# Adjustable Stiffness-Based Supination-Pronation Forearm Physical Rehabilitation
## 1 Introduction
To the best of the authors' knowledge, there is a deficit in rehabilitation robots that are compact enough, light, and powerful enough in carrying out rehabilitation exercises. Moreover, the recommendation for new developments concerning medical devices, provided by the sustainable development agenda of the United Nations, establishes that these systems must be light weight and accessible [18]. Systems with elastic joints have gained interest in the rehabilitation robotics community due to the compliant robot interaction with the patient and the therapist. Adjustable stiffness robots promise to be beneficial regarding robustness and task adaptability [19; 20], and they are developed to be passively compliant, robust, and dexterous. Systems with similar characteristics are mentioned below. Tensegrity systems are self-stressed systems defined as a set of compressed bars in a set of tensioned elements that can be either rigid or elastic cables [21]. Due to their lightness, compliance, flexibility, and architecture, tensegrity robots demonstrate structural properties that provide them with attributes similar to their biological counterparts, which make them ideal for rehabilitation [22]. In [23], a wearable and compliant robot is designed to harmonize with the human body's upper limb where a network of tensioned lines (cables) connected with motors provides augmentative and compliance forces to perform natural arm movements. Cable-driven systems have desirable features, including a large workspace, easy implementation, lightness, reconfiguration, and high-speed movement [24]. In [25], a cable-driven articulated system was designed to assist repetitive gait training by changing cable lengths. This adjustment generates a new configuration (geometrical form), producing a change in its stiffness. According to [26], control schemes that deal with both reconfiguration and adjustable stiffness in the cable-driven and tensegrity systems are difficult to obtain. The purpose of the current study is to develop and control a supination-pronation device that is able to recreate therapy exercises with or without intervention of a physical therapist. The full wrist-forearm supination-pronation ROM is shown in Figure 1, which represents the motion of interest in this paper. It is highlighted that, depending on the medical condition of each patient, the angular displacements provided by the system can be generated with smaller values than those illustrated in Figure 1.  
Since most rehabilitation exercises result in repetitive movements, these can be performed by an appropriate rehabilitation device, but only a few rehabilitation devices are capable to perform the three phases of rehabilitation [27]. One of the main objectives in rehabilitation systems is to allow flexibility in movements, forces, and a customized training regime. Then, according to different rehabilitation phases, system reconfiguration can be achieved by using cables while springs or flexible elements allow adjusting the stiffness of the system. Upper limb devices can be divided into prosthesis and orthosis: \"A prosthesis is an artificial device that is used to replace or augment a missing or impaired part of the body. An orthosis is an orthopedic appliance or apparatus used to support, align, prevent, or correct deformities or to improve the function of movable parts of the body\" [28]. Orthosis can further be divided in extension modules of the task-space arm and exoskeleton rehabilitation devices. Some rehabilitation devices focusing on the wrist and forearm joints are briefly described in Table 1. The main disadvantage of these devices is the inability to modify torques in order to recreate different rehabilitation phases.