Origin Posterior surface of middle 1/3 of ulna and interosseous membranee Insertion Base of distal phalanx of thumb Action Extends distal phalanx of thumb at carpometacarpal and interphalangeal joints Innervation Posterior interosseous nerve (C7 and C8), the continuation of the deep branch of the radial nerve (C7, C8) Arterial Supply Posterior interosseous artery