The flexor digitorum superficialis is a muscle contained within the anterior compartment of the forearm, located on the palm side of the arm. Of the eight muscles found here, five are considered to be in the superficial layer, or close to the skin, and three are in the deep layer, or close to the bone. As this muscle is so deep in the superficial layer, it is sometimes classified as an intermediate muscle of the anterior forearm and is a flexor of the four fingers as well as the wrist joint.
Situated just beneath the flexor carpi radialis, the palmaris longus, and the flexor carpi ulnaris, all flexors of the wrist, as well as the pronator teres, a rotator of the forearm, the flexor digitorum superficialis has two heads, or sections, and therefore two points of origin. The humeroulnar head, sometimes considered itself to be two separate heads, has its origin on the medial epicondyle of the humerus, one of two rounded bony protrusions at the base of the humerus bone in the upper arm, as well as on the coronoid process of the ulna, found at the top of the ulna bone in the forearm just below the elbow joint. Both are found on the medial side of the arm, the side nearest the body when the palm is facing forward. Its radial head originates on the anterior surface of the radius bone in the forearm, just below the elbow.
From these points of origin the flexor digitorum superficialis runs down the anterior forearm and forms four tendons — one for each of the four fingers — that pass through the carpal tunnel. The carpal tunnel is a narrow channel between the carpal bones in the wrist through which these and several other tendons as well as the median nerve travel to reach their attachments in the hand. These particular tendons after passing through the carpal tunnel attach to the bases of the middle phalange bones in the four fingers on their anterior sides. As such, the main function of the flexor digitorum superficialis is to curl the fingers inward toward the palm at the large joint in the middle of the fingers, the proximal interphalangeal joint. It also assists the other forearm flexors in curling the wrist when the fingers are first curled inward.
As the flexor digitorum superficialis is involved in forearm flexion and has tendons in the carpal tunnel, overuse of this muscle can contribute to carpal tunnel syndrome. Repetitive stress on the tendons of these muscles where they pass through the carpal tunnel, as in flexing the wrist and fingers to type on a keyboard, can cause swelling, inflammation, and subsequent enlargement of these tissues. Such enlargement of the tendons can lead to compression of the median nerve, which also passes through the carpal tunnel and serves the hand, and the subsequent pain, tingling, and numbness associated with this condition.