This image shows some of the deeper muscles of the dorsal aspect of the forearm.  Note the small,  candle flame shaped muscle that surrounds the proximal end of the radius.  This muscle is the supinator, and its action is to supinate the forearm with assistance from the brachioradialis.  Its origin is the lateral epicondyle of the humerus and it inserts on the proximal radius.  The abductor pollicis longus can be seen just below the supinator.  It forms a wide band of muscle that spans obliquely between the radius and ulna.  It originates from the radius and ulna and inserts on the radial side of the first metacarpal.  Its action is to extend and abduct the thumb (first digit).  Also seen on the image are the superficial extensor digitorum lateralis and extensor digitorum communis muscles.