Experimental analysis of human motion has been based on optical, magnetic, or electronic tracking techniques to determine body segment locations and orientations. The Average Coordinate Reference System (ACRS) method was developed to reduce experimental errors in human locomotion analysis. Experimentally measured kinematic data is used to conduct analysis in human modeling, and the model accuracy is directly related to the accuracy of the data. However, the accuracy is questionable due to skin movement, deformation of skeletal structure while in motion and limitations of commercial motion analysis systems.
In this study, the ACRS method is applied to an optically-tracked segment marker system, although it can be applied to many of the others as well. Many previous studies adopted redundant marker systems, using four or five optical markers, instead of the basic three marker system to provide statistically better results of body segment position and orientation. In the ACRS method, each marker can be treated independently, as the origin of a local coordinate system for its body segment. Errors, inherent in the experimental process, result in different values for the recovered Euler angles at each origin. By employing knowledge of an initial, calibrated segment reference frame, the Euler angles at each marker location can be averaged, minimizing the effect of the skin extension and rotation. Using the proposed ACRS methodology the error is reduced when compared to the general Euler angle method commonly applied in motion analysis.