border of the scapula, or shoulder blade. It is one of the seven scapulohumeral muscles that act around the glenohumeral joint to facilitate shoulder movement. The muscle fibres converge onto a single tendon that inserts into the humerus, or upper arm bone. The main function of the teres major is to produce movements of the humerus at the glenohumeral joint. It pulls the anterior surface of the humerus medially towards the trunk (internal rotation) and can also extend the arm from the flexed position. The teres major is sometimes called "lat's little helper" because of its synergistic action with the latissimus dorsi muscle.
The teres minor arises from the posterior surface of the scapula, directly adjacent to the upper part of its lateral border. The upper fibres of the muscle culminate in a tendon that inserts onto the greater tubercle of the humerus. The lower fibres of the muscle insert directly into the humerus, just below the greater tubercle. The teres minor is deep to the deltoid muscle. As a rotator cuff muscle, the teres minor stabilises the ball-and-socket glenohumeral joint by helping to hold the humeral head (the ball) into the shallow glenoid cavity of the scapula (the socket).
The teres major and teres minor muscles are supplied by the subscapular artery, a branch of the axillary artery, and the posterior circumflex humeral artery. The teres minor is also supplied by the circumflex scapular artery, another branch of the subscapular artery. The teres major is innervated by the lower subscapular nerve, a branch of the brachial plexus, while the teres minor is innervated by the axillary nerve, which also arises from the brachial plexus.
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Latissimus dorsi
The latissimus dorsi muscle, commonly known as "lats", is a large, flat muscle that stretches across the back and sides, behind the arm. It is the widest muscle in the human body and is relatively thin, covering almost all back muscles at the posterior trunk, except the trapezius. The latissimus dorsi is considered an extrinsic muscle of the back, meaning it originates from the myogenic cells in the developing upper limb buds. Embryologically, it is closely related to the teres major muscle, as both originate from the pre-muscle sheath of the arm.
The latissimus dorsi is responsible for several functions, including extension, adduction, transverse extension (also known as horizontal abduction or horizontal extension), flexion from an extended position, and medial internal rotation of the shoulder joint. It also plays a role in the extension and lateral flexion of the lumbar spine, and assists in forced expiration and inspiration. When the arms are in a fixed overhead position, the latissimus dorsi pulls the trunk upward and forward, as in climbing or performing a chin-up.
The muscle is supplied by the thoracodorsal artery, a continuation of the subscapular artery, which is a branch of the third part of the axillary artery. Additionally, the latissimus dorsi receives blood supply from the dorsal perforating branches of the inferior three posterior intercostal arteries and the superior three lumbar arteries. The thoracodorsal nerve, a branch of the posterior cord of the brachial plexus, provides innervation to the muscle.
The latissimus dorsi is closely associated with the teres major and pectoralis major muscles. Together, these muscles work to adduct, extend, and medially rotate the arm at the glenohumeral joint. They also collaborate in the depression of the arm. The latissimus dorsi assists in moving the scapula in various directions, producing movements on the shoulder joint.
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Subscapularis
The subscapularis is a large, triangular-shaped muscle that fills the subscapular fossa of the scapula (wingbone). It is the largest and strongest of the four rotator cuff muscles, which are responsible for internal rotation of the arm and stabilization of the shoulder joint. The subscapularis is innervated by the upper and lower subscapular nerves (C5-C6), which stem from the brachial plexus. The blood supply to the subscapularis comes from branches of the subclavian artery, including the axillary artery, subscapular artery, and suprascapular artery.
The subscapularis originates from the subscapular fossa, with its medial two-thirds arising from this fossa and the remaining fibres coming from an aponeurosis covering the posterior surface of the lateral third of the muscle. The muscle fibres course laterally from the costal surface of the scapula, narrowing towards a round tendon that inserts into the lesser tubercle of the humerus and the anterior part of the articular capsule of the glenohumeral joint.
The primary function of the subscapularis is internal rotation of the humerus, which is the long bone in the upper arm. It assists in shoulder adduction and extension when the arm is in certain positions. When the arm is raised, the subscapularis pulls the humerus forward and downward. When the humerus is fixed, the insertion of the subscapularis can act as an origin, resulting in the abduction of the inferior border of the scapula.
The subscapularis is susceptible to injuries, especially in throwing motions. Symptoms of subscapularis tendonitis include pain when moving the shoulder, particularly when the arm is raised above the shoulders. An overworked subscapularis can cause a person to feel unable to lift their arm and may even lead to a frozen shoulder.
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Frequently asked questions
The axilla, or armpit, includes the serratus anterior muscle and thoracolumbar fascia, the pectoral muscles, and the subscapularis, teres major and latissimus dorsi muscles.
The armpit muscles are important for the function of the shoulders and arms. Without proper utilisation of these muscles, the neck and front shoulders may be overused, leading to severe neck tension and shoulder injuries.
One exercise for the armpit muscles is called "hand pumps supine", which involves using mini balls or soup cans and rolling them away from the shoulders.
The muscles of the rotator cuff, which includes the teres minor, are common sites of injury in adults, especially those who perform overhead motions repeatedly, such as throwing a baseball or painting a ceiling.
