Full Pronation/Supination Description
[Continued from above] . . . lateral end of the humerus (upper arm bone) to the radius (short forearm bone). It assists the biceps brachii in rotating the forearm laterally (supination).
Acting oppositely, the pronator teres is a short muscle connecting the ends of the humerus (upper arm bone) and ulna to the radius (forearm bones). It functions to rotate the arm toward the inside, as when the hand is turned so the palm is facing downward (pronation). It is helped in this role by the pronator quadratus, which runs from the far end of the ulna (longest forearm bone) to the far end of the radius (shortest forearm bone).
Neither set of muscles could accomplish the task separately, nor could they do so without a special joint that allows the radius and ulna to cross each other within the arm. Yet together they make the process of pronation/supination possible so that we can hold our arms out and turn the palm upward and then downward again.