
A reference frame provides the reference for describing physical quantities. There are different ways to describe physical quantities, such as position, velocity, etc., depending on the nature of the coordinate system employed. Reference frames are basically the different perspectives of the viewer while the coordinate systems are the different ways to describe physical quantities in these perspectives. Two commonly used coordinate systems in biomechanics are the Cartesian coordinate system and the polar coordinate system:
The Cartesian coordinate system is perhaps the most commonly used system in biomechanics. It provides a unique advantage in terms of consistency and simplicity. Unfortunately, not all biomechanical problems are well adapted to solution in Cartesian coordinate system. The bottom line is: the coordinate system should be chosen to fit the problem so that it becomes more readily soluble. 
© YoungHoo Kwon, 1998 