Maximus Gage
The forearm is the section of the upper limb from the elbow to the wrist, containing twenty muscles that act on the elbow and wrist joints, as well as the joints of the hand. These muscles are divided into two compartments: the anterior (flexor) compartment and the posterior (extensor) compartment. The muscles in the anterior compartment are responsible for flexing the forearm, while the muscles in the posterior compartment are responsible for extending the forearm. The triceps brachii is the prime extensor of the forearm, with assistance from the anconeus muscle. Other muscles in the posterior compartment that aid in extending the forearm include the supinator, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, and extensor carpi ulnaris.
| Characteristics | Values |
|---|---|
| Number of muscles in the forearm | 20 |
| Number of muscular compartments | 2 |
| Types of muscular compartments | Anterior (flexor) and posterior (extensor) |
| Muscles that extend the forearm | Triceps Brachii, Anconeus |
| Muscles that extend the wrist | Extensor Carpi Radialis Longus, Extensor Carpi Radialis Brevis, Extensor Digitorum, Extensor Carpi Ulnaris |
| Muscles that extend the fingers | Extensor Digitorum, Extensor Digiti Minimi, Extensor Indicis, Extensor Pollicis Brevis, Extensor Pollicis Longus |
| Muscles that extend the thumb | Extensor Pollicis Brevis, Extensor Pollicis Longus, Abductor Pollicis Longus |
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What You'll Learn
Triceps Brachii
The triceps brachii is a three-headed muscle in the arm, with medial, lateral, and long heads. It is the only muscle in the posterior compartment of the arm, and it spans almost the entire length of the humerus. The triceps brachii is responsible for extending the forearm at the elbow joint. It is also involved in the extension and adduction of the arm at the shoulder joint.
The triceps brachii is innervated by the radial nerve, which provides a separate branch for each head. The C6 root value of the radial nerve innervates the lateral head, the C7 root value innervates the long head, and the C8 root value supplies the medial head. However, recent studies have found that in some individuals, the long head of the triceps brachii is innervated by the axillary nerve, or by both the radial and axillary nerves.
The triceps brachii can be targeted through isolation or compound elbow extension movements. Isolation exercises include cable push-downs, lying triceps extensions, and arm extensions behind the back. Compound exercises include pressing movements like push-ups, bench presses, and dips. The triceps brachii can also be contracted statically to keep the arm straight against resistance.
The triceps brachii plays a role in creating anatomical spaces that are traversed by neurovascular structures. This makes it an important surgical landmark. The medial head of the triceps brachii is positioned adjacent to the biceps brachii muscle, and the space between them forms the medial bicipital groove. This groove provides a passageway for the brachial artery and the median and ulnar nerves.
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Anconeus
The anconeus is a small, triangular muscle located at the elbow, attaching the humerus and ulna. It is situated in the superficial region of the forearm's posterior compartment. The anconeus muscle is a continuation of the triceps brachii muscle, with their fibres often being partially or completely blended together. It originates at the dorsal side of the lateral epicondyle of the humerus and inserts at the olecranon of the ulna.
Functionally, the anconeus permits extension of the forearm and provides support for both the dorsal joint capsule and the ulna itself. Its contraction leads to the extension of the forearm at the elbow. The anconeus also keeps the tension of the dorsal joint capsule, thus preventing damage during hyperextension. It is believed that the muscle has the additional function of stabilising the ulna, especially during pronation movements of the forearm.
The anconeus is supplied by a motor branch of the radial nerve (C6-C8), arising from the radial sulcus of the humerus. Trauma to the nerve supply of the anconeus muscle can usually result from a shoulder dislocation or fractures of the upper part of the humerus or around the olecranon, or any injury that damages the radial nerve. Any disease that compromises muscular functions, particularly arm extension (e.g. muscular dystrophy), will affect this muscle.
An anatomical variation of the anconeus is found in about 34% of all humans. Although most are harmless, there is one variation that is clinically relevant: the anconeus epitrochlearis muscle. This variant of the anconeus originates at the medial epicondyle of the humerus, crossing the ulnar groove of the bone. In cases of hypertrophy, it can cause a compression of the ulnar nerve, resulting in numbness of the ulnar side of the hand and pain in the elbow (cubital tunnel syndrome).
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Extrinsic muscles
Forearm muscles are divided into two compartments: the anterior (flexor) compartment and the posterior (extensor) compartment. The forearm contains 20 muscles that act on the elbow and wrist joints, as well as the carpometacarpal, metacarpophalangeal, and interphalangeal joints of the hand. The forearm muscles are further categorised into intrinsic and extrinsic muscles. The extrinsic muscles flex and extend the digits of the hand.
The extrinsic muscles include the brachioradialis, which traverses the elbow joint, running from the arm to the wrist, helping to flex the elbow. The brachioradialis is located in the superficial region of the forearm posterior compartment. It originates from the humerus and attaches to the distal end of the radius.
The supinator muscle is located in the deep region of the forearm posterior compartment. It has two heads: one originating from the humerus, the other from the ulna. Together, they attach to the radius. The supinator muscle allows for supination of the forearm, which means to turn it so that the palm faces up.
The extensor pollicis brevis is located medially and deep to the abductor pollicis longus. In the hand, its tendon contributes to the radial (lateral) border of the anatomical snuffbox. It originates from the posterior surface of the radius and interosseous membrane and attaches to the base of the proximal phalanx of the thumb. The extensor pollicis brevis allows you to make a thumbs-up signal.
The abductor pollicis longus is situated immediately distal to the supinator muscle. In the hand, its tendon contributes to the radial (lateral) border of the anatomical snuffbox. It originates from the interosseous membrane and the adjacent posterior surfaces of the radius and ulna and attaches to the lateral side of the base of metacarpal I. The abductor pollicis longus abducts the thumb, or moves it away from the palm.
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Intrinsic muscles
The forearm is the section of the upper limb from the elbow to the wrist, with its bony structure formed by the radius and ulna. The forearm muscles are divided into two compartments: the anterior (flexor) and posterior (extensor) compartment. These compartments contain twenty muscles that act on the elbow and wrist joints, as well as carpometacarpal, metacarpophalangeal, and interphalangeal joints of the hand.
The intrinsic muscles are those that function to move the forearm by pronating and supinating the radius and ulna. These muscles turn the bones inward (toward the body) and outward (away from the body). They include the pronator teres, supinator, and pronator quadratus muscles. The pronator teres is a rectangular muscle located in the superficial region of the anterior compartment, and it pronates the forearm. The supinator is located in the deep region of the posterior compartment and supinates the forearm. The pronator quadratus is a square-shaped muscle located adjacent to the wrist in the deep region of the anterior compartment, and it pronates the forearm.
The supinator muscle is located immediately distal to the abductor pollicis longus. In the hand, its tendon contributes to the radial (lateral) border of the anatomical snuffbox. The extensor pollicis brevis can be found medially and deep to the abductor pollicis longus. In the hand, its tendon contributes to the radial (lateral) border of the anatomical snuffbox.
The extrinsic muscles, on the other hand, flex and extend the digits of the hand. They include muscles like the brachioradialis, which traverses the elbow joint, running from the arm to the wrist, helping to flex the elbow. The biceps brachii flex the forearm and work with the supinator of the forearm to rotate it so the palm faces upward. The triceps brachii extend the forearm.
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Supinator
The supinator is a broad, spiral muscle located in the posterior compartment of the forearm. It is one of twenty muscles that act on the elbow and wrist joints, as well as the carpometacarpal, metacarpophalangeal, and interphalangeal joints of the hand. The forearm muscles are essential to the fine motor actions of the upper limb, allowing for complex movements of the arm, wrist, and fingers.
The supinator consists of two layers of fibres, or planes, that differ in their mode of attachment. The superficial layer arises by tendinous fibres, while the deep layer originates with already formed muscular slips. Both layers originate from the same landmarks, including several osteofibrous structures of the elbow: the lateral epicondyle of the humerus, the radial collateral ligament of the humeroulnar joint, the annular ligament of the superior radioulnar joint, the supinator crest of the ulna, and the adjacent part of the ulnar fossa. From here, the muscle wraps (or spirals) around the proximal third of the radius, inserting into the upper third of its lateral, posterior, and anterior surfaces.
The supinator is innervated by the deep branch of the radial nerve, which becomes the posterior interosseous nerve upon exiting the supinator muscle. Its nerve roots are primarily from C6, with some C5 involvement and possible additional C7 innervation. The radial nerve divides into deep and sensory superficial branches just proximal to the supinator muscle, which can lead to entrapment and compression of the deep branch and potentially result in selective paralysis of the muscles served by this nerve. This nerve syndrome is known as supinator entrapment syndrome and can be caused by compression by various soft-tissue masses surrounding the nerve or stress caused by repetitive supination and pronation.
The main function of the supinator is to supinate the forearm, or rotate the radius laterally at the proximal radioulnar joint. This action puts the radius parallel to the ulna, bringing the hand into the supine position (facing anteriorly, palm up, like when holding a bowl of soup). The supinator is the prime mover in slow and unopposed supination movements, such as rotating the hand to grab popcorn from a bowl. However, for quick, strong, or forceful supination movements, or when there is resistance, the supinator is assisted by the biceps brachii muscle. The biceps brachii cannot act as a supinator when the forearm is fully extended, so the most powerful supination occurs when the elbow joint is flexed to 90 degrees, such as when turning a screwdriver or uncorking a bottle of wine.
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Frequently asked questions
The triceps brachii is the main muscle responsible for extending the forearm. The anconeus is also involved in extending the forearm at the elbow.
The triceps brachii is located in the posterior (extensor) compartment of the arm. The anconeus is located in the superficial region of the forearm posterior compartment.
The supinator, brachioradialis, and pronator teres muscles are also involved in extending the forearm.
The triceps brachii can also act as a weak arm extensor and adductor. The anconeus moves the ulna during pronation and helps to stabilise the elbow joint. The supinator allows for supination of the forearm, or turning the palm to face upwards. The brachioradialis helps to flex the elbow and turn the arm. The pronator teres rotates the forearm.
