Thumb flexion and extension

In contrast to the fingers, the thumb can flex at only two joints - the metacarpophalangeal joint and an interphalangeal (IP) joint, since it has only two phalanges. Flexion of the thumb takes place in a plane at an angle to the other digits. When flexed, the thumb points generally toward the 5th MP joint.

Muscles: flexor pollicis brevis flexes the MP joint only, while flexor pollicis longus flexes both the MP and IP joints, and is the only muscle to flex the IP joint.

Extension of the thumb also occurs in a plane offset from that of the fingers, but the motion has the same effect of taking the thumb posteriorly in the anatomical position.

Muscles: extensor pollicis brevis extends the MP joint only, while extensor pollicis longus extends the MP and IP joints, and is the only muscle to extend the IP joint.