Origin Posterior surfaces of radius and interosseous membrane Insertion Base of proximal phalanx of thumb Action Extends proximal phalanx of thumb at metacarpophalangeal 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