Accuracy of glenohumeral joint center of rotation estimates using traditional motion capture versus bi-plane fluoroscopy

Accurate estimation of the glenohumeral joint center of rotation is critical for biomechanical modeling of shoulder motion. This study evaluated the accuracy of three predictive methods for estimating glenohumeral joint center of rotation using optical motion capture, compared against a benchmark derived from synchronous bi-plane fluoroscopy. Five healthy adults performed shoulder abduction, flexion, and external rotation in 90° abducted and adducted positions. Root mean square error and axial errors were calculated across the full range of motion. Results showed that none of the predictive methods met clinically acceptable thresholds, with absolute error exceeding 10 mm and axial errors surpassing ±5 mm. Error increased significantly with humeral elevation beyond 60° but remained stable during rotational movements. The inclusion of technical markers slightly reduced axial error but did not substantially improve overall accuracy, likely due to soft tissue artifact. Directional axial errors revealed consistent biases, with the acromion method tending to produce anterior, medial, and superior errors. These findings suggest that predictive methods may be suitable for low-elevation tasks, such as gait, but are unreliable for high-elevation activities like overhead sports. The study highlights the importance of evaluating center of rotation estimation across full motion ranges and suggests that future work should explore humeral marker placement and functional methods to mitigate soft tissue artifact and improve accuracy. Validating reliable optical motion capture-based methods against bi-plane fluoroscopy will enhance consistency and comparability in shoulder biomechanics research.