Evaluation of the reliability of reflective marker placements

This study determined the reliability of recording knee angle coordinates in the sagittal plane during erect stance by use of skin reflective markers. To this end, 32 healthy male and female subjects participated in three standardised test sessions of 1 hour duration spaced at 1- and 4-week intervals. At each session, markers were placed on three bony landmarks of the leg and their coordinates were photographed with the leg in extension and semi-flexion. The resultant knee angles, calculated from lines drawn on the photographs marking the coordinate sites, were subjected to separate repeated measures analysis of variance statistical procedures. The results showed that both sets of measurements could be carried out reliably when repeated within a single test session (R = 0.87). However, when repeated over time, neither measurement reached the criterion for acceptable reliability (set at R > or = 0.80). The magnitude of the method error was also higher between test sessions for both angles than it was for tests repeated within the sessions. These data suggest that whereas knee coordinates recorded on a single day can be reproduced reliably, they may be less useful for evaluating knee angle changes over time.