Extensor carpi radialis longus arises from the lower third of the lateral supracondylar ridge of the humerus and the lateral intermuscular septum. It has a long tendon that passes deep to the extensor retinaculum. It inserts into the base of the dorsum of the second metacarpal.
The action of extensor carpi radialis is to extend and radially-deviate the hand.
It is supplied the radial nerve (C6, C7).