SOME EFFECTS OF CAMERA PLACEMENT ON THE ACCURACY OF THE KINEMETRIX 3-DIMENSIONAL MOTION ANALYSIS SYSTEM

Objective. To assess the accuracy of the Kinemetrix motion analysis sy stem to measure horizontal movement by a single reflective marker away from two cameras with differing camera placements. Design. Trial of t he effects of nine different camera arrangements on precision of non-h uman movement. Background. In many cases the ability to arrange camera s to allow a separation of 60 degrees is not possible. Little is known about the precision of motion analysis systems for small camera angle separations. Methods. The accuracy of the Kinemetrix system was asses sed with camera horizontal separations of 15 degrees, 30 degrees and 4 5 degrees, and vertical separations of 0 degrees, 15 degrees and 30 de grees rendering nine different camera placements. The distance between the cameras and the object was always maintained at 4 m. During each test the marker was moved a known horizontal distance along a line bis ecting the horizontal angular separation of the two cameras. The mean absolute errors of the Kinemetrix measurement were calculated. Results . At the smallest camera separation tested (15 degrees horizontal, 0 d egrees vertical), the Kinemetrix was unable to calculate the three-dim ensional co-ordinate of the marker. For all other camera positions tes ted, the errors in measurements were small (mean absolute errors < 2 m m). Conclusions. Maintaining camera horizontal and vertical separation s above a sum of 30 degrees is sufficient for clinical testing. Releva nce Motion analysis systems are becoming more common for clinical eval uation where only confined testing areas are available. These confined areas often make positioning of cameras at greater than 60 degrees im possible; therefore, there is a need to explore the errors involved in placing two cameras at less than 60 degrees. (C) 1998 Published by El sevier Science Ltd. All rights reserved.