Muscles of the Forearm

1. Introduction
2. Anterior Compartment
   
   • Superficial Layer
   • Intermediate Layer
   • Deep Layer
3. Posterior Compartment
   
   • Superficial Layer
   • Deep Layer
4. Quiz

There are many muscles in the forearm which act at the elbow or the wrist. There are anterior and posterior forearm muscles that are split into separate compartments:

• Anterior Superficial - innervated by the median and ulnar nerves. This compartment contains pronator teres, flexor carpi ulnaris, flexor carpi radialis and palmaris longus.
• Anterior Intermediate - innervated by the median nerve. This compartment contains flexor digitorum superficialis.
• Anterior Deep - mainly innervated by the median nerve. This compartment contains flexor digitorum profundus, pronator quadratus and flexor pollicis longus. 
• Posterior Superficial - innervated by the radial nerve. This compartment contains brachioradialis, extensor carpi radialis longus and brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, anconeus.
• Posterior Deep - innervated by the radial nerve. This compartment contains supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, extensor indicis proprius.

Introduction

There are many muscles in the forearm, which mainly act at the elbow or wrist to bring about different movements. They can be categorised into anterior and posterior compartments, and can be challenging to learn. 

It is also very important when studying muscles that you are comfortable with anatomical terminology, as a lot is used in these articles. For more information, please check out our Anatomical Terminology Article. 

Anterior Compartment

The anterior compartment can be split into three different layers; superficial, intermediate and deep. The muscles in the anterior compartment are all innervated by the median nerve, except for flexor carpi ulnaris and the ulnar (medial) half of flexor digitorum profundus which are innervated by the ulnar nerve. 

Superficial Layer

There are four muscles in the superficial layer. They all originate from the medial epicondyle of the humerus:

Pronator Teres

This muscle attaches to the shaft of the radius and acts to pronate the forearm. Pronator teres is the most lateral of the four superficial anterior muscles.

Flexor Carpi Radialis 

This muscle attaches to metacarpals II and III. It acts to flex at the wrist whilst also contributing to abduction. It lies immediately medial to pronator teres, with palmaris longus next medially and flexor carpi ulnaris most medial. 

Palmaris Longus 

This muscle attaches to the flexor retinaculum and acts to flex at the wrist. This is the only muscle in this layer that has a name that does not directly describe its action, but it attaches to the 'palm' through the flexor retinaculum, and this means it must flex the wrist. 

Flexor Carpi Ulnaris 

This muscle attaches to the pisiform bone in the hand. It contributes to both wrist flexion and adduction. It is the most medial of the muscles in this layer. 

This layer of muscles can be visualised if the right hand is placed on the anterior side of the left arm, with the palm on the left medial epicondyle and the fingers pointing to the hand. The first finger illustrates the path of pronator teres, the middle finger the path of flexor carpi radialis, the ring finger the path of palmaris longus and the little finger the path of flexor carpi ulnaris. 

Pronator teres, flexor carpi radialis and palmaris longus are all innervated by the median nerve. Flexor carpi ulnaris is innervated by the ulnar nerve (only one with ulnar in the name). 

Diagram - This illustration highlights the muscles of the superficial layer. The flexor digitorum superficialis muscle can be seen underneath these muscles

Creative commons source by CFCF, edited by Dr. Sachin Sudhakaran [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Intermediate Layer

The intermediate layer is straightforward; there is only one muscle:

Flexor Digitorum Superficialis (FDS) 

This muscle originates from the medial epicondyle of the humerus and also from the radius. It then splits into four tendons, which each attach to the middle phalanx of a finger. It acts to flex the fingers at the metacarpophalangeal joints (MCPJs) and the proximal interphalangeal joints (PIPJs). It also contributes to wrist flexion. It's also important to note that even though it has 'superficial' in the name, it is in the intermediate layer on the anterior forearm. 

Deep Layer

The deep layer has three muscles to know:

Flexor Digitorum Profundus (FDP) 

This muscle originates from the ulna and, like FDS, splits into four tendons. These tendons, however, attach to the distal phalanx of each finger, allowing flexion of the distal IPJs  as well as the MCPJs. 

FDP has a slightly confusing innervation: the part of the muscle supplying the index and middle fingers is innervated by the median nerve (via the anterior interosseous branch), while the part supplying the ring and little fingers is innervated by the ulnar nerve. This is an important consideration when thinking about nerve lesions affecting finger flexion. 

Flexor Pollicis Longus (FPL) 

This muscle originates from the radius and attaches to the distal phalanx of the thumb. This muscle allows flexion of the IPJ and MCPJ of the thumb. 

Pronator Quadratus 

This muscle is found deep to the tendons of FDP and FPL, and lies in the distal forearm across the distal radioulnar joint. It originates from the ulna and attaches to the radius. This allows it to pronate the forearm. 

Diagram - This illustration highlights the muscles of the deep layer in the anterior compartment of the forearm

Creative commons source by CFCF, edited by Dr. Sachin Sudhakaran [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Preview the Muscles of the Forearm quiz