Flexor digitorum superficialis muscle

Flexor digitorum superficialis (flexor digitorum sublimis) is an extrinsic flexor muscle of the fingers at the proximal interphalangeal joints. The bulk of the muscle is in the intermediate layer of the anterior compartment of the forearm.

Origin and Insertion
The muscle has two classically described heads - the humeroulnar and radial - and it is between these heads that the median nerve and ulnar artery pass.

Four long tendons come off this muscle near the wrist and travel through the carpal tunnel formed by the flexor retinaculum. These tendons, along with those of flexor digitorum profundus, are enclosed by a common flexor sheath. The tendons attach to the anterior margins on the bases of the middle phalanges of the four fingers. These tendons have a split (Camper's Chiasm) at the end of them through which the tendons of flexor digitorum profundus pass.

Function
The primary function of flexor digitorum superficialis is flexion of the middle phalanges of the fingers at the proximal interphalangeal joints, however under continued action it also flexes the metacarpophalangeal joints and wrist joint.

To test flexor digitorum superficialis, one finger is flexed at the proximal interphalangeal joint against resistance, while the remaining three fingers are held fully extended (to inactive flexor digitorum profundus).