This chapter deals with the mechanical block of the human motor control scheme and it will show:

• the assumptions in modeling human movement,
• the differences between medical and technical descriptions of movement,
• the general way to describe rigid body motion with a translation vector and a rotation matrix,
• some alternatives in 3-D to derive rotation angles from the rotation matrix, e.g. Euler angles and helical axes,
• the concepts of inverse dynamics and direct (or forward) dynamics,
• how to derive the equations of motion for a system of rigid bodies in the Newton-Euler or Lagrange formulation.