3D motion analysis represents a means of collecting objective, accurate and repeatable gait data, however the high cost of equipment hampers its widespread use in routine clinical practice. The purpose of this study was to determine the marker tracking accuracy of a new low-cost optical 3D motion analysis system. A rigid cluster of four reflective markers was used to compare a low-cost Optitrack 3D motion analysis system against two more expensive systems (Vicon 612 and Vicon MX). Accuracy was measured by comparing the mean vector magnitudes (between each combination of markers) for each system, and reliability was measured through the coefficients of variation (CV). Gaps in the marker trajectories, which are considered undesirable, were also counted. With regards to accuracy, the largest disagreement between mean vector magnitudes for Optitrack and Vicon MX was 2.2%. The largest disagreement between Vicon 612 and Vicon MX was 2.1%. In terms of reliability, the mean CV was lowest in Vicon MX (0.3%) and similar in the Vicon 612 (2.5%) and Optitrack (2.3%) systems. The number of trajectory gaps for the Vicon MX, Vicon 612 and Optitrack systems were; zero, six and 11 respectively.
The study found the Optitrack system to provide an inexpensive 3D motion analysis system that can offer marker tracking accuracy and reliability which is similar to an older and still widely used system (Vicon 612). Additional development work is necessary before Optitrack can be used for full 3D gait analysis by physiotherapists and other health professionals.