Uncertainty analysis and sensitivity of scapulothoracic joint angles to kinematic model parameters.

The purposes of this study were to determine the influence of kinematic model parameter variability on scapulothoracic angle estimates, and to define which parameters of the kinematic model have the largest effect on scapulothoracic angle estimates. Nominal subject-specific kinematic models of nine participants were implemented. Fifteen parameters of the nominal models relative to the clavicle length, ellipsoid, sternoclavicular and acromioclavicular joint centers, and contact point location were altered from - 1 to 1 cm. Then, scapulothoracic angles were computed during four movements using multibody kinematic optimizations for nominal and altered models. The percentage of scapulothoracic angle variance explained by each parameter of the kinematic model was computed using Effective Algorithm for Computing Global Sensitivity Indices. When altering simultaneously the 15 parameters of the kinematic model, scapulothoracic angles varied up to 50°. For all movements and degrees of freedom, the clavicle length significantly explained the largest part of scapulothoracic angle variance (up to 25%, p < 0.01). In conclusion, kinematic model parameters need to be estimated accurately to avoid any bias in scapulothoracic angle estimates especially in a clinical context. The present sensitivity analysis may also be used as a benchmark for future works focusing on improving shoulder kinematic models. The curves represent mean scapulothoracic angles computed with the nominal model and their variability when kinematic model parameters are altered. The colormap graphs represent the percentage of scapulothoracic angle variance explained by each parameter of the kinematic model.

Effects of the soft tissue artefact on the hip joint kinematics during unrestricted activities of daily living.

Soft tissue artefact (STA) affects the kinematics retrieved with skin marker-based motion capture, and thus influences the outcomes of biomechanical models that rely on such kinematics. In order to be compensated for, the effects of STA must be characterized across a broad sample population and for different motion activities. In this study, the error introduced by STA on the kinematics of the hip joint and of its individual components, and on the location of the hip joint center (HJC) was quantified for fifteen THA subjects during overground gait, stair descent, chair rise and putting on socks. The error due to STA was computed as the difference between the kinematics measured with motion capture and those measured simultaneously with moving fluoroscopy, a STA-free X-ray technique. The main significant effects of STA were: underestimation of the hip range of motion for all four activities, underestimation of the flexion especially during phases of the motion with higher flexion, overestimation of the internal rotation, and lateral misplacement of the HJC mostly due to the functional calibration. The thigh contributed more to the STA error than the pelvis. The STA error of the thigh appeared to be correlated with the hip flexion angles, with a varying degree of linearity depending on the activity and on the phase of the motion cycle. Future kinematic-driven STA compensation models should take into account the non-linearity of the STA error and its dependency of the phase of the motion cycle.