Joint rotation between two attitudes in the spherical rotation coordinate system.

Three-dimensional joint rotation between two attitudes rather than starting from neutral position is the most common pattern of human movement. This paper presents a method for determining the joint rotations starting from a non-neutral attitude in the spherical rotation coordinate system based on a two-step rotation method. The method uses three spherical rotation angles that are consistent with clinical description of joint rotations and are mathematically proved to be sequence independent. The method can be easily understood by clinicians and applied in clinical practice. Two numerical examples of humeral rotations are presented to demonstrate the effectiveness of the method in dealing with different patterns of three-dimensional joint rotations between two attitudes.