This muscle arises from the common extensor origin on the front of the lateral epicondyle of the humerus, passes down behind and deep to its fellow longus, and by a flattened tendon is inserted into the base of the third metacarpal. The lower part of the tendon is crossed by the three extensor muscles of the thumb.
Nerve supply of extensor carpi radialis brevis:
By a branch in the cubital fossa from the posterior interosseous nerve (C6, 7), before the nerve pierces the supinator muscle.
Action of extensor carpi radialis brevis:
Its name indicates its action. It is indispensable in ‘making a fist’. Note that it and the longus are inserted into the same metacarpals as flexor carpi radialis and compare the symmetry of insertion of flexor and extensor carpi ulnaris muscles.