A flexor is a muscle that serves to bend a body part, such as a limb. Flexor muscles are responsible for decreasing the angle between bones on two sides of a joint, as in bending the elbow or knee. Several muscles in the hands and feet are named for their function as flexors, including the flexor carpi radialis and flexor carpi ulnaris, which stretch from the 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, while the flexor digitorum superficialis is closer to the surface and acts upon the midsections of the fingers. In the foot, the flexor digitorum longus and flexor digitorum brevis originate at the tibia (shinbone) and calcaneus (heel bone), respectively, and act upon the four smaller toes.

In addition to muscles, flexor tendons are cord-like extensions that connect muscles to bones and allow for movement of the fingers and thumb. Flexor tendons run from the forearm across the wrist and palm and into the fingers, allowing for the ability to bend one or more of the joints in the hand. Injuries to the flexor tendons can cause loss of the ability to bend the fingers or thumb, and can occur from a cut on the palm side of the fingers, hand, wrist, or forearm, or from a sudden and forceful pull against the tendon. Certain health conditions, such as rheumatoid arthritis, can weaken the flexor tendons and make them more likely to tear or rupture.