The flexor digitorum profundus is a deep muscle of the human forearm. It is responsible for flexing the fingers, meaning that it curls the four fingers inward from an extended position. Situated in the palm side of the lower arm, it is one of eight muscles found here. These muscles are typically classified as superficial, or lying closer to the skin, or deep, or lying closer to the bone. The flexor digitorum profundus is found in the deep layer alongside the flexor pollicis longus, which flexes the thumb, and the pronator quadratus, which rotates the forearm palm-down.
Beginning at the elbow and crossing the wrist joint to attach to the finger bones, the flexor digitorum profundus is the largest of the deep forearm muscles. It is found on the pinky side of the anterior forearm and runs parallel to the flexor pollicis longus, which is found on the thumb side. This muscle finds its origins starting just below the elbow joint, along the majority of the body of the ulna bone, particularly on its anterior or front side and on its medial or pinky side. It also arises from the interosseous membrane, which separates the forearm into anterior and posterior compartments, and from the deep fascia of the forearm, a layer of fibrous tissue that encases the muscles of the forearm like a casing on a sausage.
From its origins on the ulna, the flexor digitorum profundus crosses the wrist via elongated tendons, visible on the inside of the forearm when flexing the wrist. Along with the tendons of the flexor pollicis longus and the flexor digitorum superficialis, an intermediate muscle found just above it, these tendons pass through the carpal tunnel and insert along the phalanges, or finger bones. Specifically, the flexor digitorum profundus attaches on the anterior or palmar side of the distal phalanges, the bones in the tips of the fingers.
As it crosses all of the finger joints as well as the joints of the hands and wrist, this muscle acts as a flexor of each joint. Its primary function is to curl the bones of the four fingers at the proximal and distal interphalangeal joints, but it also flexes the metacarpophalangeal joints, as in making a fist, as well as the wrist joint, as in curling the hand inward toward the elbow. Performing these actions simultaneously and repetitively, as in typing on a keyboard, is often the cause of carpal tunnel syndrome.
Classified as a repetitive stress injury (RSI), carpal tunnel syndrome is the result of frequent and prolonged stress on the tendons of the flexor digitorum profundus and other two muscles that pass through the carpal tunnel in the wrist, an opening between the carpal bones through which the median nerve passes. Overuse of these muscles through repetitive movements like typing can cause swelling, inflammation, and subsequent enlargement of the tendons. Such enlargement of the tendons can lead to compression of the median nerve, which serves the hand, and the subsequent pain, tingling, and numbness associated with this condition.