The forearm is the area of upper extremity between the elbow and wrist. The bony structure of the forearm is formed by two bones – radius and ulna.
There are twenty forearm muscles which are arranged in two compartments – anterior and posterior. The anterior compartment contains flexor muscles and is also called the flexor compartment whereas the posterior compartment contains extensor muscles and is also called the extensor compartment.
Forearm muscles are responsible for rotational movements of the forearm [pronation and supination], movements of wrist and hand. [Just to reemphasize, the flexion-extension movements of the elbow are by muscles of the arm]. The forearm is also called antebrachium.
Muscles in the Anterior Compartment of the Forearm
Anterior compartment forearm muscles are responsible for wrist flexion and finger flexion and pronation of the forearm.
Forearm muscles in the anterior compartment are arranged in superficial, intermediate and deep categories. The respective muscles in these compartments are
Superficial compartment [4 muscles]
- Flexor carpi ulnaris
- Palmaris longus
- Flexor carpi radialis
- Pronator teres
Intermediate Compartment [1 muscle]
- Flexor digitorum superficialis
Deep compartment [3 muscles]
- Flexor digitorum profundus
- Flexor pollicis longus
- Pronator quadratus
Superficial Compartment
All these muscles originate from common flexor origin or common flexor tendon. Common flexor tendon is a tendon that arises from the medial epicondyle of the humerus bone.
Flexor Carpi Ulnaris
Origin
Flexor carpi ulnaris muscle originates from two heads, humeral and ulnar. Both heads are connected by a tendinous arch. Beneath this arch, the ulnar nerve and artery pass.
The humeral head takes origin from the medial epicondyle of the humerus by the common flexor tendon.
The ulnar head originates from
- The medial margin of the olecranon of the ulna
- The upper two-thirds of the dorsal border of the ulna by an aponeurosis.
Insertion
The flexor carpi ulnaris muscle is inserted into
- Pisiform bone
- Hamate bone by the pisohamate ligament
- Fifth metacarpal bone by the pisometacarpal ligament.
Flexor carpi ulnaris can be located in the distal forearm by flexion of the wrist against resistance. It is the medial most visible tendon. It is better visualized in thin persons.
Action
Flexion and adduction of the wrist joint.
Nerve supply
The ulnar nerve [C8, T1]
Palmaris Longus
Palmaris longus is absent in about 14 percent of people. The absence of this muscle does not affect grip strength.
The distal palmaris longus tendon can be visualized by apposing the fourth finger and thumb and flexing the wrist. The tendon, if present is visible in the midline of the anterior wrist.
Origin
Palmaris longus originates from the medial epicondyle of the humerus via the common flexor tendon. It also takes origin from the adjacent intermuscular septa and from the antebrachial fascia.
The muscle is spindle-shaped and lies on the medial side of the flexor carpi radialis.
Insertion
The thin, flat tendon passes above the flexor retinaculum of the wrist and inserts on the central part of the flexor retinaculum.
It may also send a tendinous slip to the thenar muscles.
Nerve supply
The median nerve.
Action
Weak flexor of the wrist and may contribute and assist in thumb abduction movements
Flexor carpi radialis
Origin
Flexor carpi radialis originates from the medial epicondyle of the humerus via common flexor tendon.
Insertion
In the forearm, this muscle runs lateral to flexor digitorum superficialis. It inserts on the anterior aspect of the base of the second metacarpal.
Two small slips are sent to the third metacarpal and tuberosity of trapezium.
Nerve Supply
Median nerve
Pronator Teres
The pronator teres is the muscle that forms the medial border of the cubital fossa.
Origin
It arises by two heads
- Humeral head
- Larger and superficial
- Medial supracondylar ridge immediately proximal to the medial epicondyle of the humerus
- Common flexor tendon
- Ulnar head
- Thin fasciculus
- From the medial side of the coronoid process of the ulna
Insertion
The ulnar head joins the humeral head at an acute angle. Between the two heads, the median nerve enters the forearm. The ulnar artery is separated from the median nerve by the ulnar head of the muscle.
The muscle passes obliquely, goes across the forearm and inserts on the rough impression on the radial shaft on the lateral side, just distal to the insertion of the supinator muscle. This is almost in the middle of the shaft length.
Nerve supply
Median nerve
Action
Pronates the forearm. Pronation is the movement that makes palm face downward when elbows are flexed ninety degrees.
Pronator teres muscle is aided by the pronator quadratus.
Pronator teres also acts as a weak flexor of the elbow.
Intermediate Compartment
This compartment contains flexor digitorum superficialis as sole muscle. Some authors prefer to club it with superficial muscles but generally, it is considered as between the deep and superficial muscle layers.
Flexor Digitorum Superficialis
Origin
Flexor digitorum superficialis has two origins. [see image]
- Humeroulnar
- Medial epicondyle of the humerus (common flexor tendon)
- Coronoid process of the ulna.
- Ulnar collateral ligament of elbow
- Radial
- from the oblique line on the radial shaft
Insertion
This muscle lies medial to flexor carpi radialis in the forearm.
Near the wrist, this gives off four long tendons that travel under the flexor retinaculum.
The tendons, along with the tendons of flexor digitorum profundus, are enclosed by a common flexor sheath.
The tendons of flexor digitorum superficialis are attached to the anterior margins on the bases of the intermediate phalanges of the four fingers with each slip going to that of index, middle, ring and little finger.
At the insertion, the tendon splits to let tendons of flexor digitorum profundus pass. This split is called Camper’s Chiasm.
Nerve Supply
Median nerve (C7, C8, T1)
Action
Primarily this muscle causes flexion of the middle phalanges of the four fingers at the proximal interphalangeal joints.
However, with continued action, it also flexes the metacarpophalangeal joints.
It also acts as a weak wrist flexor.
Deep Compartment
The deep compartment of the anterior forearm contains three muscles- flexor digitorum profundus, flexor pollicis longus, and pronator quadratus.
Flexor Digitorum Profundus
Origin
The muscle originates from
- Upper three –fourth of the anterior and medial surface of the ulna
- Interosseous membrane and deep fascia of the forearm
Insertion
In the arm, flexor digitorum profundus lies deep to the flexor digitorum superficialis muscle. Like superficialis, flexor digitorum profundus also ends in four tendons one to each of the second to fifth fingers. Each slip inserts on the base of the distal phalanx on the palmar aspect.
As the insertion is distal to that the superficialis muscle, the tendon of the profundus muscle goes through the split in the insertion of superficialis [Camper’s chiasm, found just before insertion of the superficialis]. The profundus slip passes through the split and attaches to the base of the distal phalanx.
Nerve supply
The muscle has dual nerve supply – median nerve via anterior interosseous nerve and ulnar nerve.
Ulnar nerve supplies medial part of the muscle that acts on the ring and little finger [C8, T1]
The median nerve supplies the lateral half of muscle acting on the index and ring finger.[C8, T1]
Action
Flexor digitorum profundus flexes the distal interphalangeal joints of the fingers. It also causes flexion of the metacarpophalangeal joints and the wrist.
Flexor Pollicis Longus
Origin
Flexor pollicis longus originates from
- The anterior surface of the radius from immediately below the radial tuberosity and oblique line
- Surrounding interosseous membrane
Sometimes slip from the coronoid process and medial epicondyle may also contribute.
Insertion
This muscle lies laterally to the flexor digitorum profundus.
The flattened tendon passes under the flexor retinaculum between the lateral head of the flexor pollicis brevis and the oblique part of the adductor pollicis,
It is inserted into the base of the distal phalanx of the thumb on the palmar aspect.
Pronator Quadratus
It is a square-shaped muscle that lies deep to the tendons of the flexor digitorum profundus and flexor pollicis longus.
Origin
It originates from the anterior distal aspect of the distal diaphysis of the ulna. The fibers run perpendicular to the forearm.
Insertion
Superficial fibers insert into anterior distal diaphysis of the radius and anterior metaphysis.
Deep fibers insert proximal to the ulnar notch.
Nerve Supply
Median nerve (via anterior interosseous branch).
Action
Pronator quadratus as the name suggests acts to pronate the forearm. It attempts to pull the radius towards itself resulting in rotatory motion.
Forearm Muscles in the Posterior Compartment
Forearm muscles in the posterior compartment of the forearm are popularly called the extensor muscles.
These muscles in general function to extend the wrist and the fingers. All of these muscles are innervated by the radial nerve.
Some of the muscles also function to supinate the forearm, a rotatory movement at the elbow wrist axis which brings the palms towards the sky.
The muscles of the forearm in the posterior compartment can be clubbed into two layers – superficial and deep.
The layers are separated by a fascia.
At the wrist, the extensor forearm muscles are compartmentalized in six osteofascial compartments [see later section- Arrangement of Forearm Muscles and Tendons in the Wrist]
Superficial Muscles
The superficial layer of the posterior forearm contains seven muscles.
- Brachioradialis
- Extensor carpi radialis longus
- Extensor carpi radialis brevis
- Extensor digitorum
- Extensor carpi ulnaris
- Extensor digiti minimi
- Anconeus
Four of these muscles, namely, extensor carpi radialis brevis, extensor digitorum, extensor carpi ulnaris, and extensor digiti minimi have a common tendinous origin at the lateral epicondyle, also called common extensor origin.
Brachioradialis
Though brachioradialis anatomically has an extensor location and is innervated by the radial nerve, it actually is a flexor of the elbow and thus is a paradoxical muscle.
The muscle can be made prominent by flexing a semipronated forearm against resistance.
Origin
Brachioradialis takes origin from the proximal aspect of the lateral supracondylar ridge of humerus.
Insertion
The muscle is inserted into the distal radius just before the radial styloid process.
Nerve Supply
Radial Nerve.
One interesting fact that deserves mention here is that out of all muscles supplied by radial nerve, only four are directly supplied by the radial nerve. The rest of the muscles receive the supply from the deep branch of the radial nerve.
Brachioradialis is one of these four muscles. Other muscles that receive innervation from the radial nerve are the triceps, anconeus, and extensor carpi radialis longus.
Action
Elbow flexion especially in the semipronated forearm.
Extensor Carpi Radialis Longus
Origin
Extensor carpi radialis longus originates from
- Lateral supracondylar ridge of the humerus
- Lateral intermuscular septum
- Lateral epicondyle of the humerus
Insertion
The fibers unite to form a flat tendon at the upper third of arm which moves down along the lateral border of the radius under the abductor pollicis longus and extensor pollicis brevis.
It lies between the brachioradialis and the extensor carpi radialis brevis and crosses to the hand via the second tendon compartment along with extensor carpi radialis brevis.
It inserts on the dorsoradial surface of the base of the second metacarpal.
Nerve Supply
Radial nerve, direct branch [C6,7]
Action
Extensor carpi radialis longus is an extensor of the wrist joint and also causes radial abduction of the wrist.
Extensor Carpi Radialis Brevis
Origin
It arises from
- lateral epicondyle of the humerus by the common extensor tendon
- From the radial collateral ligament of the elbow joint
- From aponeurosis which covers its surface
- Intermuscular septa between it and the adjacent muscles
Insertion
The muscle forms a flat tendon in the middle of the forearm. The tendon might have loose connections to the extensor carpi radialis longus. Both pass under the abductor pollicis longus and extensor pollicis brevis, beneath the extensor retinaculum.
It inserts into the dorsoradial surface of the third metacarpal bone. Some fibers also go to the dorsomedial surface of the second metacarpal bone.
Nerve Supply
The radial nerve, deep branch
Action
Extension and abduction of the wrist, just similar to extensor carpi radialis longus.
Extensor Digitorum
The extensor digitorum muscle is the main extensor of the fingers.
Origin
It originates from
- The lateral epicondyle via common extensor tendon
- Intermuscular septa between it and the adjacent muscles
- Antebrachial fascia or fascia of the forearm
Insertion
Near the wrist, it divides into four tendons, which together with that of the extensor indicis pass through the extensor tendon compartment.
Opposite the metacarpophalangeal joint, each slip gives fasciculi to the collateral ligaments and serves as the dorsal ligament of this joint.
After crossing the metacarpophalangeal joint, it forms a broad aponeurosis [reinforced by interossei and lumbricals] that covers the surface of the first phalanx.
At the proximal interphalangeal joint, the aponeurosis divides into three slips.
The middle is inserted into the base of the middle phalanx. The two lateral slips travel further to insert into the dorsal surface of the last phalanx.
Fasciculi are given from tendons at the interphalangeal joints which act as dorsal ligaments.
Nerve Supply
Radial nerve [deep branch]
Action
Extention of fingers [index, middle, ring and little] at the metacarpophalangeal and interphalangeal joints. It also contributes to wrist extension.
Extensor Digiti Minimi
The extensor digiti minimi is an extensor muscle for the little finger.
It is a slender muscle on the medial aspect of the posterior forearm.
In some people, the extensor digiti minimi muscle is fused with the extensor digitorum. Some authors consider it to originate from extensor digitorum muscle.
Origin
The muscle takes origin from the common extensor tendon and lies medially to the extensor digitorum.
Insertion
The tendon passes posterior to distal radioulnar joint and divides into two slips that join the extensor digitorum tendon.
All the three slips attach to the dorsal digital expansion of the little finger.
Sometimes a slip is also given to the ring finger as well.
Nerve Supply
Radial nerve, deep branch
Action
It causes extension of the little finger. It also contributes to the extension at the wrist.
Extensor Carpi Ulnaris
Origin
The muscle originates from the lateral epicondyle of the humerus
Insertion
It attaches to the base of the fifth metacarpal.
Nerve Supply
Radial nerve (deep branch).
Action
Extension and adduction of the wrist.
Anconeus
The anconeus is present on the medial aspect of the extensor region near the elbow and is often indistinguishable from the triceps muscle because the fibers blend with the triceps muscle.
Origin
The anconeus muscle takes origin from the lateral epicondyle.
Insertion
It attaches to the posterior and lateral part of the olecranon.
Nerve Supply
Radial nerve
Action
Anconeus muscle extends and stabilizes the elbow joint. It also abducts the ulna during pronation of the forearm.
Deep Muscles
There are five deep muscles in the deep compartment of the forearm.
- Supinator
- Abductor pollicis longus
- Extensor pollicis brevis
- Extensor pollicis longus
- Extensor indicis
While extensor indicis acts on the index finger, supinator acts in the forearm to supinate. The rest of the three muscles act on the thumb.
Supinator
Origin
The supinator muscle originates from two heads [the deep branch of the radial nerve passes between the two heads]
- Lateral epicondyle of the humerus by a tendon
- Deep muscular head
- The posterior surface of the ulna [supinator crest]
- Radial collateral ligament
- Annular radial ligament
Insertion
The superficial fibers are inserted into the lateral edge of the radial tuberosity and the oblique line of the radius.
The deep part forms a sling-like fasciculus that encircles the neck of the radius above the tuberosity and inserts into
- The posteromedial surface of radial tuberosity of its medial surface
- Posterior dorsal and lateral surfaces of the radial shaft in the middle between the oblique line and the head
Nerve Supply
Deep branch of radius [after it leaves the supinator, the deep branch of the radial nerve is also called posterior interosseous nerve].
Action
Supinates the forearm
Abductor Pollicis Longus
Abductor pollicis longus is an abductor muscle that is situated immediately distal to the supinator muscle. Distally, its tendon forms the anterior border of the snuffbox.
[Another abductor of the thumb, abductor pollicis brevis is a thenar muscle]
Origin
It originates from the interosseous membrane and the adjacent posterior surfaces of the radius and ulna.
Abductor pollicis longus originates from
- The posterior surface of the shaft below the insertion of the anconeus
- Interosseous membrane,
- Middle third of the posterior surface the radius.
Insertion
The muscle passes downward and lateralward to end in a tendon. The tendon runs along the groove on the lateral side of the lower end of the radius along with the tendon of the extensor pollicis brevis.
It inserts in the following manner
- Distal superficial part
- Radial side of the base of the first metacarpal bone
- Deep part
- Trapezium along with its joint capsule and ligaments
- Belly of abductor pollicis brevis
- Some slips to opponens pollicis.
Nerve Supply
Radial nerve (posterior interosseous branch).
Action
Abductor pollicis longus, as the name suggests, abducts the thumb.
Extensor Pollicis Brevis
Extensor pollicis brevis lies medial and deep to the abductor pollicis longus.
Origin
Extensor pollicis brevis originates from the posterior surface of the radius, interosseous membrane, and the ulna distal to the abductor pollicis longus.
Insertion
It attaches to the base of the proximal phalanx of the thumb.
Action
Extensor pollicis brevis extends the metacarpophalangeal joint and carpometacarpal joints of the thumb.
Nerve Supply
Radial nerve (Posterior interosseous branch).
Extensor Pollicis Longus
This muscle is also an extensor of thumb. It can be differentiated from the extensor pollicis brevis by its larger belly. Its tendon forms the medial border of the anatomical snuffbox in the wrist.
Origin
Extensor pollicis longus originates from the posterior surface of the ulna and interosseous membrane.
Insertion
It passes through the tendon compartment and lies on the narrow, oblique groove on the back of the lower end of the radius. At the wrist, it turns towards the thumb using Lister’s tubercle on the distal end of the radius as a pulley.
It attaches to the distal phalanx of the thumb.
Nerve Supply
Radial nerve (posterior interosseous branch).
Action
Extensor pollicis longus extends all joints [carpometacarpal, metacarpophalangeal and interphalangeal] of thumb.
Extensor Indicis Proprius
This muscle is an independent extensor muscle of the index finger.
Origin
Extensor indicis proprius originates from the posterior surface of the ulna and interosseous membrane, distal to the extensor pollicis longus.
Insertion
It runs through the fourth tendon compartment in the wrist together with the extensor digitorum. It attaches to the extensor hood of the index finger.
Nerve Supply
Radial nerve (posterior interosseous branch).
Action
Extends the index finger.
Arrangement of Forearm Muscles and Tendons in the Wrist
Here is a cross-section of the wrist showing the arrangement of tendons in the wrist and their relations to other structures.
On the extensor side, the retinaculum sends down septa which are attached to the longitudinal ridges on the posterior surface of the lower end of the radius. This creates six osteofascial compartments as shown in the cross-section above.
The structures passing through each compartment, from lateral to the medial side, are listed below.
- Compartment I
- Abductor pollicis longus
- Extensor pollicis brevis
- Compartment II
- Extensor carpi radialis longus
- Extensor carpi radialis brevis
- Compartment III
- Extensor pollicis longus
- Compartment IV
- Extensor digitorum
- Extensor indicis
- Posterior interosseous nerve and anterior interosseous artery are also present in this compartment.
- Compartment V
- Extensor digiti minimi
- Compartment VI
- Extensor carpi ulnaris
Each compartment is lined by a synovial sheath which is reflected on the contained tendons.
Clinical Significance of Forearm Muscles
Anatomical Landmarks
Anatomical landmarks are used in locating the lesion or structure during clinical examination or surgery. Important landmarks formed by forearm muscles and/or their tendons are
- Flexor digitorum superficialis is important during surgery. The median nerve and ulnar artery pass between its two heads and then travel posteriorly.
- In the distal forearm, the radial artery and nerve are sandwiched between the brachioradialis and the deep flexor muscles.
- Anteriorly, proximal to the wrist, flexor carpi radialis is the most lateral tendon visible when the wrist is brought into flexion.
- Abductor pollicis longus forms the lateral border of the anatomical snuffbox.
Golfer Elbow and Tennis Elbow
These are commonly used names for medial epicondylitis and lateral epicondylitis of the humerus respectively. As we know medial epicondyle is the origin of many flexors and lateral epicondyle is the origin site for extensors.
Overuse of the respective muscles is one of the important causes of these conditions.
Cubital tunnel syndrome
Cubital tunnel syndrome may be caused by the aponeurosis of the two heads of the flexor carpi ulnaris muscle compressing the nerve.
Pronator Teres syndrome
Pronator teres syndrome is a type of neurogenic pain caused by compression of the median nerve by the pronator teres muscle in the forearm.
Tendon transfers
In various tendon injuries of the limb or paralytic disorders resulting in loss of movements, tendon transfer surgeries using expendable working tendons are conducted to provide the maximum possible movement outcome.
Palmaris longus is completely expendable and is often used in various tendon transfers or as tendon grafts.
Tendons for transfer are decided on an individual basis depending on the tendon available and type of movement lost.
