## 1 Introduction  
The reduction of movement in specific parts of the body is caused by diseases whose origins are motor issues. In these cases, the treatment consists of rehabilitation therapies with a great number of sessions to increase the patient's quality of life. Unfortunately, there exist major obstacles beyond the disease that make therapies difficult. One of them is related to the economic burden, since health systems are hard to be sustained in the long-term because of the increasing prevalence of chronic diseases [1]. On the other hand, an inconvenience for patients (and their caregivers, in the case of dependent people) involves taking them to the hospital or the rehabilitation center. This situation has become really evident during the situation of confinement caused by COVID-19. Another, but no less important barrier, concerns the lack of motivation of patients when it comes to therapy engagement. Given that rehabilitation sessions usually consist of repeating the same exercises over and over again, this task ends up being monotonous and extremely boring. Consequently, this can lead to an eventual loss of interest in the therapy, which may result in patients quitting the rehabilitation treatment. All of these obstacles, along with others, are discussed in [2]. The last two barriers should be especially considered when dealing with patients who are children or adolescents. On the one hand, they need to be accompanied by an adult to the rehabilitation center, which entails inconveniences for the families. On the other hand, while adults are aware that therapy itself benefits them, young people tend to have less determination to perform tasks if they do not obtain some short-term reward in exchange [3]; therefore, motivation is essential for them to maintain their commitment to the exercise program [4; 5; 6]. Technological advances have offered alternatives to mitigate this problem. Among these are telemedicine-based solutions, which allows us to reduce costs and avoid the inconveniences associated with taking the patients to the hospital or the rehabilitation center [7; 8]. Additionally, in order to increase the patient's motivation, the use of serious games is considered [9; 10]. These games promote entertainment while performing the exercises. In the field of physical rehabilitation, a specific type of serious game is used: the so-called exergames. They involve physical exercises while the patient interacts with the game. There is evidence that the use of exergames improves the motivation and adherence of patients in rehabilitation treatments. In fact, it has been demonstrated that exergames help to promote physical exercise and well-being, in addition to improving cognitive functions or psychological factors, among other benefits [9; 11; 12; 13]. A significant number of exergame-based systems can be found in the literature. Most of them make use of sensors attached to the patient's body, which can be uncomfortable and even difficult to use at home [14]. This problem has been solved, at least partially, by the appearance of commercial devices, such as _Microsoft Kinect_, _Nintendo Wii_, or _Leap Motion_. These devices are capable of capturing the patient's natural movements without the use of markers or devices attached to the body. These types of devices facilitate the deployment of exergames at home [4; 5] and the execution of the exercises in a more straightforward way. Some video games support the graphic representation of the user (or player) in two or three dimensions, which is known as a virtual avatar. This type of representation makes the system work as a \"virtual mirror\", that is, it allows the patients to perceive their movements and provides them with immediate visual feedback to know their level of success. It has been proven that the use of these non-intrusive devices makes the treatment more motivating, which brings benefits to the motor recovery process [15].