Forearm extensors (posterior compartment)
Musculi antebrachii posteriores
Also known as: extensor–supinator group, common extensor origin, posterior forearm muscles, wrist extensors
The twelve muscles of the posterior compartment of the forearm. They extend the wrist and digits, abduct the thumb and supinate the forearm, and all are supplied by the radial nerve or its posterior interosseous branch.
Overview
The posterior (extensor–supinator) compartment lies behind the radius, ulna and interosseous membrane. Several superficial muscles share the common extensor origin on the lateral epicondyle.
- Superficial layer (7): brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and anconeus (often grouped with them).
- Deep layer (5): supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, extensor indicis. The three "outcropping" thumb muscles (APL, EPB, EPL) emerge between ECRB and extensor digitorum to cross the radial side of the wrist.
Innervation: the radial nerve directly supplies brachioradialis, ECRL and anconeus (above the elbow). Its deep branch supplies ECRB and supinator, then passes through supinator to become the posterior interosseous nerve (PIN), which supplies all the rest. Blood supply is from the posterior interosseous artery, the radial artery and the recurrent interosseous arteries.
At the wrist the tendons pass under the extensor retinaculum in six synovial compartments: (1) APL + EPB; (2) ECRL + ECRB; (3) EPL, which turns around Lister's (dorsal) tubercle; (4) extensor digitorum + extensor indicis; (5) EDM; (6) ECU. Strong wrist extension by these muscles is essential for a powerful grip. It puts the long finger flexors under tension, so radial nerve palsy (wrist drop) also weakens the grip.
- Location
Posterior (extensor) compartment of the forearm.
Parts & subdivisions
- Flexes the elbow in mid-pronation. Radial nerve.
- Extends and abducts the wrist. Radial nerve.
- Extends and abducts the wrist. Deep branch of the radial nerve. Main site of tennis elbow.
- Extends digits 2–5 via the extensor expansions. PIN.
- Extends the little finger. PIN.
- Extends and adducts the wrist. PIN.
- Assists elbow extension. Radial nerve.
- Supinates the forearm. Deep branch of the radial nerve passes through it.
- Abducts the thumb at the CMC joint. PIN.
- Extends the thumb MCP joint. PIN.
- Extends the thumb IP joint. PIN.
- Extends the index finger independently. PIN.
Anatomy
- Origin
Common extensor origin on the lateral epicondyle, lateral supracondylar ridge, and the posterior surfaces of the radius, ulna and interosseous membrane.
- Insertion
Metacarpal bases (wrist extensors), the extensor expansions and phalanges (digital extensors), and the radius (supinator).
- Action
Extension of the wrist and digits; radial and ulnar deviation; supination; thumb abduction and extension.
- Innervation
Radial nerve and its deep branch / posterior interosseous nerve.
- Nerve roots
C5–C8 (mainly C7–C8)
- Arterial supply
Posterior interosseous artery, radial artery, radial recurrent and recurrent interosseous arteries.
Function
- Extend the wrist and the digits
- Supinate the forearm
- Abduct and extend the thumb
- Stabilise the wrist in extension for a strong grip
Clinical significance
Lateral epicondylitis ("tennis elbow")
Degenerative tendinopathy of the common extensor origin, mainly ECRB. It causes pain over the lateral epicondyle, made worse by resisted wrist and middle-finger extension and by gripping. It is usually self-limiting with load management.
Wrist drop
Radial nerve injury in the radial groove (mid-shaft humeral fracture, "Saturday night palsy") paralyses the wrist and finger extensors, while triceps is usually spared. A PIN lesion (e.g. radial head fracture, compression by supinator) causes finger drop with radial deviation, because ECRL is spared, and no sensory loss.
De Quervain tenosynovitis
Stenosing tenosynovitis of the 1st dorsal compartment (APL + EPB), common in new parents. Pain is produced by ulnar deviation of the wrist with the thumb held in the fist (Finkelstein / Eichhoff test).
Mnemonics
Extensor compartments
Number of tendons per compartment, radial → ulnar: 2-2-1-2-1-1 (APL/EPB · ECRL/ECRB · EPL · ED/EI · EDM · ECU).
Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.