## 1 Introduction  
According to the World Health Organization (WHO), 15% of the world's population have some type of disability, representing more than one billion people. Of these one billion, almost 200 million have significant difficulties in functioning [1].  
Current disability rates are increasing, and demanding access to health rehabilitation programs will be a significant concern in the years to come. Data from the National Institute of Statistics and Geography of Mexico show that approximately 7.1 million people have a disability and, of these, 33% have a disability in the upper limbs [2].  
The essential type of rehabilitation that restores mobility and movement efficiency is a therapy based on the movement and exercise called kinesitherapy [3]. If the therapeutic movement is carried out without any collaboration from the patient, it is called passive rehabilitation [4].  
This requires an intense involvement of qualified therapists for the treatment, who conduct the whole process and perform the appropriate exercises with the patient [5]. This process is very exhausting and time-consuming for the therapist, and so the therapies are not intensive and long-lasting [6].  
The increasing number of people with functional disabilities and the importance of having good rehabilitation has led to the research, development, and introduction of robots that can contribute significantly to improving the results of these programs [7].  
The aim of these robots is the application of technology to the development of devices that assist and enhance rehabilitation therapies for people with disabilities [8]. It is shown that robotic therapy contributes to the recovery of motor skills of the affected limb [9], but the obtained functional results can be improved [8].  
Robotic devices can be used by clinical staff or by patients according to medical treatment determined by the specialist so that the physical interface allows a direct transfer of mechanical energy and the exchange of information [10]. These devices are designed to match the shape and function of the human body. The links and joints correspond to the limbs of the human body; the exoskeleton is coupled externally to the person [11]. This device considers the obstacles to optimal functioning and creates designing solutions for the user to interact successfully in their environment [12].  
One of the main qualities of robotic rehabilitation systems is that they can provide the desired number of therapy repetitions without decreasing the therapy's quality, which is very useful for users [13]. One of these devices is an exoskeleton, which is a robotic system that adheres externally to the human body to perform specific functions; their main characteristic is their high dependency on the movements of the joints and extremities of the human body. A person uses these systems in such a way that the structure of the exoskeleton leads to the direct transfer of mechanical energy [9; 14].  
These projects are considered as assistive and rehabilitative devices for the disabled or elderly. As a portable device, it must have several aesthetic and functional characteristics. Aesthetics is directly related to the size, weight/inertia, comfort of wearing, extensive range of motion, complexity, and appearance of the exoskeleton [15]. The functionality is related to generating the required torque and velocity while maintaining the robustness of operation.  
The main problems encountered when developing an exoskeleton for rehabilitation are determining the movement tasks to be performed and defining the appropriate mechanical design for the movement tasks performed by the patient since their prescription and mechanical input constrain the mechanical design and control [16]. The optimal safety and performance of a rehabilitation device require a design methodology in which everyone involved in the device design contributes to [17].