The flexor pollicis longus muscle is located in the lower half of the arm, from the elbow down. It is an anatomical part that is unique to humans. The muscle extends into the hand and aids with thumb flexion. It originates from the bottom portion of the radius, which is a bone in the forearm. It is bordered by the supinator muscle, which also extends from the radius, at the top. The muscle then runs down the side of the forearm and attaches to the interosseus membrane of the forearm, which serves as the base of attachment for other muscles. The flexor pollicis longus traverses the hand as a tendon. This tendon travels through the carpal tunnel, the point of connection between the palm and forearm, and into the thumb. It is located between two muscles that move and flex the thumb, the flexor pollicis brevis and the abductor pollicis. The end of the flexor pollicis longus is found at the tip of the thumb. If the tendon in the flexor pollicis longus muscle suffers damage or injury, recovery may take longer than other hand injuries due to the lack of blood flow to tendons.