A hip joint kinematics driven model for the generation of realistic thigh soft tissue artefacts

In human movement analysis, accuracy and robustness of the algorithms used to determine the location of centres of rotation from stereophotogrametric data depend mainly on their capacity to deal with the artefacts due to soft tissue deformation (STA). While evaluating these algorithms using a mathematical simulation approach, effectual realizations of STAs are needed. This study aimed at accomplishing this objective by modelling STAs, in twelve selected areas of the thigh, as a linear function of the hip angles, assuming no knee joint movement. The proposed model was calibrated and assessed using ex-vivo experiments. This entailed that only the component of the STA due to skin stretching was accounted for. Photogrammetric data of markers placed on the skin and on hip-bone and femur pins were recorded during passive flexion-extension, ab-adduction, rotation and circumduction of the hip joint. Artefact skin marker displacements were represented in a femur embedded anatomical frame. Model parameters were estimated by minimizing the least squares difference between measured and modelled STAs. The STA affecting a skin marker placed in a given thigh location of a given subject could be modelled with a high accuracy (median root mean square difference over 4 subjects*3 trials*12 markers*3 coordinates: 0.8mm-inter quartile range 1.0mm). This was also true for a hip joint movement different from the one used to calibrate the model. High inter-subject variability of the model parameters confirmed the subject-dependency of the phenomenon.