## 5 Discussion  
This work presents the development and usability evaluation of an upper limb rehabilitation system based on a UCD approach for active rehabilitation in post-stroke patients. One of the advantages of using UCD is the continuous feedback from therapists, which facilitates the acceptance of new technologies in clinical practice. This acceptance is necessary because the quality of rehabilitation can be affected by the rejection of this technology [50].  
A guideline that helps develop new rehabilitation technologies is necessary for a successful transfer of rehabilitation research to the clinical environment. In [51], the authors detected that usability and clinical effectiveness are relevant points for the integration of rehabilitation systems. Another essential point related to the system, is to satisfy the requirements of therapists, such as comfort and safety, and if it is easy to set up and fun to use.  
The objective of this work was to measure usability, user experience, and observe how feedback from therapists can increase the usability of a system. In addition, a general comparison was made between the use of a monitor and HMD as the visualization system; however, the system needs to be evaluated with post-stroke patients to observe its clinical effectiveness. For this evaluation, the use of specific scales for virtual environments in rehabilitation such as the Questionnaire for Satisfaction Evaluation of Virtual rehabilitation (USEQ) [52], and presence questionnaires will provide relevant information related to the performance of the system in a clinical environment, including which visualization system will be more effective in the clinic.  
With this work, it is possible to identify some elements that make a rehabilitation system with a VE usable for therapists, such as the ease of using it, the integration of GUI elements that are easy to understand, and intuitive interaction, easy-to-learn functions, and engaging virtual scenarios for patients. In addition, with identifying the requirements shown in Table 2, the system integrates valuable features for the therapist, for instance, the selection of predefined levels, the medical scales used by therapists, the best position of the patient, and the movements involved in rehabilitation.  
The therapists' involvement in developing the proposed system is reflected in the scores of usability (see Table 7), pragmatic and hedonic qualities, and attractiveness (see Table 8), resulting in a system with high usability and attractiveness to the users.  
On the other hand, the haptic device's evaluation from the volunteers show similar average scores when they use the monitor or the HMD. The most significant difference is in characteristic number eight, where the volunteers feel greater precision when using  
Figure 11: User performance with each display system.  
the HMD. This could be related to the immersion that this device provides. This difference is also reflected in the VE evaluation, where the same characteristic obtains the highest difference.  
In addition, the quantitative data obtained from the volunteers' evaluation was useful to detect any difference between the use of an HMD and a monitor. The usability results showed that there is not a remarkable difference between the two display systems. With the improvements made after the therapists' evaluation, the system obtains better results in the hedonic and pragmatic qualities, meaning that the changes made after the therapists' feedback improved the system.  
This research indicates that a VR rehabilitation system that includes the needs of the therapy and the therapist could be a helpful tool in UL rehabilitation; this allows acceptance in a clinical environment and reduces rejection of this type of technology by the therapist.