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The deep flexors 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 flexors of the forearm. 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
Flexor digitorum profundus Origin: proximal half of the anterior ulna and the interosseous membrane
Insertion: palmar side of the terminal phalanges from second to fifth finger
Innervation: median and ulnar nerves
Function: flexion of the metacarpophalangeal joints (MCP), proximal interphalangeal (PIP), distal interphalangeal joints from second to fifth finger
Flexor pollicis longus Origin: anterior side of the radius and interosseous membrane
Insertion: palmar side of the distal phalanx of the thumb
Innervation: median nerve
Function: flexion of the thumb at the MCP and interphalangeal joint of the first digit; opposition of the thumb
Pronator quadratus Origin: distal end of the ulna
Insertion: distal end of the radius
Innervation: median nerve
Function: pulls the radius medially - pronation
Clinical importance Anterior interosseous syndrome (Kiloh-Nevin syndrome I)

This article will describe all the deep flexors of the forearm in detail, together with 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.

Anatomy and supply

The deep flexors of the forearm 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

This muscle 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 side of the end phalanges of the second to fifth finger.

It is innervated by the median nerve (second and third finger, root values C8-T1) and ulnar nerve (fourth and fifth finger, root values C7-T1).

Flexor pollicis longus muscle 

The FPL has its origin at the anterior side of the radius and the interosseous membrane, sometimes also at the medial epicondyle of the humerus. Its tendon runs also through the carpal tunnel and inserts at the palmar side of the distal phalanx of the thumb.

It is innervated by the median nerve (root values C8- T1).

Pronator quadratus muscle

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

It is innervated by the median nerve (C8-T1).

Innervation of the deep flexors

As outlined above, as with (almost) all flexors of the forearm, 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

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.

Recommended video: Deep flexors of the forearm
Origins, insertions, innervation and functions of the deep flexors of the forearm.

Functions

The deep flexors 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 well as the wrist joint. 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 joing of the first digit, and opposition of the thumb, which occurs primarily at the 'saddle-like' carpometacarpal joint and metacarpophalangeal joint of the thumb. Furthermore, it flexes and radially abducts the wrist joint.
  • The pronator quadratus pulls the radius medially, thus causing a pronation at the radioulnar joint.

Clinical note

The deep flexors of the forearm can be paralyzed through a lesion of the anterior interosseous nerve (anterior interosseous syndrome or Kiloh-Nevin syndrome).

Common causes are an entrapment by the superficial flexors of the forearm (e.g. through hypertrophy) and accessory muscles, or more rarely, follwoing trauma to the arm/forearm (e.g. a fracture or elbow dislocation).

The affected patients complain about pain in the forearm and hand weakness. The most characteristic sign though is the inability of forming the “okay” sign with the fingers (pinch sign). This happens due to the fact that the flexor digitorum profundus and flexor pollicis longus are the only muscles which are able to bend the fingers at their distal interphalangeal joints.

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Show references

References:

  • M. Schünke/E. Schulte/U. Schumacher: Prometheus – LernAtlas der Anatomie – Allgemeine Anatomie und Bewegungssystem, 2nd edition, Thieme Verlag (2007), p. 308-309
  • J. E. Muscolino: The muscular system manual – The skeletal muscles of the human body, 2nd edition, Elsevier Mosby (2005), p. 592-600
  • R. H. Whitaker/N. R. Borley: Anatomiekompass, 2nd edition, Thieme Verlag (2003), p. 132
  • J. Heisel: Neurologische Differenzialdiagnostik, Thieme Verlag (2007), p. 131-132

Author:

  • Achudhan Karunaharamoorthy

Illustrators:

  • First Illustration Gallery - Yousun Koh
  • Second Illustration Gallery - Yousun Koh
  • Third Illustration Gallery - Yousun Koh, National Library of Medicine
  • Median nerve - ventral view - Begoña Rodriguez
© Unless stated otherwise, all content, including illustrations are exclusive property of Kenhub GmbH, and are protected by German and international copyright laws. All rights reserved.

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