Elbow and wrist extension. Artwork of the muscles of the arm from the front, with red arrows showing the direction of movement when extending the elbow and wrist joints. Elbow extension is the straightening of the arm from a bent position (here from 90 degrees downwards), a motion performed mainly by the triceps brachii muscle on the underside of the upper arm. Wrist extension (sometimes called dorsiflexion) is where the hand is tilted in the direction of the back of the hand, as opposed to palmar flexion. The nerve used is the radial nerve. This is the left arm. For the right arm, see C016/6791.