A survey of formal methods for determining the centre of rotation of ball joints.
Accurate location of the hip joint centre is a necessary component in biomechanical human motion analysis to measure skeletal parameters and describe human motion. In human movement analysis, the hip joint centre can be estimated using functional methods based on the relative motion of the femur to pelvis measured using reflective markers attached to the skin surface. Determination of the hip joint centre by functional methods suffers inaccuracy due to the soft tissue artefact, which is the relative motion between the markers and bone. Therefore, one of the main objectives in human movement analysis is the assessment and correction of the soft tissue artefact. Various studies have described the movement of the soft tissue artefact and minimized it invasively. The goal of this study is to present a non-invasive method to assess and reduce the soft tissue artefact effects using optical motion capture data and tissue thickness from ultrasound measurements during flexion, extension, and abduction of the hip joint. Results showed that the displacement of markers is non-linear and larger in areas closer to the hip joint. It was also found that the marker displacements are dependent on the movement type, being relatively larger in flexion movement. This quantification of soft tissue artefacts was used as a basis for a correction procedure for hip joint centre and minimizing the soft tissue artefact effects. Results showed that our method reduces the error in the functional hip joint centre from 13.65-22.54 mm to 7.9-12.82 mm.