Some effects of camera placement on the accuracy of the Kinemetrix three-dimensional motion analysis system.
OBJECTIVE
To assess the accuracy of the Kinemetrix motion analysis system to measure horizontal movement by a single reflective marker away from two cameras with differing camera placements.


METHODS
Trial of the effects of nine different camera arrangements on precision of non-human movement.


BACKGROUND
In many cases the ability to arrange cameras 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 assessed with camera horizontal separations of 15 degrees, 30 degrees and 45 degrees, and vertical separations of 0 degrees, 15 degrees and 30 degrees 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 bisecting 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 degrees vertical), the Kinemetrix was unable to calculate the three-dimensional co-ordinate of the marker. For all other camera positions tested, the errors in measurements were small (mean absolute errors < 2 mm).


CONCLUSIONS
Maintaining camera horizontal and vertical separations above a sum of 30 degrees is sufficient for clinical testing.


CONCLUSIONS
Motion analysis systems are becoming more common for clinical evaluation where only confined testing areas are available. These confined areas often make positioning of cameras at greater than 60 degrees impossible; therefore, there is a need to explore the errors involved in placing two cameras at less than 60 degrees.
Thornton
M J
MJ
Department of Health Sciences, University of East London, London, UK.
Morrissey
M C
MC
Coutts
F J
FJ