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.

Contrapposto Skeleton

Here's a before-and-after illustration of what happens to the skeleton when the body shifts to a contrapposto pose.

from Human Anatomy by Jose M. Parramon

Joint Movement

from Human Anatomy for Artists by Eliot Goldfinger

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

Movement of the Jaw

Just as the image illustrates, the jaw does not open and pivot at the corners of the mouth, but at a point behind the teeth on the upper jaw.

from Human Anatomy by Jose M. Parramon