Extensor digitorum

What it is

The main extensor of the fingers, running from the outer elbow down the back of the forearm and sending four tendons across the back of the hand. Those are the tendons you can see standing up under the skin when you straighten your fingers.

The tendons are joined to one another across the back of the hand by connecting bands, which is part of why the fingers do not extend fully independently.

Each tendon flattens over the finger into an extensor expansion or extensor hood — a triangular sheet of connective tissue that wraps the back of the finger and receives the tendons of the small hand muscles too. Finger extension is a genuinely collaborative arrangement rather than one tendon pulling one bone.

It arises from the common extensor tendon, shared with several other muscles at the outer elbow.

What it does

Extends the fingers, principally at the knuckles, and assists wrist extension.

Straightening the two outer joints of the fingers is mostly done by the small muscles of the hand pulling on the extensor expansion, not by this muscle directly.

What loads it

Resisted finger and wrist extension. It is loaded lightly in most training and rarely worked deliberately.

What is commonly got wrong about it

"Tennis elbow" as inflammation. Lateral elbow pain is usually attributed to the common extensor origin, and particularly to extensor carpi radialis brevis. The traditional name, epicondylitis, implies inflammation, and tissue studies have generally not found much — the condition is now more often called a tendinopathy, describing a change in tendon structure rather than an inflammatory process. It is a clinical problem, not a training one.

That extensor work is needed to balance grip training. The idea that the finger extensors must be trained to offset heavy gripping is widely repeated and not well supported. It is harmless and it is not established.

That the tendons on your hand are muscles. The muscle is in the forearm.