Learning Objective
By the end of this session, students should be able to identify the muscles of the posterior forearm, differentiate between the superficial and deep layers, and describe their attachments, actions, innervation, and key clinical correlations, with particular emphasis on radial nerve function and injury patterns.
The posterior compartment of the forearm contains the extensor muscles, whose primary function is extension of the wrist and fingers. All muscles in this compartment are innervated by the radial nerve or its branches.
These muscles are arranged into two layers:
- Superficial layer
- Deep layer
The two layers are separated by a layer of deep fascia. Understanding this organization is essential for both anatomical localization and clinical correlation.
Superficial Layer of the Posterior Forearm
The superficial layer consists of seven muscles, most of which share a common tendinous origin at the lateral epicondyle of the humerus—a key concept in lateral epicondylitis.
Brachioradialis
The brachioradialis is a functional paradox. Although it is innervated by the radial nerve and lies in the extensor compartment, it primarily acts as a flexor of the elbow.
- Key Feature: Most prominent when the forearm is semi-pronated and flexed against resistance.
- Neurovascular Relation: The radial nerve and radial artery pass deep to this muscle in the distal forearm.
Attachments
- Origin: Lateral supracondylar ridge of the humerus
- Insertion: Distal radius, proximal to the radial styloid
Actions
- Flexion at the elbow
- Assists in supination at the proximal radioulnar joint
Innervation
- Radial nerve
Extensor Carpi Radialis Longus & Brevis (ECRL, ECRB)
Located on the lateral aspect of the posterior forearm, these muscles contribute to both extension and abduction (radial deviation) of the wrist.
Attachments
- ECRL: Lateral supracondylar ridge
- ECRB: Lateral epicondyle
- Insertion: Bases of metacarpals II and III
Actions
- Wrist extension
- Wrist abduction
Innervation
- Radial nerve
Extensor Digitorum Communis
This is the primary extensor of digits 2–5.
Clinical Testing: Forearm pronated, fingers extended against resistance.
Attachments
- Origin: Lateral epicondyle
- Insertion: Extensor hoods of digits 2–5
Actions
- Extension at MCP and IP joints
Innervation
- Radial nerve (deep branch)
Extensor Digiti Minimi
Often partially fused with the extensor digitorum, this muscle provides independent extension of the little finger.
Attachments
- Origin: Lateral epicondyle
- Insertion: Extensor hood of the 5th digit
Actions
- Extension of the little finger
- Assists wrist extension
Innervation
- Radial nerve (deep branch)
Extensor Carpi Ulnaris
Positioned on the medial side of the posterior forearm, allowing ulnar deviation.
Attachments
- Origin: Lateral epicondyle and posterior ulna
- Insertion: Base of the 5th metacarpal
Actions
- Wrist extension
- Wrist adduction (ulnar deviation)
Innervation
- Radial nerve (deep branch)
Anconeus
A small triangular muscle located posterior to the elbow, often blending with the triceps.
Attachments
- Origin: Lateral epicondyle
- Insertion: Olecranon and proximal ulna
Actions
- Assists elbow extension
- Stabilizes the elbow
- Abducts the ulna during pronation
Innervation
- Radial nerve
Clinical Correlation: Lateral Epicondylitis (Tennis Elbow)
- Caused by the overuse of superficial extensor muscles
- Results in inflammation of the common extensor tendon
- Peak incidence: 40–50 years
- Pain localized to the lateral epicondyle
Deep Layer of the Posterior Forearm
The deep compartment contains five muscles, most of which act on the thumb and index finger. All are innervated by the posterior interosseous nerve (except the supinator).
Supinator
Located on the floor of the cubital fossa, with the deep branch of the radial nerve passing between its two heads.
Attachments
- Origin: Lateral epicondyle and posterior ulna
- Insertion: Posterior surface of the radius
Action
- Supination of the forearm
Innervation
- Radial nerve (deep branch)
Abductor Pollicis Longus
Forms part of the lateral border of the anatomical snuffbox.
Attachments
- Origin: Posterior radius, ulna, interosseous membrane
- Insertion: Base of metacarpal I
Action
- Thumb abduction
Innervation
- Posterior interosseous nerve
Extensor Pollicis Brevis
Deep and medial to APL; also contributes to the snuffbox.
Attachments
- Origin: Posterior radius and interosseous membrane
- Insertion: Proximal phalanx of thumb
Action
- Extension at the MCP and CMC joints
Innervation
- Posterior interosseous nerve
Extensor Pollicis Longus
Its tendon passes around Lister’s tubercle, acting as a pulley.
Attachments
- Origin: Posterior ulna and interosseous membrane
- Insertion: Distal phalanx of thumb
Action
- Extension of all thumb joints
Innervation
- Posterior interosseous nerve
Extensor Indicis Proprius
Allows independent extension of the index finger.
Attachments
- Origin: Posterior ulna
- Insertion: Extensor hood of the index finger
Action
- Index finger extension
Innervation
- Posterior interosseous nerve
Clinical Correlation: Wrist Drop
Wrist drop results from radial nerve injury proximal to the elbow, commonly due to:
- Axillary compression or humeral neck fractures
- Mid-shaft humeral fractures (radial groove)
Pathophysiology
- Paralysis of wrist extensors
- Unopposed wrist flexion by median-innervated flexors
- Characteristic flexed wrist posture
