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The armpit, also known as the axilla, is a pyramid-shaped area located between the shoulder girdle and thorax. It is an important space for neurovascular and lymphatic structures to travel through to reach the upper extremity from the neck. The axilla includes the axillary space, an anatomical space within the shoulder girdle between the arm and the thoracic cage. The soft skin covering the lateral axilla contains many hair and sweat glands. The axilla contains several muscles, including the serratus anterior, latissimus dorsi, teres major, and pectoralis major and minor. These muscles are important for the function of the shoulders and arms, and their improper utilisation can lead to overuse of the neck and front of the shoulders, causing severe neck tension and contributing to shoulder injuries.
| Characteristics | Values |
|---|---|
| Area | Directly under the shoulder joint |
| Muscles | Serratus Anterior, Pectoralis Major, Pectoralis Minor, Subscapularis, Teres Major, Latissimus Dorsi, Biceps Brachii, Coracobrachialis |
| Functions | Movement of the shoulder blades, arm rotation, attaching arms to the body |
| Nerves | Brachial Plexus, Medial and Lateral Pectoral, Medial Cutaneous of Arm and Forearm, Upper and Lower Subscapular, Thoracodorsal |
| Vessels | Axillary Vein, Axillary Artery, Cephalic Vein, Basilic Vein |
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What You'll Learn
Pectoralis major and minor muscles
The axilla, or armpit, is the area on the human body directly under the shoulder joint. The axilla includes the axillary space, an anatomical space within the shoulder girdle between the arm and the thoracic cage. The pectoral muscles are the group of skeletal muscles that connect the upper extremities to the anterior and lateral thoracic walls. The pectoral region is located on the anterior chest wall and contains four muscles that exert a force on the upper limb: the pectoralis major, pectoralis minor, serratus anterior, and subclavius.
The pectoralis major is the superior-most and largest muscle of the anterior chest wall. It is a thick, fan-shaped muscle that lies underneath the breast tissue and forms the anterior wall of the axilla. The pectoralis major is the most superficial muscle in the pectoral region. There are two heads of the pectoralis major, the clavicular and the sternocostal, which reference their area of origin. 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 upper and lower fibres of the pectoralis major insert into the crest of the greater tubercle of the humerus. The two heads of the pectoralis major have different nervous supplies, with the clavicular head deriving its nerve supply from the lateral pectoral nerve.
The pectoralis minor lies underneath its larger counterpart muscle, the pectoralis major. Both muscles form part of the anterior wall of the axilla region. The pectoralis minor originates from the third to fifth ribs and inserts into the coracoid process of the scapula. It is crucial in stabilising the scapula by pulling it downward and anteriorly against the thoracic wall. The pectoralis minor is much thinner and triangular in shape compared to the pectoralis major.
The serratus anterior is located more laterally in the chest wall and forms the medial border of the axilla region. It originates from the lateral aspects of ribs 1-8 and attaches to the costal surface of the medial border of the scapula. The function of the serratus anterior is to rotate 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 some 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 fibres follow the course of the ribs onto which they attach. The serratus anterior is also referred to as the "big swing muscle" or the "boxer's muscle". This is 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 serratus anterior is located in the underarm/back area. 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 rib cage when at rest and during movement. The muscle is divided into three parts: the superior, intermedius, and inferior. The lowest four digitations of the serratus anterior interdigitate with the fibres of the external oblique. The inferior part of the muscle is the most prominent and powerful one.
The innervation of the serratus anterior is supplied by the long thoracic nerve, a branch of the brachial plexus. The vascular supply to the serratus anterior comes from the superior and lateral thoracic arteries (branches of the axillary artery) as well as branches from the thoracodorsal artery (branch of subscapular artery).
Weakness of the serratus anterior leads to an altered line of pull of the rotator cuff muscle, which could increase the risk of subacromial impingement syndrome. The most common causes of serratus muscle pain include tension, stress, and overuse. This pain may also result from serratus anterior myofascial pain syndrome (SAMPS), a rare myofascial pain syndrome.
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Latissimus dorsi and teres major muscles
The axilla, also known as the armpit, underarm or oxter, includes the axillary space, an anatomical space within the shoulder girdle between the arm and the thoracic cage. The axilla is bounded by several muscles, including the latissimus dorsi and teres major muscles.
The latissimus dorsi is a large, powerful muscle on the back of the torso. It covers much of the back and has a broad area of attachment, connecting to the vertebrae, lumbar vertebrae, sacrum, iliac crest, and ribs. The latissimus dorsi is important for maintaining balanced movement and stability of the shoulder complex. It also aids in the medial rotation and adduction of the humerus.
The teres major is a small muscle that assists the latissimus dorsi in some of its actions. It originates on the inferior angle of the scapula and connects from the torso to the humerus. The function of the teres major is to extend, medially rotate, and adduct the humerus, as well as play a role in stabilizing the humeral head.
Both the latissimus dorsi and teres major muscles are involved in the medial rotation of the humerus when the arm is abducted. They work together synergistically, activating as one muscle unit. These muscles are also used in tendon transfer procedures to treat weakness or injuries in the shoulder or rotator cuff.
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Subscapularis, one of the rotator cuff muscles
The subscapularis is a large, strong triangular shoulder muscle located in the subscapular fossa of the scapula. It is one of the four muscles of the rotator cuff, along with the supraspinatus, infraspinatus, and teres minor. The rotator cuff is a group of muscles and tendons around the shoulder joint that help with movement and rotation of the arm and shoulder.
The subscapularis muscle is the only medial (internal) rotator of all the rotator cuff muscles. It is responsible for the internal rotation of the arm and stabilization of the shoulder joint. The precise action of the subscapularis is to internally rotate the arm on the shoulder joint, allowing the arm to be held outstretched, away from the body. It also contributes to the creation of concavity compression, a stabilizing mechanism that prevents the dislocation of the humerus by translating forces.
The subscapularis is an important stabilizer of the shoulder joint when the deltoid, pectoralis major, biceps, and triceps brachii muscles are active. These muscles are responsible for several overhead movements of the arm. The subscapularis muscle is innervated by the upper and lower subscapular nerves (C5-C6), which stem from the superior and posterior cords of the brachial plexus, respectively. The primary blood supply to the subscapularis muscle comes from a series of branches of the subclavian artery, including the axillary artery and its branches.
An overworked subscapularis muscle may result in difficulty lifting the arm or a frozen shoulder. This can be diagnosed through tests such as the lift-off test and the Belly Press Test, which assess the patient's ability to internally rotate the arm and maintain its position. If a subscapularis tear or dysfunction is suspected, conservative management includes rest, activity modification, analgesia, ice, and physical therapy.
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Biceps Brachii muscle
The biceps brachii muscle, commonly known as the biceps, is a large, thick muscle on the upper arm's ventral portion. The muscle is composed of two heads: a short head and a long head. The long head is located on the lateral side of the biceps brachii, while the short head is located on the medial side. The short head originates from the apex of the coracoid process of the scapula, and the long head originates from the supraglenoid tubercle of the scapula.
The biceps brachii works across three joints: the glenohumeral, elbow, and radio-ulnar joints. It is involved in various tasks, including lifting, sports involving throwing and racket use, and gesturing. As a result, biceps tendinopathy is a common condition seen in this muscle, often caused by muscle overuse or trauma. For example, the "popeye deformity," commonly seen in baseball pitchers, is caused by a ruptured long head tendon due to chronic wear and tear.
The biceps brachii is supplied by the musculocutaneous nerve (C5-C7), a branch of the brachial plexus. The primary arterial blood supply for the biceps brachii muscle comes from the muscular branches of the brachial artery. This artery is the axillary artery's continuation as it exits the axilla at the teres major's inferior margin. The brachial artery can be palpated deep to the biceps and is important for locating before taking blood pressure to ensure accurate measurements.
The biceps brachii's main functions include the flexion and supination (outward rotation) of the forearm. In the shoulder joint, the two muscle heads partially enforce opposite movements. The long head pulls the arm away from the trunk (abduction) and turns it inwards (inward rotation), while the short head pulls the arm back towards the trunk (adduction). When both heads contract simultaneously, it leads to an arm bend (flexion). In the elbow joint, the muscle bends the forearm (flexion) and rotates it outwards (supination).
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Frequently asked questions
The axilla, or armpit, includes the axillary space, an anatomical space within the shoulder girdle between the arm and the thoracic cage. The axilla contains the serratus anterior muscle, thoracolumbar fascia, pectoral muscles, subscapularis, teres major and latissimus dorsi muscle.
The muscles in the armpit are important for the function of the shoulders and arms. Without proper utilisation of these muscles, the neck and the front of the shoulders may be overused, leading to severe neck tension and shoulder injuries.
One exercise for the armpit muscles is called "hand pumps supine". This exercise uses mini balls, but you can substitute them with two soup cans. Place your hands on the sides of the cans and roll them away from your shoulders.
