Vectors have four characteristics.
Given a movement of the human body, Name the joint at which the movement occurs. Determine what segment is moving , what segment is stable, and the joint where the movement takes place. Determine the plane of movement in which the movement occurs, and the axis around which the joint moves in this plane. Draw a simple diagram that illustrates the body in the plane you have named. Even if the movement takes place in more than one plane, you can analyze only one plane at a time.