Anatomy Guide › Upper limb
Extensor Digitorum
Make a fist, then spring your fingers open: the muscle doing the opening is the extensor digitorum, the main extensor of all four fingers. It is an economical design — a single muscle belly on the back of the forearm that splits into four tendons, visible as the ridges that fan across the back of your hand when you stretch your fingers wide.
The tendons pass the wrist through a fibrous tunnel system, then flatten over each finger into an elegant structure called the extensor expansion — a triangular hood that wraps the finger's back and sides. This hood is where hand anatomy gets clever: the small muscles of the hand (lumbricals and interossei, driven largely by the ulnar nerve) insert into the same hood, so straightening a finger fully is a cooperative act between forearm and hand muscles. The tendons are also cross-linked to each other on the back of the hand by slanting bands, which is why your ring finger cannot lift far on its own when the others are folded — a party trick that is really an anatomy demonstration.
The system's failure points are known to every hand clinic. A jammed fingertip can snap the tendon's final slip, leaving a droopy 'mallet finger'; damage to the hood's central band produces the boutonnière deformity, where the finger buttons through its own hood. Both are diagnosed at a glance by the shape the finger assumes.
In Anamo, the extensor digitorum spans a long lane from the outer elbow to the fingertips. It sits among a crowd of similarly named extensors — indicis, digiti minimi, pollicis — and distinguishing them is the forearm region's real test. Warm readings along the back of the forearm usually mean you are in this family.
- One muscle belly extends all four fingers via four tendons
- Its tendons form the visible ridges on the back of the hand
- Cross-links between tendons limit the ring finger's solo lift
- 'Mallet finger' is a rupture of its final tendon slip
Think you could find the extensor digitorum on a 3D model? Play Anamo and test yourself — every guess is color-coded by how close it lands to the mystery structure.