A method to calculate the centre of the ankle joint: a comparison with the Vicon Plug-in-Gait model.

BACKGROUND In gait analysis, calculation of the ankle joint centre is a difficult task. The conventional way to calculate the ankle joint centre is using the Vicon Plug-in-Gait model. The present study proposes a new model, which calculates the joint centre from two markers positioned over the medial and lateral malleoli (i.e. Two-marker-model). METHODS In order to compare the proposed model with Plug-in-Gait model, gait data from healthy and patient subjects were captured using a motion capture system. The ankle joint centres were calculated by the two models. A test-retest experiment was carried out to check reliability and repeatability for Two-marker-model. FINDINGS Two ankle joint centres produced by two models were significantly different. The distances between two ankle joint centres were approximately 16.8 (mm), and the differences in the posterior-anterior, medial-lateral and inferior-superior directions were approximately 6.3, 7.7 and 8.2 (mm). Further error analysis highlighted that the probability of producing errors in Two-marker-model is lower than that in Plug-in-Gait model due to the Two-marker-model's simple and reliable marker positioning. The reliability and repeatability coefficients for the new model were greater than 0.9. INTERPRETATION In principle, the Plug-in-Gait model is more likely to produce errors than the Two-marker-model, because the former employs multiple markers from the pelvis to calf to define the ankle joint centre with marker positions being very user-dependent. The results suggest that the Two-marker-model can be considered an alternative to Plug-in-Gait model for calculating ankle joint centre.