The forgotten forearms (Posterior compartment)

Last but not least, the muscles located at the posterior compartment of the forearms.

Superficial posterior compartment

The extensor digitorum muscle helps in the movements of the wrists and the elbows. It extends the phalanges, then the wrist, and finally the elbow. It acts principally on the proximal phalanges. It tends to separate the fingers as it extends them.

Extensor digitorum

Origin:

  1. lateral epicondyle via the CET (common extensor tendon).
  2. antebrachial fascia.

Insertion:

  1. the base of middle phalanx of each of the four fingers (central band).
  2. the base of distal phalanx of each of the four fingers (2 lateral bands).

The extensor digiti minimi is a two joint muscle. It acts as an extensor in both joints. It extends the wrist, which means it moves the back of the hand toward the back of the forearm. It also extends the little finger, which means it straightens the little finger from a fist. When the muscle moves, it forces the little finger to bend and stretch. Sudden or unexpected movement of the finger or trauma may damage the muscle. Traction to keep the little finger from moving is typically recommended to treat the injury. Sprain of this muscle is common in athletes but is not considered to be a serious injury.

Extensor digiti minimi

Origin:

  1. lateral epicondyle via the CET (common extensor tendon).
  2. antebrachial fascia.
  3. the ulnar aspect of extensor digitorum.

Insertion:

  1. the base of middle phalanx of the 5th digit (central band).
  2. the base of distal phalanx of the 5th digit (2 lateral bands).

The extensor carpi ulnaris muscle allows the wrist, or carpus, to extend and bend. It works in conjunction with the flexor carpi ulnaris muscle during the adduction of the wrist, meaning when the wrist bends toward the body’s midline. However, it is the only muscle responsible for ulnar deviation. This refers to the movement of the hand sideways in the direction of the pinky. The extensor carpi ulnaris muscle is the primary muscle used when you accelerate your motorcycle.

Extensor carpi ulnaris

Origin:

  1. 1st head – lateral epicondyle via the CET (common extensor tendon).
  2. 2nd head – the posterior body of the ulna.
  3. antebrachial fascia.

Insertion:

medial side of the base of the 5th metacarpal.

A common injury to the extensor carpi ulnaris is tennis elbow. This injury occurs in people that participate in activities requiring repetitive arm, elbow, and wrist, especially when they are tightly gripping an object. Some symptoms include pain when shaking hands or when squeezing/gripping an object. The pain worsens when a person moves their wrist with force. The pain intensifies because the extensor carpi ulnaris has an injury near the elbow area and as a person moves their arm, the muscle contracts, thus causing it to move over the lateral epicondyle of the humerus. This causes irritation to the already existing injury.

The brachioradialis flexes the forearm at the elbow. It enables flexion of the elbow joint. The muscle also assists with pronation and supination of the forearm. These two movements allow the forearm and hand to turn so that the palm faces up or down. The arms are the only part of the body with this ability. The muscle is used to stabilize the elbow during rapid flexion and extension while in a mid position, such as in hammering.

Brachioradialis

Origin:

  1. the upper lateral supracondylar ridge of the humerus (between the triceps and brachialis muscles).
  2. the lateral intermuscular septum of the humerus.

Insertion:

  1. the superior aspect of the styloid process of the radius.
  2. the lateral side of the distal 1/2 to 1/3 of the radius.
  3. antebrachial fascia.

The extensor carpi radialis longus is a long muscle that connects the outside of the elbow to the bone at the base of the first finger. It extends the wrist and abducts the hand.

Extensor carpi radialis longus

Origin:

  1. lower lateral supracondylar ridge (below the brachioradialis).
  2. the lateral intermuscular septum of the humerus.

Insertion:

the base of 2nd metacarpal.

The extensor carpi radialis brevis muscle aids in moving the hand. Specifically, it abducts and extends the hand at the wrist joint. It is an extensor, and an abductor of the hand at the wrist joint. That is, it serves to manipulate the wrist so that the hand moves away from the palm and towards the thumb.

Extensor carpi radialis brevis

Origin:

  1. lateral epicondyle via the CET (common extensor tendon).
  2. radial collateral ligament.
  3. antebrachial fascia.

Insertion:

base of 3rd metacarpal.

Deep posterior compartment

Supinator consists of two planes of fibers, between which the deep branch of the radial nerve lies. Its function is to supinate the forearm. Supinator always acts together with biceps, except when the elbow joint is extended.

Supinator

Origin:

  1. lateral epicondyle of humerus.
  2. supinator crest of ulna.
  3. radial collateral ligament.
  4. annular ligament.
  5. antebrachial fascia.

Insertion:

the proximal portion of the anteriorlateral surface of the radius

The extensor indicis extends the index finger, and by its continued action assists in extending the wrist and the mid carpal joints. Because the index finger and little finger have separate extensors, these fingers can be moved more independently than the other fingers.

Extensor indicis

Origin:

  1. the posterior surface of ulna (distal to extensor pollicis longus).
  2. interosseous membrane.
  3. antebrachial fascia.

Insertion:

the base of the middle and distal phalanx of the index finger

The abductor pollicis longus muscle is one of three muscles in the forearm that facilitate the movements of the thumb. The others are the extensor pollicis brevis and extensor pollicis longus. These three muscles, along with the extensor indicis, make up the group of muscles called the deep extensors.The abductor pollicis longus lies immediately below the supinator and is sometimes united with it. The chief action of abductor pollicis longus is to abduct the thumb. It also assists in extending and rotating the thumb.

Abductor pollicis longus muscle

Origin:

  1. posterior surfaces of ulna and radius.
  2. interosseous membrane.
  3. antebrachial fascia.

Insertion:

lateral aspect of base of 1st metacarpal

The extensor pollicis brevis muscle is located on the dorsal side of the forearm. In a close relationship to the abductor pollicis longus, the extensor pollicis brevis both extends and abducts the thumb.

Abductor pollicis brevis muscle

Origin:

  1. posterior surfaces of radius (below abductor pollicis longus).
  2. interosseous membrane.
  3. antebrachial fascia.

Insertion:

the base of proximal phalanx of thumb (often a slip inserts into extensor pollicis longus tendon)

The extensor pollicis longus extends the terminal phalanx of the thumb. When moving the thumb, the muscle uses the radial tubercle as a pulley.

Extensor pollicis longus

Origin:

  1. posterior surface of ulna.
  2. interosseous membrane.
  3. antebrachial fascia.

Insertion:

distal phalanx of the thumb.

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