The extrinsic muscles are located in the anterior and posterior compartments of the forearm. They control crude movements and produce a forceful grip.
- The muscles in the anterior compartment of the forearm are organised into three layers:
- Superficial: flexor carpi ulnaris, palmaris longus, flexor carpi radialis, pronator teres.
- Intermediate: flexor digitorum superficialis.
- Deep: flexor pollicis longus, flexor digitorum profundus and pronator quadratus.
This muscle group is associated with pronation of the forearm, flexion of the wrist and flexion of the fingers.
They are mostly innervated by the median nerve (except for the flexor carpi ulnaris and medial half of flexor digitorum profundus, which are innervated by the ulnar nerve), and they recieve arterial supply from the ulnar artery and radial artery
Anterior Compartment: Extrinsic Hand Muscles
Superficial Compartment
The superficial muscles in the anterior compartment are the flexor carpi ulnaris, palmaris longus, flexor carpi radialis and pronator teres.
They all originate from a common tendon, which arises from the medial epicondyle of the humerus.
1.Flexor Carpi Ulnaris
Attachments: The flexor carpi ulnaris has two origins. The humeral head originates from the medial epicondyle of the humerus with the other superficial flexors, whilst the ulnar head originates from the olecranon of the ulnar.
The muscle tendon passes into the wrist and attaches to the pisiform bone, hook of hamate, and base of the 5th metacarpal
Actions: Flexion and adduction at the wrist.
Innervation: Ulnar nerve.
2.Palmaris Longus
This muscle is absent in about 15% of the population.
Attachments: Originates from the medial epicondyle, attaches to the flexor retinaculum of the wrist.
Actions: Flexion at the wrist.
Innervation: Median nerve.
3.Flexor Carpi Radialis
Attachments: Originates from the medial epicondyle, attaches to the base of metacarpals II and III.
Actions: Flexion and abduction at the wrist.
Innervation: Median nerve.
4.Pronator Teres
The lateral border of the pronator teres forms the medial border of the cubital fossa, an anatomical triangle located over the elbow.
Attachments: It has two origins, one from the medial epicondyle, and the other from the coronoid process of the ulna. It attaches laterally to the mid-shaft of the radius.
Actions: Pronation of the forearm.
Innervation: Median nerve.
Intermediate Compartment
The flexor digitorum superficialis is the only muscle of the intermediate compartment. It can sometimes be classed as a superficial muscle, but in most individuals, it lies between the deep and superficial muscle layers.
The muscle is a good anatomical landmark in the forearm – the median nerve and ulnar artery pass between its two heads, and then travel posteriorly.
Attachments: It has two heads – one originates from the medial epicondyle of the humerus, the other from the radius. The muscle splits into four tendons at the wrist, which travel through the carpal tunnel, and attaches to the middle phalanges of the four fingers.
Actions: Flexes the metacarpophalangeal joints and proximal interphalangeal joints at the 4 fingers, and flexes at the wrist.
Innervation: Median nerve.
Deep Compartment
There are three muscles in the deep anterior forearm: flexor digitorum profundus, flexor pollicis longus, and pronator quadratus.
1.Flexor Digitorum Profundus
Attachments: Originates from the ulna and associated interosseous membrane. At the wrist, it splits into four tendons, that pass through the carpal tunnel and attach to the distal phalanges of the four fingers.
Actions: It is the only muscle that can flex the distal interphalangeal joints of the fingers. It also flexes at metacarpophalangeal joints and at the wrist.
Innervation: The medial half (acts on the little and ring fingers) is innervated by the ulnar nerve. The lateral half (acts on the middle and index fingers) is innervated by the anterior interosseous branch of the median nerve.
2.Flexor Pollicis Longus
This muscle lies laterally to the FDP.
Attachments: Originates from the anterior surface of the radius and surrounding interosseous membrane. Attaches to the base of the distal phalanx of the thumb.
Actions: Flexes the interphalangeal joint and metacarpophalangeal joint of the thumb.
Innervation: Median nerve (anterior interosseous branch).
3.Pronator Quadratus
A square shaped muscle found deep to the tendons of the FDP and FPL.
Attachments: Originates from the anterior surface of the ulna and attaches to the anterior surface of the radius.
Actions: Pronates the forearm.
Innervation: Median nerve (anterior interosseous branch).
Posterior Compartment Of Extrinsic Hand Muscles
- The muscles in the posterior compartment of the forearm are commonly known as the extensor muscles.
The general function of these muscles is to produce extension at the wrist and fingers. They are all innervated by the radial nerve.
Anatomically, the muscles in this compartment can be divided into two layers; deep and superficial. These two layers are separated by a layer of fascia.
Superficial Muscles
The superficial layer of the posterior forearm contains seven muscles. Four of these muscles – extensor carpi radialis brevis, extensor digitorum, extensor carpi ulnaris and extensor digiti minimi share a common tendinous origin at the lateral epicondyle.
1.Brachioradialis
The brachioradialis is a paradoxical muscle. Its origin and innervation are characteristic of an extensor muscle, but it is actually a flexor at the elbow.
The muscle is most visible when the forearm is half pronated, and flexing at the elbow against resistance.
In the distal forearm, the radial artery and nerve are sandwiched between the brachioradialis and the deep flexor muscles.
Attachments: Originates from the proximal aspect of the lateral supracondylar ridge of humerus, and attaches to the distal end of the radius, just before the radial styloid process.
Actions: Flexes at the elbow.
Innervation: Radial nerve.
2.Extensor Carpi Radialis Longus and Brevis
The extensor carpi radialis muscles are situated on the lateral aspect of the posterior forearm. Due to their position, they are able to produce abduction as well as extension at the wrist.
Attachments: The ECRL originates from the supracondylar ridge, while the ECRB originates from the lateral epicondyle. Their tendons attach to metacarpal bones II and III.
Actions: Extends and abducts the wrist.
Innervation: Radial nerve.
3.Extensor Digitorum Communis
The extensor digitorum is the main extensor of the fingers. To test the function of the muscle, the forearm is pronated, and the fingers extended against resistance.
Attachments: Originates from the lateral epicondyle. The tendon continues into in the distal part of the forearm, where it splits into four, and inserts into the extensor hood of each finger.
Actions: Extends medial four fingers at the MCP and IP joints.
Innervation: Radial nerve (deep branch).
4.Extensor Digiti Minimi
The extensor digiti minimi is thought to originate from the extensor digitorum muscle. In some people, these two muscles are fused together. Anatomically, the extensor digiti minimi lies medially to the extensor digitorum.
Attachments: Originates from the lateral epicondyle of the humerus. It attaches, with the extensor digitorum tendon, into the extensor hood of the little finger.
Actions: Extends the little finger, and contributes to extension at the wrist.
Innervation: Radial nerve (deep branch).
5.Extensor Carpi Ulnaris
The extensor carpi ulnaris is located on the medial aspect of the posterior forearm. Due to its position, it is able to produce adduction as well as extension at the wrist.
Attachments: Originates from the lateral epicondyle of the humerus,and attaches to the base of metacarpal V.
Actions: Extension and adduction of wrist.
Innervation: Radial nerve (deep branch).
6.Anconeus
The anconeus is situated medially and superiorly in the extensor compartment of the forearm. It is blended with the fibres of the triceps brachii, and the two muscles can be indistinguishable.
Attachments: Originates from the lateral epicondyle, and attaches to the posterior and lateral part of the olecranon.
Actions: Extends and stablises the elbow joint. Abducts the ulna during pronation of the forearm.
Innervation: Radial nerve.
Deep Muscles
There are five muscles in the deep compartment of the posterior forearm – the supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus and extensor indicis.
With the exception of the supinator, these muscles act on the thumb and the index finger.
1.Supinator
The supinator lies in the floor of the cubital fossa. It has two heads, which the deep branch of the radial nerve passes between.
Attachments: It has two heads of origin. One originates from the lateral epicondyle of the humerus, the other from the posterior surface of the ulna. They insert together into the posterior surface of the radius.
Actions: Supinates the forearm.
Innervation: Radial nerve (deep branch).
2.Abductor Pollicis Longus
The abductor pollicis longus is situated immediately distal to the supinator muscle. In the hand, its tendon contributes to the lateral border of the anatomical snuffbox.
Attachments: Originates from the interosseous membrane and the adjacent posterior surfaces of the radius and ulna. It attaches to the lateral side of the base of metacarpal I.
Actions: Abducts the thumb.
Innervation: Radial nerve (posterior interosseous branch).
3.Extensor Pollicis Brevis
The extensor pollicis brevis can be found medially and deep to the abductor pollicis longus. In the hand, its tendon contributes to the lateral border of the anatomical snuffbox.
Attachments: Originates from the posterior surface of the radius and interosseous membrane. It attaches to the base of the proximal phalanx of the thumb.
Actions: Extends at the metacarpophalangeal and carpometacarpal joints of the thumb.
Innervation: Radial nerve (posterior interosseous branch).
4.Extensor Pollicis Longus
The extensor pollicis longus muscle has a larger muscle belly than the EPB. Its tendon travels medially to the dorsal tubercle at the wrist, using the tubercle as a ‘pulley’ to increase the force exerted.
The tendon of the extensor pollicis longus forms the medial border of the anatomical snuffbox in the hand.
Attachments: Originates from the posterior surface of the ulna and interosseous membrane. It attaches to the distal phalanx of the thumb.
Actions: Extends all joints of the thumb: carpometacarpal, metacarpophalangeal and interphalangeal.
Innervation: Radial nerve (posterior interosseous branch).
5.Extensor Indicis Proprius
This muscle allows the index finger to be independent of the other fingers during extension.
Attachments: Originates from the posterior surface of the ulna and interosseous membrane, distal to the extensor pollicis longus. Attaches to the extensor hood of the index finger.
Actions: Extends the index finger.
Innervation: Radial nerve (posterior interosseous branch).
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