Pronation of the forearm. Artwork of an anterior (frontal) view of a human left forearm with relaxed hand and thumb and fingers outstretched, showing the limb's external appearance 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 (see C016/2851).