1.1. Rehabilitation of Upper Limb Disorders
Upper limb disorders (ULD) represent a major concern for healthcare systems because of the considerable economic burden they generate [
1]. Most patients with ULD are limited in their activities of daily living (ADL) and require a continuous rehabilitation process [
2], whose aim is to restore the lost capabilities and range of motion. Due to the complex nature of this type of injury [
3], there are discrepancies about the most effective intervention therapies for ailment. However, most physical intervention methodologies agree that the patient must perform strength and repetition tasks [
4,
5,
6]. Such tasks enable the patient to work on various parts of the body in a structured and separate manner. However, strength and repetition tasks, by their own nature, do not train the patient in the complications inherent in the organization of whole limb movements. This fact hinders functional improvement in ADL and delays the eventual return of the patient to a productive and fulfilling life.
One of the major concerns involved in this type of rehabilitation is that the patients need to experience enough repetitions to induce the underlying neuroplastic adaptations necessary for their improvement [
6]. For many patients, such a volume of therapy is economically unfeasible when it must be done in specialised centers and under the supervision of trained physicians. This is why conventional home-based therapy has become an alternative to extend the practice hours outside the clinic sessions and helps to establish a steady regime. However, most home-based therapy programs are monotonous and repetitive, resulting in a lack of motivation and reduced commitment from the patients [
7]. This causes low adherence to treatment, which is crucial for achieving patient recovery. On top of that, such programs also lack the supervision of a therapist that can properly ensure that the exercises are correctly performed. Put together, these two facts are the reason that most home-based programs fail to influence the rehabilitation results in a noticeable way [
8].
To try to mitigate this problem, several telerehabilitation solutions have been proposed. However, most are limited to offline monitoring by the therapist, telephone calls, or in some cases, videoconferencing [
9,
10,
11]. There are more elaborate approaches that involve the installation of equipment in the patient’s home [
12], designed and custom built specifically for this purpose, and they are therefore, expensive. To avoid this high cost, some authors have tried to use off-the-shelf products, such as video game consoles and peripherals. These solutions, due to their origin in gaming, were expected to increase the patient’s engagement. However, they are less than ideal for rehabilitation tasks, since commercial games and their peripherals have not been initially designed for the specific movements that the patient must perform [
13,
14] and are not yet sufficiently engaging for all patients [
13,
15]. In some cases, this gap cannot be bridged, due to the inability to develop custom software solutions or modify already available games for proprietary systems, such as game consoles. On top of that, these games are difficult to operate for patients [
16].
On the other hand, the use of virtual reality (VR) systems used as complementing tools for rehabilitation and motor retraining have proven successful in promoting patient adherence to therapy and optimising therapeutic gains [
17]. Indeed, virtual reality promotes adherence to physical activity (PA), whether commercial or customisable games are used. One study [
18] pointed out that the adoption of home-based virtual rehabilitation at home for patients with neuromotor disabilities could be enhanced by using commercial games, but only if these detect compensatory movements, integrate social platforms, provide kinematic reports, and track the patient’s progress. However, the development of purpose-built VR games allows one to provide task-oriented exercises, and visual and auditory feedback regarding the performance of the patient’s PA [
19], which could lead to neuromuscular reeducation and functional improvement [
20]. The studies of Wu et al. (2019) [
21] and Tarakci, E. et al. (2019) [
22] evaluated the effectiveness and feasibility of their VR solutions for hand and upper extremity rehabilitation, respectively, using Leap Motion, a commercial device that enables hand recognition. Their findings pointed out that movements learned in VR environments can be transferred to real-world equivalent tasks in most cases, and that VR engages the player to increase the rehabilitation intensity.
Taking all these facts into account, the benefits that VR telerehabilitation solutions can provide are usually dampened by the use of expensive equipment and/or the need to rely on commercial games. Therefore, it is necessary to build solutions that (a) use off-the-self low-cost hardware VR devices instead of costly and custom equipment, and (b) employ software applications specifically designed for the rehabilitation tasks instead of commercial games.
The main novelty of this work is that it objectively defines the actions and ranges of movement that must be achieved in each VR game for upper limb rehabilitation of bimanual ADL movements. This clarity in the design represents a guideline not found to date in the literature for the construction of virtual environments (VEs) for functional rehabilitation. Additionally, of course, this VR application is supported by a real-time remote system using a novel feasible methodology for video compression.
1.2. Hardware Selection Considerations
Concerning the hardware, most existing game-based therapies focus on two fundamental features: to induce as much immersion as possible and to accurately track the movements of the patient. The former improves the user’s engagement in a task that is eminently physical (increases the adherence to treatment), whereas the latter is necessary for assessing if the movements have been properly performed (reduces the need for supervision by a therapist).
To accurately capture the movements of the patient during the rehabilitation session, it is desirable to use sensors with six degrees of freedom (movement and rotation in the three axes). This is why the Kinect™ device is one of the most widely-used systems for motion capture in the field of physiological therapy [
23,
24].
Virtual reality devices, by their own nature, are capable of very accurate motion capture, since precise tracking of the user’s head and hand movements is required for an immersive simulation, which is precisely the other criterion listed before. Therefore, VR provides reliable and precise data about the hands and head motions, and at the same time, produces an immersive simulation. On top of that, unlike other commercial closed solutions, such as video game consoles, it is easy to develop and distribute custom software for most VR devices.
Although VR headsets have their drawbacks (for instance, they may be challenging to use for some stroke survivors and cerebral palsy patients [
25]), rehabilitation solutions based on immersive environments are more effective than traditional [
26] because they:
Improve the engagement of the patient: Exergames increase the energy expenditure of the patient and involve both cognitive and physiologicaly rewarding tasks [
27], improving the adherence to treatment.
Provide physical fidelity to real movements: The patient performs motions similar to those needed for analogous daily life situations.
Provide cognitive fidelity to a real situation: The patient completes activities inside an environment designed to be similar to the real world.
Therefore, it is expected that an immersive VR solution for telerehabilitation will increase the patient’s adherence to treatment and improve the fidelity of the movements performed. However, such a solution requires not only the use of a VR head-mounted display (HMD), but also a high-performance computer that renders the virtual experience and sends the movement and session information to the therapist. Such a computer could be a considerable expense—an economic barrier to access that limits this solution.
1.3. 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 sees a 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.