Extensor Digitorum Communis
The Extensor Digitorum Communis is the principle extensor of the medial (ulnar) four digits. It occupies much of the posterior surface of the forearm. It extends the proximal phalanges and, through its collateral reinforcements, the middle and distal phalanges.
To test the extensor digitorum communis, the forearm is pronated and the fingers
extended. The patient attempts to keep the fingers extended at the metacarpophalangeal
joints as pressure is exerted on the proximal phalanges in the direction of
flexion. The tendons are rendered prominent on the dorsum of the hand.