Viewpoint Effects on Markerless Upper Limb Kinematics During Musculoskeletal Rehabilitation Exercises

This study investigates the performance of upper-limb kinematic reconstruction obtained using a low-cost, skeleton-based markerless motion capture system with RGB cameras, compared to a laboratory-grade markerless reference system. Kinematic data were collected from healthy participants performing selected upper-limb tasks commonly adopted in musculoskeletal rehabilitation protocols. Two camera configurations, i.e., frontal and lateral, were analyzed to evaluate their influence on upper-limb kinematic estimation.Preliminary results indicate that reconstruction accuracy is task- and viewpoint-dependent. For movements primarily occurring in the sagittal plane, the lateral configuration showed improved agreement with the reference system (Mean Absolute Error (MAE) equal to 2.7° in the best case), whereas for smalleramplitude or multi-planar movements the frontal configuration provided more consistent estimates (MAE ranging between 4°- 16° depending on the task and variable).Overall, the observed errors suggest the potential feasibility of low-cost RGB-based setups for upper-limb functional assessment in controlled environments. These findings highlight the importance of camera viewpoint selection and provide preliminary indications for optimizing acquisition setups in clinical and rehabilitation applications.