Wrist extension is mediated by which nerve and muscles?

• A. C6, C7, C8, Radial nerve - extensor carpi radialis longus and brevis, extensor carpi ulnaris
• B. C5, C6 - deltoid
• C. C7, C8 - flexor carpi ulnaris
• D. C8, T1 - first dorsal interosseous

Answer: (A)

Wrist extension relies on the posterior forearm extensor muscles, all innervated by the radial nerve with roots around C6 to C8. The primary muscles producing this action are the extensor carpi radialis longus and brevis, which extend the wrist (and help with radial deviation), and the extensor carpi ulnaris, which extends the wrist toward the ulnar side. The radial nerve fibers supplying these muscles come from C6–C8, making this muscle–nerve–root combination the best match for wrist extension.

The deltoid is for shoulder abduction and is innervated by the axillary nerve (C5–C6), not involved in extending the wrist. The flexor carpi ulnaris acts to flex and adduct the wrist and is innervated by the ulnar nerve (C8–T1), not extend the wrist. The first dorsal interosseous is a hand muscle that abducts the index finger and is innervated by the deep branch of the ulnar nerve (C8–T1), also not responsible for wrist extension.