Reference Systems in Space
The 3 CARDINAL PLANES (and their associated AXES of ROTATION) move ALONG WITH THE BODY.
Motions can be described using the 3 cardinal planes:
-The forearm flexed by 30º
-The angle at the knee – between the longitudinal axis of the thigh
and the leg is 90º – ie. the knee has been flexed 90º
The above are termed RELATIVE ANGLES.
Motions can also be described WITH REFERENCE TO a FIXED SYSTEM of REFERENCE
Usually using the CARTESIAN CO-ORDINATE SYSTEM comprising 3 mutually perpendicular lines which meet at the ‘origin’ of the ‘global reference system’ (GRS) positioned in some part of the ‘motion capture volume’,
-The co-ordinates of the elbow joint center with respect to the GRS is (100, 120, 150) along the x, y and z axes respectively. The units of measurement could be cm, m etc.
The above are termed ABSOLUTE ANGLES.