Smartphone-Based Video Analysis for Guiding Shoulder Therapeutic Exercises: Concurrent Validity for Movement Quality Control  
[PERSON]  
1 Centro de Investigacao em Reabilitacao (CIR), Escola Superior de Saide, Instituto Politecnico do Porto, [PERSON], 400, 4200-072 Porto, Portugal; [EMAIL_ADDRESS] (M.L. ); [EMAIL_ADDRESS] (B.C.) 2 Centro de Investigacao em Atividade Fisica, Saude e Lazer (CIAFEL), Faculdade de Desporto, Universidade do Porto, Rua Dr.  
process is extended to home. Recent studies have highlighted both the advantages and challenges associated with home-based rehabilitation [4; 7; 8]. Home-based rehabilitation offers logistical benefits, optimizing resources, reducing travel time, and providing flexibility in appointment scheduling [9]. This rehabilitation modality has shown potential in enhancing users' performance in daily activities, improving functional capacity, and enhancing overall quality of life. The home environment, rich in context-dependent learning opportunities and the use of familiar objects, increases the likelihood of transferring acquired skills to daily living activities [10]. However, the effectiveness of home-based rehabilitation may be influenced by several factors. The absence of direct clinical oversight, despite efforts to include continuous monitoring, pain education, and feedback in the home setting, could impact the overall success of the intervention. The adherence to prescribed home-based exercise programs has been posed as one of the main challenges in home-based rehabilitation. The non-monitorization of critical parameters such as frequency, intensity, sets, repetitions, rest time, and exercise speed can contribute to this fact [4; 11; 12; 13]. Therefore, further research to optimize home-based implementation is required. To address these limitations, among the various proposed solutions, one that shows high potential is the use of smartphones. Smartphones could be an interesting strategy as they can increase the attraction and involvement of users in the rehabilitation process. Given that it is a device that is easily used in daily life and available anywhere, it allows the bridging of gaps related to time, space, and costs and, more importantly, allows remote monitoring by the physiotherapist [4; 10; 14; 15; 16]. Several studies pointed to the use of smartphones to support rehabilitation as being very promising [17] since it would increase the effectiveness of exercise-based physiotherapy interventions in this setting and, thus, encourage health gains [4]. Two-dimensional (2D) systems that are incorporated in smartphone's cameras are simple to use, easily accessible, and affordable. However, their performance when compared to other systems, such as three-dimensional (3D) systems, is not yet well established [18]. Motion capture systems, widely used to quantify human movement, and 3D motion capture systems are considered the gold standard in human movement analysis in terms of accuracy and reliability [18]. However, 3D analysis methods are expensive and cannot be used in in home-based settings. On the other hand, the overall performance of 2D motion capture systems is not yet well established, which could justify the lack of broad utilization of these systems in both research and clinical contexts [18]. Given the aforementioned, there is a clear need to provide satisfactory evidence for the validity and reliability of smartphones' 2D video system as a tool to guide movement rehabilitation through movement quality control. Considering its portability, real-time data responsiveness, and the standardization of its usage, this smartphone application featuring a 2D camera system has the potential to offer benefits to the scientific community, patients, and rehabilitation professionals, ultimately enhancing treatment adherence and overall quality of life. Thus, the present study aims to validate a smartphone application which, through video recording monitors, supervises the execution of shoulder therapeutic exercises and gives the user feedback regarding the movement quality.