## 4 Discussion  
The purpose of this study was to assess the concurrent validity of a smartphone application employing a 2D video system for supervising shoulder complex exercises in comparison to a gold-standard optoelectronic 3D system. Our hypothesis centered on the feasibility of employing an affordable and portable system as a kinematic tool with reasonable accuracy for guiding home-based rehabilitation exercises when compared to the gold standard. Our findings indicate that agreement varies depending on the body segment and exercise in question. Starting with the analysis of the HEAD segment, we observed a wide range of limits of agreement in both exercises, representing significant variation in the percentage difference between the two methods. The Bland-Altman analysis also reveals a consistent bias trend. As the movement quality index of each subject decreased, the percentage difference between the two methods increased. However, upon closer examination of the Bland-Altman plot for the HEAD segment during the M90 exercise, it becomes evident that an agreement between methods exists when the movement quality index exceeds 90%, as evidenced by mean differences approaching zero [34]. This result may suggest that there is an alignment between the application and the 3D system when participants perform the guided and non-guided exercises almost identically. Future studies should assess the application in populations more prone to compensations, such as those with associated symptoms, as our participants generally demonstrated good movement quality. Regarding the trunk segment, which plays a pivotal role in both exercises, we observed narrower ranges of limits of agreement and bias values compared to the head segment. The bias trend was again evident in the Bland-Altman analysis and confirmed by the regression analysis. However, in the context of the D1 exercise and considering the analysis of the trunk segment, the average variation in the movement quality index ranged from 70% to 100%, and for the M90 exercise, it ranged from 90% to 100%. This finding suggests that, overall, participants displayed consistent performance in both guided and non-guided versions of the exercise. Consequently, the observed trend associated with the trunk segment in both exercises may have limited clinical significance, as the small variation in the average movement quality control (\(x\)-axis) indicated an optimal kinematic relationship between GE and NGE. In the shoulder segment during exercise D1, a narrower range of limits of agreement was noted, indicating reduced disparities between the 2D and 3D kinematic analyses. This phenomenon may be linked to the lower bias values observed in this segment and exercise. Furthermore, in this exercise, the shoulder segment displayed a narrower range of variation, with the movement quality index varying between 90% and 100%. The diagonal movement performed by the shoulder complex is recognized as a functional motion commonly employed in numerous daily activities [31,35]. This may explain the participants' higher proficiency in executing this movement without direct supervision from a physiotherapist, closely mirroring their performance in the guided exercise. This suggests that the observed bias trend may not have clinical significance. However, in the M90 exercise, the shoulder segment, despite showing higher values of limits of agreement and bias compared to the same segment in the D1 exercise, did not exhibit the consistent bias observed in the other segments and showed no statistical differences between methods