The extensor pollicis longus is a muscle located in the forearm. It is responsible for extending the thumb at the metacarpophalangeal and interphalangeal joints. This muscle originates from the posterior surface of the ulna and the interosseous membrane, and it inserts into the base of the distal phalanx of the thumb. This muscle plays a crucial role in hand movements, particularly in activities that require thumb extension, such as grasping and pinching. It is innervated by the radial nerve, which is essential for its function. Injuries or conditions affecting the extensor pollicis longus can lead to difficulties in thumb mobility.