## 1 Introduction  
Spinal cord injury (SCI) is mechanical damage to the spinal cord. It can be traumatic when the damage results from physical traumas (car accidents, work accidents, gunshots, falls, sports injuries, etc.) or non-traumatic when it is caused by an underlying pathology (infection, tumors, musculoskeletal disease, etc.) [1]. SCI causes an alteration of sensory and motor function since there is an interruption of nervous tissue that performs the communication between the brain and the rest of the body [2]. According to the level of the injury, there are different types of paralysis that may be grouped as tetraplegia (arms, trunk, legs, and pelvic organs are affected) and paraplegia (trunk, legs, and pelvic organs are affected). Moreover, SCI can be classified according to the severity in incomplete injury when nerve communications are partially interrupted (some sensory and/or motor activity is retained) and in complete injury when the damage completely impedes nerve communications (no functions retained). Therefore, determining the cord segments affected by the spinal cord injury appears to be crucial in order to understand the gravity of the situation and to adapt the rehabilitation plan [3]. The worldwide incidence of SCI lies between 10 and 83 per million inhabitants per year [4]. In Spain, approximately 1000 new cases of SCI per year occur due to trauma, which is the main cause, half due to traffic accidents, and the rest due to falls, blows, sport accidents, or other injuries [5]. Most of these people have permanent damage. It is observedthat people with SCI have difficulties and limitations in mobility (96.9% of cases), in self-care (81.1% of cases), and in carrying out tasks of domestic life (84.3% of cases). Regarding the economic aspect, the lifetime financial burden brought on by SCI conditions can be massive for patients, insurance companies, and hospital systems. The costs associated are greatly affected by both the patient's extent of injury and the subsequent degree of disability, regarding the acute and post-acute phases as well as the chronic phase within the context of longer-term rehabilitation [6]. Reducing therapy cost by including lower cost technologies to improve the quality of life for SCI injured patients is suggested. It is estimated that cervical SCI accounts for approximately more than one-third of all people living with SCI [4]. The loss of upper limb (UL) function and in particular the use of the hands is one of the most devastating losses an individual can experience [7]. In contrast with lower limbs, upper limbs comprise different joints that allow execute several fine movements thanks to complex neuromuscular control. This results in a considerable and crucial functionality of these elements. Consequently, upper limb and trunk motion impairment caused by SCI has a high impact on the independence and quality of life of the affected person [8; 9]. Each small improvement in upper limb motor control is fundamental and translates to important amelioration in the patient's independence and confidence in everyday life [10]. In particular, the role of the hands appears to be fundamental in everyday life and could reduce dependencies on the caregiver: Even with few movements of the finger, a tetraplegic person could accomplish different useful tasks such as pressing a switch, interacting with a keyboard to communicate, or holding an object [11]. Among others, pinching and grasping movements are essential gestures in everyday life situations. In rehabilitation, functional neuro-recovery is emphasized, which aim is promoting the restoration of function through use of the affected limbs [12]. The goal is to promote plasticity, that is, the ability of neurons to rearrange their anatomical and functional connectivity in response to environmental input, thereby achieving new or modified outputs, namely, behaviors.