Schematic of the lower leg and force plate, showing coordinate systems and heel point.

<p>A global coordinate system <i>XYZ</i> is defined, where the origin <i>O</i> is situated in a corner of the force plate. The direction <i>X</i> is lateral, <i>Y</i> is anterior and <i>Z</i> is vertical up. A local coordinate system <i>xyz</i> is defined, where the origin <i>L</i> is situated at an LED kinematic marker. This is the lower of two markers placed directly on the skin over the medial surface of the tibia. The local <i>z</i> direction points to the second LED marker, the y direction is perpendicular to the <i>z</i> axis and lies in the plane of progression (defined as a plane parallel to the global <i>YZ</i>-plane), the <i>x</i> direction is defined orthogonal to the <i>yz</i>-plane to complete the local right-handed orthogonal coordinate system. The position of the heel point <i>H</i> is derived from the location of the center of pressure as obtained from the force plate data during heel strike, as measured in the global coordinate system <i>XYZ</i>. These global center of pressure points are transformed to the local coordinate system <i>xyz</i>. The median <i>xyz</i>-position of these transformed local center of pressure points is taken as the position of the heel point <b><i>r</i></b>_<i>H</i>, which is a time-invariant vector in the local lower-leg coordinate system <i>xyz</i>. The global position of the heel point <b><i>R</i></b>_<i>H</i> is time variant, due to the movement of the local coordinate system in the global coordinate system.