Anatomy Guide › Upper limb
Flexor Digitorum Profundus
Curl just the last joint of a finger — the fingertip alone — and you have isolated the flexor digitorum profundus, the only muscle in the body that can bend the fingers' end joints. 'Profundus' means deep: it lies against the ulna and the forearm's interosseous membrane, beneath its partner the flexor digitorum superficialis, and sends four tendons through the carpal tunnel into the fingers, where each tendon threads through a split in the superficialis tendon to reach the final bone.
That threading arrangement — one tendon perforating another — is one of anatomy's neatest mechanical details, and it underlies the clinical tests every hand surgeon performs: hold the middle joint straight and ask the patient to bend the fingertip, and you are testing profundus alone. The muscle also has a split personality in its wiring: the index and middle finger portions are driven by the median nerve, the ring and little finger portions by the ulnar nerve — so a single nerve injury weakens half the muscle and spares the rest, a pattern that helps localize the damage.
The profundus is the backbone of power grip. Hanging from a bar, carrying a suitcase, a rock climber crimping an edge — all end-joint flexion under load runs through these tendons. Climbers know the muscle's tendon-pulley system intimately, because 'flexor pulley' ruptures are the sport's signature injury. Rugby and football contribute another classic: 'jersey finger,' where a grabbing fingertip is forcibly straightened and the profundus tendon tears off the bone — most often in the ring finger.
In Anamo, the profundus occupies the deepest layer of the front forearm, hugging the ulna. It contrasts with the extensor family on the back of the forearm — a warm-versus-hot comparison between the two sides quickly settles which surface of the forearm the mystery part lives on.
- The only muscle that bends the fingertips' end joints
- Its tendons thread through splits in the superficialis tendons
- Half is wired by the median nerve, half by the ulnar nerve
- 'Jersey finger' is its tendon tearing off the bone mid-tackle
Think you could find the flexor digitorum profundus on a 3D model? Play Anamo and test yourself — every guess is color-coded by how close it lands to the mystery structure.