Section 2, Chapter 1, All the world's a stage: geometry of the environment for human action

For vertebrates, almost all action involves the movement of some of the bones of the skeleton that result in moving the individual relative to the environment. To simplify the problem, we shall focus on creatures that utilize motion of the limbs in order to move over the land. To further simplify the problem, we shall narrow our focus on human locomotion. Most of our discussion will consider actors performing on a stage.

By default, or as initial position, we shall assume that the actor is standing upright.