Extensor carpi radialis brevis
is an extensor, and an abductor of the hand at the wrist joint
It lies medial to longus, and is slightly shorter. It starts lower down on the humerus at the lateral epicondyle, and inserts into the base of the 3rd metacarpal.
Like all the muscles in the posterior forearm, ECR brevis is supplied by a branch of the radial nerve.