Marker position uncertainty in joint angle analysis for normal human gait — A new error-modelling approach

Measurement uncertainty is present when using optical motion capture to investigate joint angles during human gait. The aim of this work is to analyse the effect of uncertainty in the position of optical markers on the calculation of joint angles during human gait. The uncertainty is based on known errors of the measurement of marker positions (e. g. measurement inaccuracies, marker placement errors and soft tissue artefacts) and is modelled with epistemic uncertainty. Based on optical motion capture data of normal human gait, the epistemic uncertainty is propagated through the joint angle calculations using the Graph Follower algorithm. While being well suited to model the uncertainty present in motion capture, epistemic uncertainty has not yet been considered to model errors in the marker positions. The main contribution of this work is a new model for marker position errors and a new method to analyse the effects of these errors on the subsequent joint angle calculations during gait. This allows for an efficient worst case analysis, based on a real dataset, providing information on the largest possible error in the joint angle calculation based on given marker position errors. It is also capable of providing the opposite information, i. e. a given limit on joint angle errors can be used to determine acceptable marker errors. Further, it is also possible to investigate the effect of specific markers and their contribution to the joint angle analysis.