9.1.1. Global coordinate system

To express any coordinate, we need a coordinate system. A coordinate system is composed of an origin (the point in space everything is expressed relative to) and a set of axes. In human movement biomechanics, we usually use a cartesian system composed of three orthonormal axes (x, y and z).

In newton dynamics and at the human scale, it is totally acceptable to define a global, non-moving coordinate system everything can be referenced to. In Fig. 9.2, we define such a fixed system:

• The origin is approximately at the hip level and posterior to the person;

• The x axis points forward;

• The y axis points upward;

• The z axis points to the right.

This coordinate system is completely arbitrary: any other origin or set of orthonormal axes would still be perfectly valid, as long as we are consistent during the whole analysis. This is the one we chose here, the one every global coordinate will refer to.

Fig. 9.2 A global coordinate system.