Measuring small linear displacements with a three-dimensional video motion analysis system: determining its accuracy and precision.
OBJECTIVE
To determine accuracy, precision, and smallest detectable difference for a three-dimensional (3D) video motion analysis system specifically configured for measuring small and slow displacements within a small measurement volume (0.7 x 0.5 x 0.3 m).


METHODS
Repeated measurements with random sequence of conditions.


METHODS
Rehabilitation research laboratory.


METHODS
A reference sliding device was used to control cyclic displacements of two reflective markers over 5 calibrated ranges (1, 3, 10, 30, and 60 mm). Nine cycles were performed for each of 9 conditions (3 directions x 3 zones). Four cameras recorded all trials on tapes, which were digitized with a Kinemetrix system.


METHODS
Change in distance of the moving markers relative to a third static marker was averaged over 50 frames per trial. Mean error, mean absolute error, and intertrial and intratrial standard deviations (SDs) were calculated for each zone and direction.


RESULTS
For 810 trials, mean error and absolute error were, respectively, .034 mm and .094 mm. The mean intertrial and intratrial SDs and 99% confidence interval were .047 mm (CI = +/- .121 mm) and .030 mm (CI = +/- .077 mm). The corresponding smallest detectable differences were .171 mm and .109 mm.


CONCLUSIONS
Motion analysis configured for registration within small volumes allows measurement of minuscule displacements with great accuracy and may therefore be suitable for many applications in rehabilitation research other than gait analysis.
Everaert
D G
DG
Department of Rehabilitation Sciences, Faculty of Physical Education and Physiotherapy, University of Leuven, Belgium.
Spaepen
A J
AJ
Wouters
M J
MJ
Stappaerts
K H
KH
Oostendorp
R A
RA
D004867:Equipment Design; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000295:instrumentation*; D008161:Lumbosacral Region; D009068:Movement; D012680:Sensitivity and Specificity; D014741:Video Recording / Q000295:instrumentation*