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Posterior Forearm Muscles Anatomy
Posterior Forearm Muscles Anatomy
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The muscles in the posterior compartment of the forearm are called the extensor muscles. Their main job is to extend the wrist, fingers, and thumb, and help stabilize the forearm during hand motions.
All of these muscles are controlled by the radial nerve and its branches.
These muscles are arranged in two layers:
Superficial layer - closer to the skin.
Deep layer - closer to the bones.
The superficial layer contains six muscles, flexes the elbow (brachioradialis), and extends as well as moves the wrist.
The deep layer contains five muscles, supinates the forearm (supinator), and moves the thumb (abduction and extension).
A layer of fascia separates the superficial and deep muscles.
Superficial Muscles
Superficial Muscles
The superficial group has seven main muscles: Brachioradialis, Extensor carpi radialis longus, Extensor carpi radialis brevis, Extensor digitorum, Extensor digiti minimi, Extensor carpi ulnaris, and Anconeus.
Most of these muscles start from the lateral epicondyle of the humerus (a bony bump on the outer elbow) using a common extensor tendon.
Together, they help with:
Flexion at the elbow.
Extension, abduction, and adduction at the wrist.
Extension of the fingers (digits 2–5).
Brachioradialis
Brachioradialis
The brachioradialis flexes the elbow, especially when the forearm is half-turned. Although it’s in the extensor compartment and innervated by the radial nerve, it acts as a flexor.
The brachioradialis flexes the elbow, especially when the forearm is half-turned. Although it’s in the extensor compartment and innervated by the radial nerve, it acts as a flexor.
Origin: Lateral supracondylar ridge of the humerus.
Insertion: Near the styloid process of the radius.
Innervation: Radial nerve.
Extensor Carpi Radialis Longus
Extensor Carpi Radialis Longus
The extensor carpi radialis longus extends and abducts (moves outward) the wrist.
The extensor carpi radialis longus extends and abducts (moves outward) the wrist.
Origin: Lateral supracondylar ridge of the humerus.
Insertion: Base of the 2nd metacarpal.
Innervation: Radial nerve.
Extensor Carpi Radialis Brevis
Extensor Carpi Radialis Brevis
The extensor carpi radialis brevis extends and abducts the wrist.
The extensor carpi radialis brevis extends and abducts the wrist.
Origin: Lateral epicondyle via the common extensor tendon.
Insertion: Base of the 3rd metacarpal.
Innervation: Deep branch of the radial nerve.
Anconeus
Anconeus
The anconeus assists the triceps in extending the elbow and stabilizing the joint.
The anconeus assists the triceps in extending the elbow and stabilizing the joint.
Origin: Lateral epicondyle.
Insertion: Olecranon of the ulna.
Innervation: Radial nerve.
Extensor Digitorum
Extensor Digitorum
The extensor digitorum is the main extensor of the fingers and also helps extend the wrist.
The extensor digitorum is the main extensor of the fingers and also helps extend the wrist.
Origin: Lateral epicondyle.
Insertion: Extensor hoods of digits 2–5.
Innervation: Deep branch of the radial nerve.
Extensor Digiti Minimi
Extensor Digiti Minimi
The extensor digiti minimi extends the little finger and helps extend the wrist.
The extensor digiti minimi extends the little finger and helps extend the wrist.
Origin: Lateral epicondyle.
Insertion: Extensor hood of the little finger (digit 5).
Innervation: Deep branch of the radial nerve.
Extensor Carpi Ulnaris
Extensor Carpi Ulnaris
The extensor carpi ulnaris extends and adducts (moves inward) the wrist.
Origin: Lateral epicondyle and posterior border of the ulna.
Insertion: Base of the 5th metacarpal.
Innervation: Deep branch of the radial nerve.
Quick Reference Table: Muscle Name, Origin, Insertion & Innervation
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Deep Muscles
Deep Muscles
The deep layer has five muscles: the Supinator, Abductor pollicis longus, Extensor pollicis brevis, Extensor pollicis longus, and Extensor indicis.
All are supplied by the posterior interosseous branch of the radial nerve.
Except for the supinator, these muscles mainly move the thumb and index finger.
Abductor Pollicis Longus
Abductor Pollicis Longus
The abductor pollicis longus is located just below the supinator. It has a long tendon that travels toward the base of the thumb and forms the lateral border of the anatomical snuffbox, a small triangular depression on the thumb’s dorsal side.
Origin: Posterior surfaces of the radius, ulna, and interosseous membrane.
Insertion: Base of the 1st metacarpal.
Innervation: Posterior interosseous nerve.
Extensor Pollicis Longus
Extensor Pollicis Longus
The extensor pollicis longus is quite large and has a long tendon that travels around Lister’s tubercle, a bony bump on the radius that acts as a pulley to increase its mechanical leverage.
The extensor pollicis longus is quite large and has a long tendon that travels around Lister’s tubercle, a bony bump on the radius that acts as a pulley to increase its mechanical leverage.
Its tendon forms the medial (ulnar) border of the anatomical snuffbox.
Origin: Posterior ulna and interosseous membrane.
Insertion: Base of the thumb’s distal phalanx.
Innervation: Posterior interosseous nerve.
Supinator
Supinator
The supinator lies deep in the upper (proximal) forearm. Supinates the forearm, meaning it rotates the radius so the palm faces upward. Works with biceps brachii during strong or fast supination (like turning a doorknob). It has two heads:
A superficial head from the lateral epicondyle.
A deep head from the supinator crest of the ulna.
The deep branch of the radial nerve passes between these two heads, forming a tunnel called the Arcade of Frohse, a common site of nerve compression.
Origin: Lateral epicondyle and supinator crest of the ulna.
Insertion: Upper third of the radius (wraps around it).
Innervation: Deep branch of the radial nerve.
Extensor Indicis
Extensor Indicis
The extensor indicis lies next to the extensor pollicis longus in the distal forearm. Its tendon joins the extensor hood of the index finger, medial to the tendon of extensor digitorum.
It extends the index finger independently of the other fingers (this allows pointing or delicate finger movements).
Origin: Posterior ulna and interosseous membrane
Insertion: Extensor expansion of the index finger
Innervation: Posterior interosseous nerve.
Extensor Pollicis Brevis
Extensor Pollicis Brevis
The extensor pollicis brevis extends the thumb at its base joint and helps abduct the wrist.
Origin: Posterior radius and interosseous membrane.
Insertion: Base of the thumb’s proximal phalanx.
Innervation: Posterior interosseous nerve.
Need a recap? Take a look at the Posterior Forearm Muscles anatomy notes here.
Need a recap? Take a look at the Posterior Forearm Muscles anatomy notes here.
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References
References
Gray, H. (2009). Anatomy of the human body, part 2 (LibriVox Volunteers, Narr.) [Audiobook]. LibriVox. https://librivox.org/anatomy-of-the-human-body-part-2-by-henry-gray/ (Original work published 1858)
J Gordon Betts, Desaix, P., Johnson, E., Johnson, J. E., Korol, O., Kruse, D., Poe, B., Wise, J., Womble, M. D., & Young, K. A. (2013). Anatomy & physiology. Openstax College, Rice University. https://openstax.org/details/books/anatomy-and-physiology
Based on OpenStax, Anatomy and Physiology (2013), licensed under CC BY 4.0.
Access for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.
Content paraphrased; adaptations were made.
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