Otto Fischer was during the late 19th and early 20th century the founder of 3-D human gait analysis. From motion recordings he calculated by hand the inverse dynamics of humans in motion, for which he discovered and used the principal vectors of a system of moving bodies. With the principal vectors the equations of motion and the kinetic energy can be written in a specific simple form with full geometric meaning and with reduced mass models with which system dynamics can be investigated in a simple way at link level. Fischer applied his theory mainly in its planar form. He also presented the theory of the spatial form by example of a serial two-link chain, however the explanations in the original texts in German are challenging to understand. This paper presents Fischer’s spatial form in a modern and understandable way.