The flexor digitorum superficialis is an extrinsic muscle that allows the four medial fingers of the hand to flex. These fingers include the index, middle, ring, and pinkie fingers. The term extrinsic means that the muscle is located in the forearm. This muscle has two distinct heads, which both originate in the forearm. The humeroulner head originates at the medial epicondyle of humerus, which refers to a knuckle-like projection on the part of the upper arm bone that is closest to the elbow. This head also originates from the ulnar collateral ligament and coronoid process of ulna, which refers to a triangular projection on the ulna. The ulna is one of the bones of the forearm. The other head, known as the radial head, originates from the back portion of the radius, a bone of the forearm. Four tendons arise from this muscle close to the wrist and pass via the carpal tunnel. The tendons split and insert on the sides of the middle phalanges of the four medial fingers. In many cases, the tendon is absent from the little finger. This is known as an anatomical variant. In turn, this may result in problems with the diagnosis of an injury of the little finger. Each of the four medial fingers contains three bones. These are the distal phalanges at the fingertips, the middle phalanges, and the proximal phalanges which are closest to the palm. The primary action of the flexor digitorum superficialis is to flex the fingers at the proximal interphalangeal joints. These hinge joints are located between the between the middle and proximal phalanges. The secondary role of the muscle is to flex the metacarpophalangeal joints. These are located between the proximal phalanges and the metacarpal bones of the palm.
The muscle receives oxygen-rich blood from the ulnar artery. It is innervated by the median nerve.