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Outcropping muscles are a group of muscles that appear to emanate from the depths of the distal posterior forearm. They are also referred to as the extensor muscles. The tendons of these outcropping muscles are located below the extensor retinaculum, a thin band of fibrous connective tissue that runs across the posterior aspect of the distal forearm. The three thumb muscles that outcrop together are the abductor pollicis longus, extensor pollicis brevis, and the extensor pollicis longus.
| Characteristics | Values |
|---|---|
| Definition | Outcropping muscles are a group of muscles that appear to emanate from the depths of the distal posterior forearm. |
| Muscle Groups | Extensor digitorum, extensor carpi radialis longus, extensor carpi radialis brevis, abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus. |
| Location | Forearm and hand |
| Tendon Location | Lateral inferior portion of the forearm |
| Innervation | Posterior interosseous nerve (PIN) |
| Function | Abducting the thumb and assisting in wrist flexion |
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What You'll Learn
Outcropping muscles are deeply placed
The extensor indicis proprius is another outcropping muscle that arises from the distal ulna and joins the tendons of the extensor digitorum. The tendons of these three outcropping muscles appear on the lateral inferior portion of the forearm. These deep extensor muscles attach proximally on the distal half of the radius and the interosseus membrane, and they run to the thumb.
The abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus are three thumb muscles that outcrop together. The extensor pollicis longus is also considered the remaining outcropping muscle that makes up the medial border of the anatomical snuffbox.
The radial nerve descends from the posterior compartment of the arm and crosses into the cubital region by passing anterior to the lateral epicondyle of the humerus. The radial artery, a branch of the brachial artery, runs through the cubital fossa, then down the lateral aspect of the forearm, deep to the brachioradialis muscle.
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They appear to emanate from the distal posterior forearm
Outcropping muscles are a group of muscles in the forearm. The forearm contains a mix of muscles, including the supinator, which arises from the lateral epicondyle of the humerus and inserts into the radius. The distal end of the pronator teres is where the distal end of the supinator is located. The extensor indicis proprius arises from the distal ulna and runs distally to join the tendons of the extensor digitorum.
The extensor digitorum is a muscle with a common tendon origin at the lateral epicondyle. It inserts into the middle and distal phalanges' bases through four tendon bands. The radial nerve innervates it, and its function is to extend the MCP/IP joint. The extensor carpi ulnaris, which also originates from the lateral epicondyle, is another muscle in the forearm. The ulnar side of the fifth metacarpal's base is where the insertion takes place. It is innervated by the radial nerve and is responsible for extending and adducting/ulnar deviating the wrist.
The extensor digiti minimi is another muscle in the forearm that originates from the lateral epicondyle and inserts into the base of the fifth proximal phalanx. The posterior interosseous (radial) nerve innervates it, and it functions to extend the fifth MP joint. The abductor pollicis longus, extensor pollicis longus, and extensor pollicis brevis are further examples of outcropping muscles. The extensor pollicis longus is the remaining outcropping muscle that makes up the medial border of the anatomical snuffbox.
The distal posterior forearm contains deeply placed muscles that appear to emanate from it. The extensor indicis proprius is one such muscle that arises from the distal ulna and runs distally to join the tendons of the extensor digitorum. These deep extensor muscles attach proximally on the distal half of the radius and the interosseus membrane, and their tendons can be seen on the lateral inferior portion of the forearm. Therefore, they appear to emanate from the distal posterior forearm and are sometimes referred to as "outcropping muscles".
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The tendons of outcropping muscles are located below the extensor retinaculum
Outcropping muscles are a group of muscles in the forearm. They are called outcropping muscles because they appear to emanate from the depths of the distal posterior forearm. The extensor retinaculum is a thin band of fibrous connective tissue that runs across the posterior aspect of the distal forearm. The tendons of the outcropping muscles are located below the extensor retinaculum.
The function of the extensor retinaculum is to keep the tendons of the extensor and outcropping muscles from bowing out when the muscles are contracted and tension on the tendons occurs. The three outcropping muscles are the abductor pollicis longus, extensor pollicis brevis, and the extensor pollicis longus. These muscles converge at the base of the thumb. The abductor pollicis longus is innervated by the posterior interosseous nerve and is responsible for abducting the thumb. It may also weakly assist in wrist flexion. The extensor pollicis longus inserts onto the distal phalanx of the thumb.
The tendons of the outcropping muscles can be observed through the medial border of the anatomical snuffbox. The radial artery runs across the floor of this "snuffbox," and the superficial branch of the radial nerve can be found here as well. The radial nerve descends from the posterior compartment of the arm and crosses into the cubital region by passing anterior to the lateral epicondyle of the humerus. The radial artery forms as a branch of the brachial artery as it passes through the cubital fossa. It runs down the lateral aspect of the forearm, deep to the brachioradialis muscle, and then leaves the forearm by passing posterolaterally to run obliquely along the floor of the anatomical snuffbox.
The outcropping muscles emerge from under the extensor digitorum and run superficial to the tendons of the extensor carpi radialis longus and brevis. The tendons of the extensor carpi radialis longus and brevis run medial to the radial artery along the floor of the anatomical snuffbox to insert onto the base of the 2nd and 3rd metacarpals.
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Examples include the extensor pollicis longus and abductor pollicis longus
Outcropping muscles are a group of muscles that appear to emanate from the depths of the distal posterior forearm. These muscles include the extensor pollicis longus and the abductor pollicis longus.
The extensor pollicis longus is a skeletal muscle located dorsally on the forearm. It is one of the deep forearm extensors that control the movement of the thumb. The muscle arises from the dorsal surface of the ulna and from the interosseous membrane, next to the origins of the abductor pollicis longus and extensor pollicis brevis. The tendon of the extensor pollicis longus is supplied by branches from various arteries, including the anterior interosseous artery and the radial artery. The muscle receives innervation from the posterior interosseous nerve, which is a continuation of the deep branch of the radial nerve.
The abductor pollicis longus is also a deep extensor muscle in the forearm. It is found deep to the extensor digitorum and lateral to the extensor pollicis longus muscle. The tendon of the abductor pollicis longus passes beneath the extensor retinaculum and inserts onto the base of the first metacarpal and trapezium bones. The muscle helps to pull the thumb away from the palm and is involved in abduction and extension movements of the thumb.
Both the extensor pollicis longus and the abductor pollicis longus play important roles in thumb movement and contribute to the complex anatomy of the hand and forearm.
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They are innervated by the posterior interosseous nerve
Outcropping muscles are a group of three muscles that appear to emanate from the depths of the distal posterior forearm. They are called outcropping muscles because their tendons appear to outcrop or emerge from under the extensor digitorum. These muscles are the extensor indicis proprius, which arises from the distal ulna, and the two other deep extensor muscles that attach proximally on the distal half of the radius and the interosseus membrane and run to the thumb.
The posterior interosseous nerve is a nerve in the forearm that innervates outcropping muscles. It is the continuation of the deep branch of the radial nerve, after it has crossed the supinator muscle. The nerve fibres originate from cervical segments C7 and C8 in the spinal column. It descends along the interosseous membrane, anterior to the extensor pollicis longus muscle, to the back of the carpus, where it presents a gangliform enlargement from which filaments are distributed to the ligaments and articulations of the carpus.
The posterior interosseous nerve supplies all the muscles of the posterior compartment of the forearm, except the anconeus muscle, brachioradialis muscle, and extensor carpi radialis longus muscle. It innervates the supinator, extensor carpi radialis brevis, extensor digitorum communis, and extensor digiti minim. The nerve may be entrapped at the arcade of Frohse, which is part of the supinator muscle. This nerve can be injured in Monteggia fractures due to dislocation of the proximal head of the radius bone.
Posterior interosseous nerve syndrome can be caused by compression injuries to the upper extremity, mostly in the arcade of Frohse. It is the area where the nerve enters the supinator muscle and is the most common place for nerve compression. This syndrome can also be caused by trauma, repetitive strain, inflammation, or following surgical procedures. It results in paresis or paralysis of the digital and thumb extensor muscles, leading to an inability to extend the thumb and fingers at their metacarpophalangeal joints.
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Frequently asked questions
Outcropping muscles are a group of muscles in the forearm that appear to emanate from the depths of the distal posterior forearm.
There are three outcropping muscles: the abductor pollicis longus, extensor pollicis brevis, and the extensor pollicis longus.
The abductor pollicis longus is responsible for abducting the thumb and may also assist in wrist flexion. The extensor pollicis longus inserts onto the distal phalanx of the thumb.
