## 1 Introduction
In rehabilitation, considerable amounts of practice are required to induce neuoplastic changes and functional recovery of neurological motor deficits [24]. SCI rehabilitation works through mechanisms of experience-dependent plasticity [25]. The process of plasticity consists of changes in the activation pattern, either of structure or function of neural connections. These changes involve the modification of the strength of existing connections or the creation of new neural connections. Although it is likely that some of the reorganization after central nervous system injuries take place in the cortex, plasticity changes may also occur in subcortical structures, such as the spinal cord [26]. Therefore, neuroplasticity is essential in the SCI rehabilitation process. Nowadays, technology is typically involved in rehabilitation. It makes use of high cost devices, such as exoskeletons or robotic devices [27], or low cost devices, such as the Leap Motion, Kinect, or Novint Falcon, among others [28]. In this paper, we make use of the Novint Falcon device, a desktop haptic robot with 3 degrees of freedom (see Section 2). Several studies found in the literature have investigated the use of the Novint Falcon in healthy subjects: [PERSON] et al. analyzed the motor performance during six reaching movement sessions varying the force levels [29]. The subjects had to reach eight different objectives placed on a circumference (vertical and horizontal line and in each diagonal). [PERSON] et al. analyzed the response to different force feedback in healthy population with an educational purpose [28]. [PERSON] et al. used the Novint Falcon to train fine movements in a group of healthy developing children and noted significant improvements in handwriting performance [30]. However, there is no evidence of research studies with the Novint Falcon in spinal cord injured patients. Nonetheless, some studies have been performed with the Novint Falcon to analyze the UL function in patients who have suffered stroke [31; 32]. These studies have been conducted with a rehabilitation purpose. [PERSON] et al. evaluated the effects of repetitive arm movements with a group of eight post-stroke subjects by using the commercial virtual games included in Novint Falcon [33]. These games were oriented to reach UL functional tasks, following the rehabilitative principles. The patient performance was compared between pre and post condition by using only clinical tests. Unfortunately, in those studies, smoothness was not analyzed. Additionally, the use of technology in rehabilitation in combination with virtual reality (VR) environments has increased in the last years [34; 35; 36; 37], mainly because the quantity of therapy that patients receive increases. VR has many advantages for intervention, such as enabling the grading of activities, obtaining precise performance measures, providing a safe and ecologically valid environment, and being enjoyable and motivating [24; 38]. There is evidence about the acceleration in the rehabilitation process by the combination of these VR treatments with conventional rehabilitation [39]. Moreover, the use of VR environments can increase the intensity of the therapy through the adequate repetitions number [27]. VR reality games increase motivation in patients both in occupational and physical therapy [40]. Therefore, the health care sector is showing steadily increasing interest in serious games [41]. The work presented in this paper is centered in reaching the patient's assessment in an objective way by means of the smoothness movement characteristic. Therefore, the aim of the present paper, an extension of [42], is to develop three serious games based on UL functional tasks, manipulated by the Novint Falcon device, and to collect quantitative data about SCI patients for analyzing movement smoothness. Moreover, this research involves the novelty of applying force feedback to SCI patients while analyzing the movement smoothness.