THE EFFECTS OF MARKER SIZE ON THE ACCURACY OF THE ARlEL PERFORMANCEANALYSIS SYSTEM (APAStm)

Abstract

INTRODUCTION In order to increase the efficiency of today's motion analysis systems, most systems are capable of auto digization. In order for the APAS system to auto digitize, retroreflective markers must be used for accurate tracing of the movement. During auto digization, the computer's software will search a given area for pixels that are above a pre-determined light threshold. From these threshold pixels, the software will calculate the center of the marker. To increase the accuracy of the measurements, it has been suggested that the larger the marker, the more accurate the measurement. The purpose of this study was to examine the effects of marker size on the accuracy of the APAS system. METHODS Eight retroreflective markers were placed upon an inverted T-shaped pendulum. Tbelve reference angles were calculated from these markers. Four different starting positions were used (static, a 45 deg, a 90 deg, and a 120 deg. release position) while using four different marker sizes (0.45,0.95, 1.27, and 1.91 cm.) for a total of 16 different conditions. Ten trials were done for each condition. Tbenty frames were auto-digitized from each trial. Frame 10 represented the low point of the trajectory for the dynamic conditions. Deviations were calculated by taking the average angle over the 20 frames and subtracting it from the reference angle. The data was analyzed using a Mixed Effects ANOVA. Inter-trial variability was determined by subtracting the reconstructed angle from the reference angle on a frame by frame basis. RESULTS Significance was found in the ANOVA of marker [F (3,1893) = 6.39, p < 0.00031 and position [F (3,1893) = 350.41, p < 0.00011. Contrast tests indicate that markers 1-3 were all statistically different from marker 4 (p < 0.001, p < 0.0003, p < 0.0106). The least square means of marker showed that as the marker size increased the deviation on average decreased (0.4339 deg. to 0.3883 deg.). Examination of the inter-trial variability showed that the variability increased from the static position up to the 120 deg. position. CONCLUSION Marker size was shown to have a significant effect upon the accuracy of the APAS system. Examination of the least square means seems to indicate that the smallest marker size had the most error and the largest marker size had the least error. It should be noted though that regardless of marker size the error was no greater than 0.5 degrees. For most motion analysis procedures an error of this magnitude is probably an acceptable error.