A forearm flexor is any of a group of muscles situated in the anterior compartment of the forearm that is responsible for flexing the wrist joint. Eight muscles are found here, six of which are involved with wrist flexion, or the action of pulling the palm of the hand toward the elbow. Many of these also perform other actions in the forearm and hand, namely adduction and abduction, or waving the hand side to side, or flexing the fingers.
The eight muscles in the anterior compartment can be divided into superficial and deep muscles, or those making up the layer closest to the skin, and those making up the layer closest to the bone. Found in the superficial layer are the flexor carpi radialis, the palmaris longus, the flexor carpi ulnaris, and the pronator teres, which rotates the forearm palm-down but is not a forearm flexor. In the deep layer are the flexor digitorum superficialis, the flexor digitorum profundus, the flexor pollicis longus, and the pronator quadratus. This last muscle also performs pronation of the hand at the wrist and is likewise not a forearm flexor.
Of the three forearm flexors found superficially in the anterior compartment, the palmaris longus, which is centrally located on the inside of the forearm, has the sole responsibility of flexing the forearm. The flexor carpi radialis, found laterally or to the outside of the palmaris longus, is in addition to flexion involved in abduction of the wrist joint, or waving the hand toward the thumb side. To the inside of the palmaris longus is the flexor carpi ulnaris, which flexes and adducts the wrist, or waves the hand toward the side of the pinky finger.
The forearm flexors situated deep in the anterior compartment perform actions in the hand in addition to those at the wrist. Though a flexor of the wrist joint, the flexor digitorum profundus is largely responsible for flexing the four fingers and is located toward the pinky side of the forearm. Similarly, the flexor digitorum superficialis, which technically is found intermediate to the superficial and deep muscles, acts on both the fingers and the wrist. The flexor pollicis longus, located thumb-side, flexes the thumb and assists as a forearm flexor.
Overuse of these three muscles in particular, as in flexing the wrist and fingers to type on a keyboard, is often the cause of carpal tunnel syndrome. Repetitive stress on the tendons of these muscles where they pass through the wrist, through an opening between carpal bones known as the carpal tunnel, 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.