flexor muscle, any of the muscles that decrease the angle between bones on two sides of a joint, as in bending the elbow or knee. Several of the muscles of the hands and feet are named for this function. The flexor carpi radialis and flexor carpi ulnaris stretch from the humerus (upper-arm bone) along the inside of the forearm to the metacarpal bones of the hand and flex the wrist. The flexor digitorum profundus is a deep muscle that originates at the ulna (bone of the forearm) and acts to bend the fingers near their tips. The flexor digitorum superficialis is closer to the surface; it originates at two points, one at the junction of the humerus and ulna and the other along the radius (bone of the forearm), and acts upon the midsections of the fingers. Also in the hand are the flexor pollicis longus and flexor pollicis brevis, long and short flexors of the thumb, originating in the forearm and base of the hand, respectively. The flexor digiti minimi brevis manus acts upon the little finger.

In the foot are the flexor digitorum longus and flexor digitorum brevis, originating at the tibia (shinbone) and calcaneus (heel bone), respectively, and acting upon the four smaller toes. The flexor hallucis longus and flexor hallucis brevis originate in the calf and near the heel, respectively, and flex the great toe. The flexor digiti minimi brevis pedis acts upon the smallest toe. Compare extensor muscle.