title: A method to calculate the centre of the ankle joint: a comparison with the Vicon Plug-in-Gait model.
abstract: 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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Institute of Motion Analysis and Research, Department of Orthopaedic and Trauma Surgery, TORT Centre, Ninewells Hospital and Medical School, University of Dundee, Dundee, United Kingdom.
authors.forename: Syam Prabhakaran
authors.initials: SP
authors.lastname: Nair
authors.forename: Sheila
authors.initials: S
authors.lastname: Gibbs
authors.forename: Graham
authors.initials: G
authors.lastname: Arnold
authors.forename: Rami
authors.initials: R
authors.lastname: Abboud
authors.forename: Weijie
authors.initials: W
authors.lastname: Wang
mesh_terms: D000293:Adolescent; D000328:Adult; D000465:Algorithms; D000843:Ankle Joint / Q000033:anatomy & histology / Q000502:physiology*; D000886:Anthropometry; D001696:Biomechanical Phenomena; D002648:Child; D005260:Female; D005684:Gait; D006801:Humans; D008297:Male; D008875:Middle Aged; D008962:Models, Theoretical; D015203:Reproducibility of Results