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The extensor carpi ulnaris muscle originates from the common extensor origin on the lateral epicondyle of the humerus. As it passes distally, on the surface it forms part of the ulnar border of the forearm. At the wrist, it passes through a groove between the head and styloid process of the ulna. At this point it is within a separate compartment of the extensor retinaculum to the other extensors.
It inserts into the posterior surface of the base of the fifth metacarpal.
It acts to extend the hand and deviate it at the wrist to the ulnar side.
Extensor carpi ulnaris is supplied by the posterior interosseous nerve (C7,8), a branch of the radial nerve.