Flexor pollicis longus is a muscle that is found in the forearm region and is responsible for flexing the thumb. Some of the synonyms for this muscle include the long flexor of the thumb, flexor pollicis longus muscle, and the long thumb flexor. Other synonyms for this muscle include the deep flexor of the thumb, the pronator Radii teres, and brachialis muscle. The flexor pollicis longus muscle is an essential muscle in the hand and forearm and is involved in many important activities. These include gripping, holding, and manipulating objects and tools. It is important to maintain strong and healthy flexor pollicis longus muscles for optimal hand and forearm function.