Origin Posterior surfaces of ulna, radius and interosseous membrane Insertion Base of 1st metacarpal Action Abducts thumb and extends it at carpometacarpal joint Innervation Posterior interosseous nerve (C7 and C8), the continuation of the deep branch of the radial nerve (C7, C8) Arterial Supply Posterior interosseous artery Clinical Relevance De Quervain's Tenosynovitis