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Deep anterior forearm muscles

Recommended video: Anterior compartment forearm muscles [11:53]
Attachments, innervation, functions and related clinical anatomy of the muscles of the anterior compartment of the forearm.

You perhaps know a lot about your forearm already - you know that it allows you to perform your daily activities, give high fives or even have a go at some arm wrestling every once in a while. Therefore, it is quite important. However, have you ever wondered what lies underneath the skin that allows you to perform all those movements?

An important group of anatomical structures are the deep anterior forearm muscles. Lying at the ventral aspect of the forearm, deep to the flexor digitorum superficialis, they flex your wrist and finger joints. The impulse for those actions are given via the median nerve.

Key facts about the deep anterior forearm muscles
Flexor digitorum profundus Origins: proximal half of anterior surface of ulna, interosseous membrane
Insertions: palmar surfaces of distal phalanges of digits 2-5
Innervation: digits 2-3: Anterior interosseous nerve (from median nerve C8, T1);
Digits 4-5: Ulnar nerve (C8, T1)
Function: metacarpophalangeal and interphalangeal joints 2-5: Finger flexion
Flexor pollicis longus Origins: anterior surface of the radius and interosseous membrane
Insertions: base (palmar side) of the distal phalanx of the thumb
Innervation: anterior interosseous nerve (from median nerve C7, C8)
Function: metacarpophalangeal and interphalangeal joint 1: Thumb flexion,
Pronator quadratus Origins: distal end of anterior surface of the ulna
Insertions: distal end of anterior surface of the radius
Innervation: anterior interosseous nerve (from median nerve C7, C8)
Function: pronation of the forearm

This article will describe all the deep anterior forearm muscles in detail, their origins, insertions, innervations, and functions. At the end you will also find out some clinical relevant information about them, putting the learned knowledge into perspective.

Contents
  1. Anatomy and supply
    1. Flexor digitorum profundus muscle
    2. Flexor pollicis longus muscle 
    3. Pronator quadratus muscle
    4. Innervation of the deep anterior forearm muscles
  2. Functions
  3. Clinical note
  4. Sources
+ Show all

Anatomy and supply

The deep anterior forearm muscles are three muscles lying at the ventral/anterior forearm. They run deep to the flexor digitorum superficialis, very closely to the radius and ulna, and for that reason they are difficult to palpate. In detail they are:

Flexor digitorum profundus muscle

The flexor digitorum profundus originates at the proximal half of the anterior ulna and the interosseous membrane. Its four tendons run through the carpal tunnel and between the split end tendons of the flexor digitorum superficialis at the height of the middle phalanges. Distally, they insert at the palmar surface of the distal phalanges of the second to fifth fingers.

It is innervated by the anterior interosseus nerve, a branch of the median nerve (second and third finger, root values C8-T1) and ulnar nerve (fourth and fifth finger, root values C8-T1).

Flexor pollicis longus muscle 

The flexor pollicis longus has its origin at the anterior surface of the radius and the interosseous membrane of the forearm. Its tendon runs also through the carpal tunnel and inserts at the palmar surface of the distal phalanx of the thumb.

It is innervated by the anterior interosseus nerve, a branch of the median nerve (root values C7- C8).

Pronator quadratus muscle

The pronator quadratus arises from the distal anterior surface of the ulna and extends horizontally to the distal anterior surface of the radius giving the muscle a square-shaped appearance. It is the deepest muscle in the anterior forearm.

It, too, is innervated by the anterior interosseus nerve, a branch of the median nerve (C7-C8).

Innervation of the deep anterior forearm muscles

As outlined above, as with (almost) all deep anterior forearm muscles, these three muscles are supplied by the median nerve. The specific innervating branch, the anterior interosseous nerve, arises approximately 5 cm underneath the medial epicondyle of the humerus from the median nerve.

Median nerve (anterior view)

From there, it courses between the flexor digitorum profundus and flexor pollicis longus along the interosseous membrane and ends distally at the pronator quadratus.

As an exception, the flexor digitorum profundus receives a double innervation through both the median and ulnar nerves.

At the wrist, the tendons of the flexor digitorum profondus and flexor pollicis longus run through the carpal tunnel, a passage formed by the carpal bones dorsally and a tight densification of the antebrachial fascia (flexor retinaculum) anteriorly. Along with these tendons, the carpal tunnel contains the median nerve as well as the four tendons of the flexor digitorum superficialis.

Functions

The deep anterior forearm muscles are mainly responsible for flexion of the wrist and finger joints. 

  • The contraction of the flexor digitorum profundus leads to flexion of the metacarpophalangeal (MCP), proximal interphalangeal (PIP) and distal interphalangeal joints (DIP) of the second to fifth fingers. As this muscle is permanently tensed, the fingers are always slightly bent while at rest.
  • The flexor pollicis longus is responsible for flexion of the thumb at the metacarpophalangeal joint and interphalangeal joint of the first digit. Some sources also state that it may assist in opposition of the thumb, occuring primarily at the 'saddle-like' carpometacarpal joint and metacarpophalangeal joint of the thumb. Furthermore, it may assist in flexion and radial abduction the wrist joint.
  • The pronator quadratus causes pronation of the forearm, acting upon the radioulnar joints. Remember, pronation of the forearm results in the palm of the hand facing backwards or downwards. 

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