## 2 Materials and Methods
### Therapists and Therapy Requirements Analysis  
For the development of the system using UCD, for the first step, a literature review in UL motor rehabilitation was conducted in order to design the first questionnaire. The aim of it was to obtain information regarding traditional rehabilitation characteristics and therapists' requirements. The questionnaire consists of two parts (See Table 1). The first one provides information related to the therapists and therapy requirements. The second one looks after information about the use of technological tools in rehabilitation, such as video-games or robots. The questionnaire was applied to 16 therapists with experience in stroke rehabilitation. Six therapists belong to the Clinica Multidisciplinaria de la Salud from the Autonomous University of the State of Mexico, and the other 10 to the Centro Estatal de Rehabilitacion y Educacion Especial; both institutions are located in Toluca, Mexico. From the answers, the movements performed by the patients in post-stroke rehabilitation were identified. Therapists emphasized that the evaluation of the patient's rehabilitation progress should employ clinical scales, which became an essential requirement in developing this system. Additionally, they mentioned other metrics that could help them, such as time, range of motion, hits, and errors. With this questionnaire, a global idea about the clinical scales was obtained. With the analysis of the questionnaire results, seven requirements (RQ1-RQ7) were identified, as shown in Table 2. Three requirements were related to the therapists' needs, and the others related to the rehabilitation requirements. To fulfill RQ1 (see Table 2), therapists should have predefined levels instead of modifying the parameters of virtual scenarios, such as object velocity, duration time, number of hits, etc. Therefore, each level should increase in difficulty, according to feedback provided by the therapists. To cover RQ2, a screen with medical scales was designed. It is worth mentioning that the scales integrated into the virtual environment do not add information according to the patient's performance by themselves, and it is necessary that the therapists manually modify them (as explained in Section 2.2.2). For RQ3, the information related to the patient's performance and medical scales were saved in a CSV file with the user's ID. The ID was assigned to them after the therapists created an account for each patient. The last four requirements, RQ4-RQ7, relate to therapy needs; these requirements allow the system to have similarities with traditional rehabilitation to achieve the same objectives. To cover them, the user should not perceive a delay between the haptic device and the VE (RQ4) [41]. Moreover, according to the questionnaire answers, the patient must be sitting or recumbent during the rehabilitation; therefore, the proposed rehabilitation system requires the user to sit in a correct posture (RQ5). Finally, the haptic device allows the free movement of the shoulder, elbow, and wrist of the patient (RQ6), including the identified movements involved in traditional rehabilitation (RQ7), these movements are focused on active rehabilitation, which is why the movements of the fingers were not considered.