Flexor digitorum profundus muscleThe flexor digitorum profundis is a muscle in the forearm that flexes the fingers. It is called an extrinsic muscle, because it action is at a different location to the main body of the muscle.
Flexor digitorum profundis, along with flexor digitorum superficialis have long tendons that run down the arm and through the carpal tunnel that attach to the palmar side of the phalanges of the fingers. This muscle originates from the anterior side of the ulna.
Flexor digitorum profundis lies deep to superficialis, but it attaches more distally. To get around this problem profundus's tendons go through the tendons of superficialis, and end up attaching to the distal phlanx.