## 1 Introduction
### Designing Gamified Virtual Reality Content For Rehabilitation  
Despite there being currently no standardised criteria for the selection of commercial games for rehabilitation, some literature reviews have succeeded in establishing general guidelines to support the selection of systems [28] and games based on therapist and patient needs [5, 18]. These guidelines are designed to evaluate how well existing VR commercial games can be adapted to a rehabilitation scenario; these frameworks focus on characterising the degree to which software and task parameters can be controlled so the therapeutic goals are achieved. As such, they do not aim to guide the design of newly developed VR applications for rehabilitation. However, the aspects they focus on offer valuable lessons that must be taken into account. This is especially important when we consider that clear strategies for the development of home-based rehabilitation therapies based on VR technologies have yet to be determined.  
Taking these lessons into account, we can see that VR scenarios must implement motivational strategies based on self-determination and task-oriented challenges [19, 29]. Such strategies stimulate the self-improvement of the individual and increase the patient's autonomy [30, 31] during the practice. Game mechanics based on them have shown that increasing the user's engagement leads to more practice and also to higher quality practice [18].  
Concrete guidelines for the design of effective rehabilitation games that go further than that are still not present in the literature. This lack of guidelines in the design of VR and augmented reality (AR) content has also been pointed out in the field of physical exercise for the older population [32]. Moreover, many rehabilitation solutions present in the literature that use the term \"VR\" are non-immersive applications in which the user seesa representation of their own body on a computer monitor [26; 27; 33; 34; 35]. This terminology problem is exacerbated when looking for design guidelines for what is now being called \"fully-immersive VR\" in the literature, since these solutions are not thoroughly explored in the field of functional rehabilitation.  
On top of that, the VR-based solutions present in the literature are not focused on bimanual training. This is especially important for this work, since most ADL tasks typically involve asymmetrical bimanual movement [36]. There was an study [37] that focused on bimanual training, but again, not with a VR solution. Game design approaches for bimanual rehabilitation systems in VR are still unexplored, and there is a lack of guidelines that can guide their development.  
This work introduces a VR telerehabilitation system that uses off-the-shelf hardware and a newly developed application: FarmDay. This application aims to enhance the bimanual upper extremity motor function of patients with ULD. Its design, due to the aforementioned gap in the literature, was guided by the expert opinions of practicioners and evaluated by them. This way, a set of design principles for the development of VR-based rehabilitation solutions and a collection of valuable lessons learned can begin to form.