Axis of motion of the humeroradial joint
The capitulum of humerus and the articular facet of the radius head comprise this joint. It is a small ball and socket joint that is capable of allowing the forearm to pronate and supinate, and to extend and flex. When the forearm is fully flexed, the articular facet of the radial head is off the capitulum of humeral head and rests on the radial fossa of humerus.