There are two groups of muscles of hand – extrinsic muscles and intrinsic muscles.

Intrinsic muscles of the hand are those muscles which are located within the hand itself, in contrast to extrinsic muscles which originate proximally in the forearm and insert into the hand by long tendons.

Extrinsic muscles are responsible for crude movements of hand whereas intrinsic muscles control fine movements.

Hand movement is enhanced, complemented and strengthened by wrist movements. Wrist muscles originate proximally and travel along extrinsic muscles of the hand and are considered together.

Extrinsic Muscles of Hand and Wrist

The extensor muscles are all extrinsic, except for the interosseous-lumbrical complex, which is involved in interphalangeal joint extension.

The extrinsic flexors consist of 3 wrist flexors and a larger group of thumb and digit flexors. These muscles are supplied by median nerve, except for the flexor carpi ulnaris and the flexor digitorum profundus to the small and ring finger, which are innervated by the ulnar nerve.

Extrinsic flexor muscles are also superficial and deep.

Flexor carpi radialis, palmaris longus, flexor carpi ulnaris and the flexor digitorum superficialis belong to superficial group whereas flexor digitorum profundus and flexor pollicis longus are deep.

Superficial Extrinsic Extensor Muscles

All of them take origin from common extensor origins at the tip of lateral condyle of the humerus. All of them are supplied by the radial nerve.

These muscles are

• Extensor carpi radialis longus.
• Extensor carpi radialis brevis
• Extensor digitorum
• Extensor digiti minimi
• Extensor carpi ulnaris

Extensor Carpi Radialis Longus

Origin

• Lower one of the lateral supracondylar ridge of the humerus
• Common extensor origin, and some from
• Lateral intermuscular septum.

Insertion

It is inserted into the dorsal surface of the base of the second metacarpal bone, on its radial side.

Nerve Supply

It is innervated by the radial nerve.

Actions of extensor carpi radialis longus are

• Extension of the wrist (along with the extensor carpi ulnaris).
• Abduction of the wrist (along with the extensor carpi radialis brevis).
• Assists movements of the digits by fixing the wrist.

Extensor Carpi Radialis Brevis

Origin

This muscle originates from common extensor origin and radial collateral ligament of the elbow

Insertion

Dorsal aspect of bases of second and third metacarpal bones.

Nerve Supply

Innervation is by posterior interosseous nerve, a branch of the radial nerve.

Actions are similar to those of extensor carpi radials longus.

The tendons of these two muscles pass through the same compartment of the extensor retinaculum and have a common synovial sheath.

Extensor Digitorum

Origin

Common extensor origin.

Insertion

Tendon of the muscle splits into four parts, one for each digit except the thumb.

The extensor digitorum and the extensor indicis [described later] pass through the same compartment of the extensor retinaculum and have a common synovial sheath.

The four tendons of the extensor digitorum emerge from under of the extensor retinaculum and fan out over the dorsum of the hand.

The tendon to the index finger is joined its medial side by the tendon of the extensor indicis, and the tendon to the little finger is joined on its medial side by the two tendons of the extensor digiti minimi.

The dorsal digital expansion is a small triangular aponeurosis covering the dorsum of the proximal phalanx with its base at the metacarpophalangeal joint. The main tendon of the extensor digitorum occupies the central part of the expansion and is separated from the metacarpophalangeal joint by a bursa.

The posterolateral corners of the extensor expansion are joined by tendons of the interossei and of a lumbrical muscle.

Deep transverse metacarpal ligament is attached at the corners.

Close to the proximal interphalangeal joint, the extensor tendon divides into a central slip and two collateral slips.

• The central slip is joined by some fibers from the margin of the expansion, crosses the proximal interphalangeal joint and is inserted on the dorsum of the base of the middle phalanx.
•  The two collateral slips are joined by the remaining thick margin of the extensor expansion. They then join each other and are inserted on the dorsum of the base of the distal phalanx.

At the metacarpophalangeal and interphalangeal joints, the extensor expansion forms the dorsal part of the fibrous capsule of the joints.

The retinacular ligaments extend from the side of the proximal phalanx, and from its fibrous flexor sheath to the margins of the extensor expansion to reach the base of the distal phalanx.

Nerve supply

Posterior interosseous nerve.

Actions

Extension of interphalangeal metacarpophalangeal and wrist joints.

Extensor Digiti Minimi

Origin

Common extensor origin.

Insertion

The tendon passes through a separate compartment of the extensor retinaculum, behind the radioulnar joint.

The tendon lies medial to the tendon of the extensor digitorum for the fifth digit.

The tendon divides into two slips which join the extensor expansion of the little finger on.

Nerve supply

Posterior interosseous nerve.

Action

Extension of the little finger at the interphalangeal and metacarpophalangeal joints.

Extensor Carpi Ulnaris

Origin

• Common extensor origin.
• Posterior border of the ulna ( by an aponeurosis common to it and the flexor carpi ulnaris and the flexor digitorum profundus).

Insertion

The tendon passes through a separate compartment of the extensor retinaculum in the groove between the head and styloid process ulna and inserts on to the medial side of the base of the fifth metacarpal bone.

Nerve Supply

Posterior interosseous nerve.

Actions

• Extension of wrist (along with the extensor carpi radialis longus brevis).
• Fixes the wrist during forceful movements of the hand.

Deep Extrinsic Extensor Muscles

These are as follows.

• Abductor pollicis longus
• Extensor pollicis longus.
• Extensor pollicis longus.
• Extensor indicis proprius

All of them arise from the radius, the ulna, and the interosseous membrane, and none of them crosses the elbow joint.

Abductor Pollicis Longus

Origin

It originates from upper parts of the posterior surfaces of the ulna and the radius and from the interosseous membrane.

The tendon usually splits into two parts. One part is attached to the lateral side of the base of the first metacarpal and other to the trapezium.  Fasciculi may be sent to the opponens pollicis, or with the abductor pollicis brevis.

Nerve Supply

Posterior interosseous nerve.

Action

Abduction and extension of the thumb at the carpometacarpal joint.

Extensor Pollicis Longus

Origin

• Posterior surface of the ulna below the origin of the abductor pollicis longus
• Interosseus membrane.

The abductor pollicis longus and the extensor pollicis brevis are deep to the superficial extensors in the upper part of the forearm. They become superficial by emerging between the extensor carpi radialis brevis and the extensor digitorum. They then cross the tendons of the extensor carpi radialis longus and brevis to reach their insertion. [see the image below]

The muscle may be absent or completely fused with the abductor pollicis longus.

Action

extension at all joints of the thumb.

Nerve Supply

Nerve supply is by the posterior interosseous nerve.

Extensor Pollicis Brevis

Origin

• Posterior surface of the radius below the origin of the abductor pollicis longus
• The interosseus membrane.

Insertion

Dorsal surface of the base of the proximal phalanx of the thumb.

Nerve Supply

Posterior interosseous nerve.

Action

Extends the proximal phalanx and metacarpal of the thumb.

Extensor Indicis

Origin

The posterior surface of the ulna below the origin of the extensor pollicis longus and from the interosseous membrane.

Insertion

The tendon joints the ulnar side of the tendon of the extensor digitorum for the index finger.

Nerve supply

Posterior interosseous nerve.

Action

Extension of the index finger.

Extensor Retinaculum

The extensor retinaculum or dorsal carpal ligament is formed by thickened fascia at the neck of the wrist. It is about 2 cm broad and holds the extensor tendons in place and prevents bowstringing of tendons at the wrist level.

[ know more about bowstringing]

It attaches laterally, to the lower part of the anterior border of the radius. Medial attachments are the styloid process of the ulna, the triquetral, and the pisiform bones.

The retinaculum sends down septa which are attached to the longitudinal ridges on the posterior surface of the lower end of radius to create six osteofascial compartments are formed on the back of the wrist [The cross-sectional image below would explain better]

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.

Flexor Carpi Radialis

Origin

Flexor carpi radialis originates from the medial epicondyle of the humerus

Insertion

Flexor carpi radialis inserts into the palmar surface of the bases of the second and third metacarpal bones. In some cases, the muscle may have additional proximal slips and distally may send insertions on the flexor retinaculum, trapezium, and the 4^th metacarpal bone.

Nerve Supply

The muscle is supplied by the median nerve.

Actions

• Flexor of the wrist (along with the flexor carpi ulnaris and the Palmaris longus).
• Abductor of the wrist (along with the extensor carpi radialis muscles)

Palmaris Longus

Origin

Palmaris longus also takes origin from medial epicondyle of the humerus.

Insertion

It inserts into distal half of flexor retinaculum and the apex of the palmar aponeurosis.

It is supplied by the median nerve.

It flexes the wrist makes the palmar aponeurosis tense.

This muscle is not significant and also absent in about 10% subjects. It is a clinical marker of the underlying median nerve at wrist and could be used for tendon transfer when available.

The palmar aponeurosis represents the distal part of the tendon of the palmaris longus.

Flexor Carpi Ulnaris

Origin

It originates from

• Humeral head from the medial epicondyle of the humerus.
• Ulnar head from the olecranon, and from the posterior border of the ulna. The ulnar nerve passes between the humeral and ulnar heads.

Insertion

The insertion is primarily into the pisiform bone, but the pull the muscle is transmitted through the pisohamate and pisometacarpal ligaments to the hook of the hamate and the base of the 5^th metacarpal bone.

Therefore, the pisiform bone is considered a sesamoid bone in the tendon of this muscle.

Nerve supply

Ulnar nerve

Actions

• Flexion of the wrist (along with the flexor carpi radialis and the Palmaris longus).
• Adduction of the wrist (along with the extensor carpi ulnaris).
• Fixes the pisiform bone during contraction of the hypothenar muscles.

Flexor Digitorum Superficialis

Origin

This is a large muscle which originates from

• Humeroulnar head from the medial epicondyle of the humerus, the ulnar collateral ligament, and a tubercle on the medial border of the coronoid process.
• Radial head arises from the anterior border of the radius up to the insertion of the pronator teres.

The tendons enter the hand by passing deep to the flexor retinaculum within the common synovial sheath. [discussed later]

The muscle ends in four tendons one each for the medial four fingers. Opposite the proximal phalanx the tendon for each digit splint into medial and lateral slip which are inserted on the corresponding sides of the middle phalanx.

At the wrist the four tendons are arranged in two pairs, the superficial pair for the middle and ring finger, and the deep pair for the index and little fingers. The tendons lie medial to the palmaris longus and lateral to the ulnar vessels and nerve.

Nerve supply

Median nerve.

Action

The flexor digitorum superficialis is the main flexor of the proximal interphalangeal joints. Secondarily, it may also flex the metacarpophalangeal and wrist joints.

Deep Extrinsic Flexor Muscles

Flexor Digitorum Profundus

Flexor digitorum profundus is the most powerful, and most bulky, the muscle of the forearm and hand. It provides the main gripping power of the hand.

Origin

The muscle takes origin from three fourths of the anterior and medial surfaces of the ulna, interosseous membrane and deep fascia of the forearm.

Insertion

The muscle fans out into four tendons (one to each of the second to the fifth digit) to the palmar base of the distal phalanx.

Flexor digitorum profundus lies deep to the superficialis, but it attaches more distally. Therefore, profundus’ tendons go through the tendons of superficialis, and end up attaching to the distal phalanx.

Nerve Supply

The medial aspect of the muscle (which flexes the 4th and 5th digit) is supplied by the ulnar nerve^.

The lateral aspect (which flexes the 2nd and 3rd digit) is innervated by the median nerve through the anterior interosseous branch.

Flexor Pollicis Longus

Origin

• Grooved anterior surface of the body of the radius, extending from immediately below the radial tuberosity and oblique line to within a short distance of the pronator quadratus
• Part of the interosseous membrane of the forearm, and generally by a fleshy slip from the medial border of the coronoid process of the ulna.

Sometimes, it gets a slip from medial epicondyle of the humerus.

Insertion

The flattened tendon passes under the flexor retinaculum of the hand through the carpal tunnel. It is lodged between the lateral head of the flexor pollicis brevis and the oblique part of the adductor pollicis and is inserted into the base of the distal phalanx of the thumb.

Actions

Flexor pollicis longus flexes interphalangeal joint of the thumb. When the thumb is fixed, it assists by flexing the wrist.

Nerve Supply

Anterior interosseous branch of the median nerve

Synovial Sheath of Flexor Tendons

The long flexor tendons of the fingers are enclosed in a common synovial sheath while passing deep to the flexor retinaculum. The sheath has a parietal layer lining the walls of the carpal tunnel, and a visceral layer closely applied to the tendons.

The synovial sheath extends upwards for 2 or 3 inches into the forearm and downwards into the palm up to the middle of the shafts of the metacarpal bones. Lower medial end is continuous with the digital synovial sheath of the little finger.

The synovial sheath of the tendon of flexor pollicis longus is usually separate. Inferiorly, it extends to the distal phalanx of the thumb superiorly reaches as far as common sheath extends.

It may communicate with the common sheath behind the retinaculum.

Distal synovial sheaths

These sheaths enclose the flexor tendons in the fingers. The digital sheath of the little finger is continuous with the ulnar bursa and that of the thumb with the radial bursa. However the distal sheaths of the index, middle and ring fingers are separate and independent.

Vincula Longa and Brevia

The vincula longa and brevia are synovial folds which transmit vessels to the tendons.

Intrinsic Muscles

Hand contains 20 muscles as follows.

Thenar muscles [4]

• Abductor pollicis brevis
• Flexor pollicis brevis
• Opponens pollicis
• Abductor pollicis

Hypothenar muscles [4]

• Plamaris brevis
• Abductor digit minimi
• Flexor digit minimi
• Oppenens digit minimi

Lumbricals [4]

Palmar interossei [4]

Dorsal interossei [4]

Thenar Muscles

Abductor Pollicis Brevis.

Origin

• Tubercle of the scaphoid.
• Tubercle of the trapezium.
• Flexor retinaculum.

Insertion

• Lateral side of the base of the proximal phalanx of the thumb.
• Some fibres are inserted into the dorsal digital expansion.

Nerve supply

Median nerve

Action

It causes abduction of the thumb at the metacarpophalangeal and carpometacarpal joints. Abduction is also associated with medial rotation.

Flexor pollicis Brevis

Origin

Superficial  head originates  from

• The tubercle of the trapezium.
• The flexor retinaculum.

The deep head arises from the trapezoid and capitates bones.

Insertion

• Lateral side of the base of the proximal phalanx.

Nerve supply

Superficial head is supplied by the median nerve and the deep head by the deep branch of the ulnar nerve.

Action

Flexion of the thumb.

Opponens Pollicis

1. Tubercle of trapezium.
2. Flexion retinaculum.

Insertion

Lateral half of the surface of the first metacarpal bone.]

Nerve supply

Median nerve

Action

Opposition of the thumb. This is a combination of flexion and medial rotation.

Adductor Pollicis

Origin

The muscle has two heads—oblique and transverse.

The oblique head arises from:

• The capitate bone.
• The bases of the 2^nd and 3^rd metacarpal bones.

The transverse head arises from the palmar protect of the third metacarpal bone.

Insertion

Medial side of the base of the proximal phalanx of the thumb. Some fibers are inserted into dorsal digital expansion.

Nerve supply

Deep branch of ulnar nerve

Action

The muscle adducts the thumb from the fixed or abducted position. The movement is used in forceful gripping.

Movements of Thumb

The movements of the thumb take place in planes at right to those of the other digits because the thumb is rotated medially through 90 degrees.

• Flexion and extension of the thumb take place in the plane of the palm
• Abduction and adduction at right angles to the plane of the palm.
• Movement of the thumb across the palm to touch the other digit is known as opposition. This movement is a combination of flexion and medial rotation.

Hypothenar Muscles

Palmaris Brevis

This muscle is superficial and lies just under the skin.

Origin

From flexor retinaculum and palmar aponeurosis.

Insertion

Skin along the medial border of the hand.

Nerve supply

Superficial branch of the ulnar nerve.

Action

It helps in gripping by marking the hypothenar eminence more prominent, and by wrinkling the skin over it.

Abductor Digiti Minimi

Origin

The muscle arises from the pisiform bone. The origin extends on to tendon of the flexor carpi ulnaris and on to the pisohamate ligament.

Insertion

Ulnar side of the base of the proximal phalanx of the little finger.

Nerve supply

Deep branch of Ulnar nerve.

Action

Abduction of little finger at the metacarpophalangeal joint.

Flexor Digiti Minimi

Origin

• Hook of the hamate bone.
• Flexor retinaculum.

Insertion

Ulnar side of the base of the proximal phalanx of the little finger.

Nerve supply

Deep branch of Ulnar nerve.

Action

Flexes the fifth metacarpal and rotates it laterally.

Opponens Pollicis

Origin

• Flexor retinaculum of the hand
• Tubercle of the trapezium.

Insertion

It passes downward and laterally and is inserted into the whole length of the metacarpal bone of the thumb on its radial side.

Nerve Supply

Median Nerve.

Function

Opposition of the thumb

Lumbricals

These are four small muscles that take origin from the tendons of the flexor digitorum profundus.

Origin

• First lumbrical – Radial side of the tendon for the index finger
• Second lumbrical – Radial side of the tendon for the middle finger.
• Third lumbrical – Contiguous sides of the tendons for the middle and ring fingers.
• Fourth lumbrical –  contiguous sides of the tendons for the ring and little fingers.

Insertion

First, second, third and fourth lumbricals pass backwards on the radial side of the second, third, fourth and fifth digits and insert into the dorsal digital expansion of the corresponding digits at metacarpophalangeal joints.

Nerve supply

• First, second lumbricals – median nerve.
• Third and fourth lumbricals – the deep branch of the ulnar nerve.

Actions

Each lumbrical muscle flexes the metacarpophalangeal joint, and extend the interphalangeal joints of the digit.

Palmar Interossei

These are four small muscles placed between the shafts of the metacarpal bones. They are numbered from lateral to the medial side.

Origin

• First palmar interosseus – Medial side of the base of the first metacarpal bone.
• Second palmar interosseus – Medial half of the palmar aspect of the shaft of the second metacarpal bone.
• Third palmar interosseus – Lateral part of the palmar aspect of the shaft of the fourth metacarpal bone.
• Fourth palmar interosseus – lateral part of the palmar aspect of the shaft of the fifth metacarpal bone.

Insertion

Each muscle is inserted into the dorsal digital expansion of one digit. It may also be attached to the base of the proximal phalanx of the same digit. The digits into which individual palmar interossei are inserted are as follows.

• First palmar interossei –  Medial side of the thumb.
• Second palmar interossei –  Medial side of the index finger.
• Third palmar interossei –  Lateral side of the fourth digit.
• Fourth Muscle – Lateral side of the fifth digit

Actions

All palmar interossei adduct the digit to which they are attached to the middle finger. In addition, they flex the digit at the interphalangeal joints.

Nerve Supply

Deep branch of the nerve.

Dorsal Interossei

Like the palmar interossei, the dorsal interossei are four small muscles placed between the metacarpal bones and are numbered from lateral to the medial side.

Origin

• First dorsal interosseus –  shafts of first and second metacarpals.
• Second dorsal interosseus –  shafts of second and third metacarpal.
• Third dorsal interosseus – shafts of third and fourth metacarpal
• Fourth dorsal interosseus shafts of fourth and fifth metacarpal

Insertion

Each muscle is inserted into the dorsal digital expansion of the digit, and into the base of the proximal phalanx muscles are inserted are as follows.

• First: Lateral side of index finger [second digit]
• Second: Lateral side of the middle finger [third digit]
• Third: Medial side of middle finger [third digit]
• Fourth: Medial side of ring finger [fourth digit]

Note that the middle finger receives one dorsal interosseus muscle on either side and the first and fifth digits do not receive any insertion. [Note the difference from palmar interossei where the middle finger does not receive any origin or insertion.]

Action

All dorsal interossei are abductors of the digits away from the line of the line of the middle finger. Movement of the middle finger to rather the medial or lateral side constitutes the abduction.

[First and fifth digits do not require dorsal interossei as they have their own abductors.]

Dorsal interossei also flex the metacarpophalangeal joint of the digit concerned and extend the interphalangeal joints.

Nerve supply

Deep branch of the ulnar nerve.

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