Pronation of the forearm. Artwork of an anterior (frontal) view of a human left forearm with clenched hand and protruding thumb, showing the underlying bone structure during pronation. The forearm is rotated while keeping the upper arm and elbow stationary. This joint movement, turning the palm from facing up to facing down, is known as pronation. It causes the two forearm bones, the ulna and radius, to cross (here, the ulna passes below the radius). Also visible are the wrist bones, part of the metacarpals of the hand, and part of the humerus (upper arm bone).