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It is common in motion analysis to define one or more angles (user-angles) based on the marker coordinates. Included in the user-angle category are the projected joint angles, body lean angle, velocity vector angle, etc. Proper combination of the local reference frames and the points & vectors computed will provide flexibility and allow the user to define a variety of user-angles to fulfill the analysis-specific needs. The major issues related to the user-angle computation are

	Angular Distance vs. Relative Angular Position
	Continuous Angle-Time Curve
	Initial Value Problem

User-angles can be defined from points and/or lines (vectors). The legitimate entries for the points are the markers, joints and the segmental CMs while those for the lines are unit coordinate vectors of the global and local reference frames, segment vectors (the vector drawn from the proximal end to the distal end in a segment), and any kinematic and kinetic vectors computed through data processing. Kwon3D provides 5 different ways to define a user-angle:

	4 points
	3 points
	1 line vs. 2 points
	2 points vs. 1 line
	2 lines

See the Software part for the details of the user-angle definition methods.

� Young-Hoo Kwon, 1998-