Page 9b -- Anatomical Calibration

The purpose of anatomical calibration is to identify the anatomical landmarks of interest (e.g. greater trochanter, lateral condyle, and lateral malleolus on the femur) with respect to an easily identifiable group of markers which are more convenient to track with the motion analysis system.

A plate containing three or more markers firmly attached to a segment of interest can be used with the three point method to identify an oblique system of coordinates which defines the body segment.

A calibration procedure is described which facilitates anatomical landmark identification. A wand containing markers of known distance separation, placed at an anatomical landmark, indicates the global coordinates of this landmark once the orientation of the wand is known.

The anatomical position is fixed with respect to the remainder of the body, and therefore a transformation from the body segment (or local 1) coordinate system to a new coordinate system based on the anatomical points of interest (or local 2) can be created. This method eliminates the need for additional anatomical markers to be affixed to the skin.

Applying the three point method to anatomical points of interest, we can define a set of coordinate axes embedded in the body segment. Often it is best to define axes parallel to the axes of joint rotation for biomechanics applications. For example, the medial and lateral condyles and the greater trochanter would best describe the axes of the femur.

This applet will attempt to save a virtual (x,y) landmark from the frontal plane, and rotate with the body. In addition, the transformation matrix from anatomical to local coordinates will be shown.