Concurrent Validity and Agreement of Viewpoint-Dependent Markerless Motion Tracking in Upper-Limb Assessment: A Validation Against Optoelectronic Motion Capture.

Quantitative upper-limb assessment in rehabilitation traditionally relies on expensive optoelectronic reference tracking. While single-camera markerless systems offer an accessible alternative, their viewpoint-dependent accuracy and depth ambiguity remain critical concerns. This study systematically validates an integrated MediaPipe tracking pipeline (Pose and Hands) against an 8-camera optoelectronic system. Ten healthy males performed four clinical upper-limb tasks, recorded simultaneously across three spatial camera viewpoints (frontal, oblique elevated, and oblique side). To isolate spatial accuracy, validations utilized direct unmodeled geometric tracking vectors rather than clinical joint models. Results demonstrate that agreement and accuracy are systematically dependent on movement geometry and camera perspective. Planar tasks like shoulder abduction exhibited moderate to good peak range-of-motion agreement (ICC = 0.63 - 0.82) and precision (RMSE = 9.00∘ - 11.23∘). For transverse plane movements like shoulder rotation, an oblique elevated perspective improved peak range-of-motion agreement (ICC = 0.68). Out-of-plane tracking and fine distal rotations (forearm pronation) exhibited poor agreement (ICC ≤ 0.34) and substantial trajectory instability (RMSE > 30∘). Ultimately, while single-camera monocular RGB tracking provides sufficient validity for gross range-of-motion screening during planar upper-limb tasks, quantitative assessment of multi-planar and distal rotational kinematics requires multi-view camera fusion, depth sensing, or specialized 2D shape-regression architectures.