Region · Upper limb
Muscles of the forearm: flexor and extensor compartments
The muscles of the forearm form an anterior flexor group and a posterior extensor group, each in layers. Learn how they move the wrist, the fingers and the thumb.
The forearm has two muscle compartments: an anterior compartment of flexors and pronators on the palm side, and a posterior compartment of extensors and supinators on the back. Most of these muscles start near the elbow and become long tendons at the wrist, which is why the forearm is fleshy near the elbow and slim near the hand.
At a glance
| Muscle | Origin | Insertion | Action |
|---|---|---|---|
| Brachioradialis | Humerus | Radius | Flexes the forearm at the elbow |
| Extensor carpi radialis longus | Humerus | Second metacarpal bone | Extends and abducts the wrist |
| Extensor carpi radialis brevis | Humerus | Third metacarpal bone | Extends and abducts the wrist |
| Extensor digitorum | Humerus | Middle phalanges of fingers 2–5, Distal phalanges of fingers 2–5 | Extends the fingers and the wrist |
| Extensor carpi ulnaris | Humerus, Ulna | Fifth metacarpal bone | Extends and adducts the wrist |
| Abductor pollicis longus | Radius | First metacarpal bone | Abduction |
| Extensor pollicis brevis | Radius | Proximal phalanx of thumb | Extension of the extremities |
| Extensor pollicis longus | Ulna | Distal phalanx of thumb | Extension of the extremities |
| Extensor indicis | Ulna | Distal phalanx of index finger | Extension of the extremities |
| Pronator teres | Humerus, Ulna | Radius | Pronates the forearm; flexes the elbow |
| Flexor carpi radialis | Humerus | Second metacarpal bone, Third metacarpal bone | Flexes and abducts the wrist |
| Flexor carpi ulnaris | Humerus, Ulna | Pisiform bone, Hamate bone, Fifth metacarpal bone | Flexes and adducts the wrist |
| Flexor digitorum superficialis | Humerus, Ulna, Radius | Middle phalanges of fingers 2–5 | Flexes the middle phalanges of the fingers; flexes the wrist |
| Flexor digitorum profundus | Ulna, Interosseous membrane | Distal phalanges of fingers 2–5 | Flexes the distal phalanges of the fingers; flexes the wrist |
| Flexor pollicis longus | Radius, Ulna | Distal phalanx of thumb | Finger flexion |
| Pronator quadratus | Ulna | Radius | Pronates the forearm |
From the anatomical data behind Anatomy Atlas 3D (Z-Anatomy, CC BY-SA 4.0, reviewed). Names link to the detailed page when there is one.
How they are organized
The radius and the ulna, joined by the interosseous membrane, divide the forearm into front and back. This page covers 16 muscles: seven at the front and nine at the back.
Anterior compartment: flexors and pronators
Many of the front muscles share a common flexor origin on the medial epicondyle of the humerus, the bump on the inner side of the elbow. They form three layers:
- Superficial layer: the pronator teres, the flexor carpi radialis and the flexor carpi ulnaris. A fourth, the palmaris longus, sits between the two wrist flexors in most people, but it is absent on one or both sides in a fair number of people.
- Intermediate layer: the flexor digitorum superficialis, a broad muscle whose four tendons go to the fingers.
- Deep layer: the flexor digitorum profundus, the flexor pollicis longus (the long flexor of the thumb) and the pronator quadratus, a small square muscle just above the wrist.
Most of the anterior compartment is supplied by the median nerve. The flexor carpi ulnaris and the half of the flexor digitorum profundus that serves the ring and little fingers are supplied by the ulnar nerve.
Posterior compartment: extensors
Many of the back muscles share a common extensor origin on the lateral epicondyle of the humerus. They form two layers:
- Superficial layer: the brachioradialis, the extensor carpi radialis longus and brevis, the extensor digitorum and the extensor carpi ulnaris. The small extensor digiti minimi and anconeus also belong here.
- Deep layer: three thumb muscles, the abductor pollicis longus, the extensor pollicis brevis and the extensor pollicis longus, plus the extensor indicis for the index finger. The supinator also lies in this layer, wrapped around the upper radius.
The whole posterior compartment is supplied by the radial nerve and its deep branch.
What they do together
- Wrist. The wrist has flexors and extensors on both sides: the flexor carpi radialis and the two extensor carpi radialis muscles on the thumb side, the flexor and extensor carpi ulnaris on the little-finger side. The flexors bend the wrist, the extensors straighten it, and the muscles of one side, working together, tilt the hand toward the thumb or the little finger.
- Fingers. The flexor digitorum superficialis bends the fingers at the middle joints and the profundus at the fingertips. The extensor digitorum straightens all four fingers; the extensor indicis lets the index finger point on its own.
- Thumb. The flexor pollicis longus bends the tip of the thumb; the three deep extensor muscles pull it back and out.
- Turning the forearm. The pronator teres and pronator quadratus turn the palm down. Supination, palm up, comes mainly from the supinator and the biceps brachii.
- Elbow. The brachioradialis flexes the elbow, most strongly with the thumb pointing up. It is a flexor even though it sits with the extensors and shares their radial nerve.
A strong grip needs both compartments: the flexors close the fingers while the wrist extensors hold the wrist slightly back, which gives the finger flexors their best length.
How to learn them
- The hand trick for the superficial flexors. Put the base of your opposite thumb on the bump on the inner side of the elbow (the medial epicondyle) and lay your fingers, spread apart, down the front of the forearm. From the thumb side, your index, middle, ring and little fingers lie roughly over the pronator teres, flexor carpi radialis, palmaris longus and flexor carpi ulnaris.
- Superficialis splits, profundus passes. Each superficialis tendon splits in two near the finger, and the profundus tendon passes through the gap to reach the fingertip.
- The anatomical snuffbox. On the back of the wrist, below the thumb, the tendons of the abductor pollicis longus and extensor pollicis brevis form one edge and the extensor pollicis longus the other. Lift your thumb and you can see it.
- Nerves by compartment: median at the front (with an ulnar exception), radial at the back.
Exams often ask for the layers of the anterior compartment in order, or for the tendons that border the snuffbox. In the app, peel the superficial flexors to reveal the deep layer, isolate a muscle to follow its tendon into the hand, and use the upper limb quiz to find each muscle by tapping it on the 3D body.
Frequently asked questions
How many layers do the forearm flexors have?
Three: superficial, intermediate (the flexor digitorum superficialis alone) and deep.
Is the brachioradialis a flexor or an extensor?
It flexes the elbow, but it belongs to the posterior compartment and is supplied by the radial nerve, like the extensors.
Does everyone have a palmaris longus?
No. It is missing on one or both sides in some people, which is one of the best-known muscle variations.
Which nerve supplies the forearm extensors?
The radial nerve, through its deep branch.
