Arm Movement

This video shows a simple movement of the arm.  Notice the opposing motions of the biceps and triceps.  When one muscle pulls and tenses, the other one relaxes and stretches.  The two muscles are working in concert to move the lower arm in relation to the upper.

Bones of the Forearm

Look how the ulna and radius seem to twist as the wrist rotates.  Remember, the radius is the bone that is at the base of the thumb and the ulna is the bone that creates the "large bump" on the pinky side of the hand.

from Human Anatomy by Jose M. Parramon