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Flexor digitorum superficialis muscle: want to learn more about it?

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Flexor digitorum superficialis muscle

Flexor digitorum superficialis (Musculus flexor digitorum superficialis)

Flexor digitorum superficialis is the largest muscle of the anterior compartment of the forearm. It belongs to the superficial flexors of the forearm, together with pronator teres, flexor carpi
radialis, flexor carpi ulnaris and palmaris longus. Some sources alternatively classify this muscle as an independent middle/intermediate layer of the anterior forearm, found between the superficial and deep groups.

Based on its origin sites, flexor digitorum superficialis is divided into two heads; a humeroulnar head and radial head. Its large muscular belly courses distally towards the wrist, where it splits into four tendons and attaches to the middle phalanges of the second through fifth digits of the hand. Due to their superficial location, these tendons can be easily palpated on the distal part of the forearm. 

The main action of this muscle is flexion of the digits 2-5 at both the metacarpophalangeal and proximal interphalangeal joints.

Key facts about the flexor digitorum muscle
Origin Humeroulnar head: Medial epicondyle of humerus, coronoid process of ulna
Radial head: Proximal half of anterior border of radius
Insertion Sides of middle phalanges of digits 2-5
Action Metacarpophalangeal and proximal interphalangeal joints 2-5: Finger flexion
Innervation Median nerve (C8, T1)
Blood supply Ulnar artery, radial artery, median artery

This article will discuss the anatomy and function of the flexor digitorum superficialis.

Origin and insertion

Flexor digitorum superficialis arises at two origin points/heads:

  • Humeroulnar head: as the name suggests, this head originates from both the medial epicondyle of humerus via the common flexor tendon, as well as the medial margin of the coronoid process of ulna. Some fibers also originate from the anterior band of ulnar collateral ligament of the elbow joint.
  • Radial head: arises from the upper two-thirds of the anterior border of the radius. It has a long linear origin extending from the radial tuberosity to the insertion of pronator teres in the distal forearm.

From these origin points, the muscular heads of the flexor digitorum superficialis courses distally across the anterior forearm. Usually, the muscle is arranged into superficial and deep layers, or strata. About halfway along the forearm, each layer gives off two tendons. The tendons of the superficial layer are directed to digits 3 and 4, whereas the tendons of the deep layer insert into digits 2 and 5. Additionally, the deep layer gives off a small slip that joins the tendon of the superficial layer directed to the digit 2. At the wrist joint, the tendons pass deep to the flexor retinaculum through the carpal tunnel, after which they diverge into two pairs.  

As the tendons of flexor digitorum superficialis enter the hand, they course superficial to the corresponding flexor digitorum profundus tendon. Upon passing the metacarpophalangeal joint, the two tendons enter the digital flexor sheath. Before reaching the proximal interphalangeal joint, each tendon of flexor digitorum superficialis bifurcates to pass around the corresponding tendon of flexor digitorum profundus.

An easy way to remember this little fact is to keep in mind the following mnemonic.

  • Superficialis Splits in two, to Permit Profundus Passing through

The split tendon of flexor digitorum superficialis reunites deep to the tendon of flexor digitorum profundus, around which it forms a loop. The flexor digitorum superficialis tendons then insert into the palmar surface of the base of the middle phalanx.

Relations

Flexor digitorum superficialis lies in the anterior compartment of the forearm lying superficial to flexor digitorum profundus and flexor pollicis longus, and deep to pronator teres, palmaris longus, flexor carpi radialis and flexor carpi ulnaris muscles. The two heads of flexor digitorum superficialis form a muscular arch, through which the median nerve and ulnar artery pass. 

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Innervation

Flexor digitorum superficialis is innervated by muscular branches of the median nerve, derived from roots C8 and T1 that arises from the medial and lateral cords of the brachial plexus. The skin that overlies the muscle is supplied by roots C6-8 and T1.

Blood supply

The primary arterial blood supply to the flexor digitorum superficialis is derived from the ulnar artery and its anterior recurrent branch. In addition to branches of the ulnar artery, the anterior and lateral surfaces of the muscle are supplied by branches of the radial artery; and its posterior surface also receives branches from the median artery.

Function

The main function of flexor digitorum superficialis is flexion of the digits 2-5 at the proximal interphalangeal and metacarpophalangeal joints. Unlike the flexor digitorum profundus, flexor digitorum superficialis has independent muscle slips for all four digits. This allows it to flex the digits individually at their proximal interphalangeal joints. In addition, flexor digitorum superficialis aids the aids flexion of the wrist.

Flexor digitorum superficialis muscle: want to learn more about it?

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“I would honestly say that Kenhub cut my study time in half.” – Read more. Kim Bengochea Kim Bengochea, Regis University, Denver

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