What is the primary muscle responsible for thumb flexion?

The primary muscle responsible for thumb flexion is the flexor pollicis longus. This muscle originates from the anterior surface of the radius and the adjacent interosseous membrane and extends to the base of the distal phalanx of the thumb. Its specific anatomical placement allows it to effectively flex the thumb at both the carpometacarpal and interphalangeal joints.

In comparison, the flexor carpi radialis is primarily involved in wrist flexion and abduction, allowing for movements at the wrist rather than directly influencing thumb flexion. The flexor digitorum superficialis primarily flexes the middle phalanges of the fingers and does not directly facilitate thumb movement. Lastly, the extensor pollicis brevis is involved in thumb extension rather than flexion, making it functionally opposite to the desired action. Thus, the flexor pollicis longus is the key muscle for performing thumb flexion.