# Adjustable Stiffness-Based Supination-Pronation Forearm Physical Rehabilitation
## 3 Adjustable Stiffness Rehabilitation Device
According to existing wrist-forearm rehabilitation protocols for supination-pronation exercises (see [38; 39; 40]), the following procedures for the use of the adjustable stiffness device reported in this paper are suggested:  
* Grasp the patient's wrist, supporting the hand with the handle, and stabilize the elbow and the forearm whether sitting or standing;
* Supination and pronation should be performed with a \(90^{\circ}\) elbow/forearm flexion or with a \(90^{\circ}\) shoulder/arm flexion;
* Supinate or pronate the forearm until a slight pain arises with previously established trajectories;
* Apply resistance through stiffness control actions as the patient evolves in his/her rehabilitation. Figure 4: Linear actuator, load cell, spring, and cable assembling. Figure 3: Handle and pulley-actuator interconnection. To achieve full forearm rehabilitation, the adjustable stiffness device must be prepared for the implementation of the three aforementioned rehabilitation phases by means of an accurate trajectory tracking process and the possibility of adding or reducing loads during an exercise. Thus, three different command phases for wrist-forearm supination-pronation rehabilitation are identified as follows:  
1. **Passive:** A trajectory, previously established by a therapist as a function of the patient's medical condition, is tracked at different angular speeds by the adjustable stiffness rehabilitation. In this therapy phase, the patient will not make any effort to perform the movement because the actuators will be in charge. 2. **Active-Assisted:** The therapist defines a desired trajectory, and the patient is asked to move his/her wrist-forearm from the neutral position (\(\theta=0\)) toward a desired one. If the patient is not able to perform the desired movement, the rehabilitation device completes the task. 3. **Active:** In this resisted phase, the desired trajectory and the tension values are previously established by the therapist. The rehabilitation device measures angular positions and tensions in the cables and provides the required forces to successfully complete the exercises. The control delivers a part of the required tension through springs deformations. An important rehabilitation robot problem is related to the fact that the robot must fit around the respective human limb. This means that the robot must be ergonomic and provide comfort in treatment. Moreover, robotic joints and links must, physically, avoid human joints and links while performing motion in order to prevent pressure points (sores). Some exoskeletons have used ring bearings to enclose the forearm [33; 34], allowing the rotation axis to lie within the forearm's envelope. The forearm supination-pronation movement results in a rotation axis inside the forearm. The one DOF of the adjustable stiffness rehabilitation system is associated with the rotation of the device about this axis. The device reported in this paper is adapted to the anthropomorphic measurements of the wrist-forearm, which have been analyzed in [41]. To allow proper training, the adjustable stiffness system must be able to move the patient's wrist and forearm throughout the entire supination-pronation ROM (see Figure 1). For wrist-forearm rehabilitation, the system configuration is as follows: The system has an initial pre-stress level such that tensions are equal (\(F_{10}=F_{20}=F_{30}=F_{40}=F_{0}\)) when the device is in its initial or neutral position (see \(\theta=0\) in Figure 3). The linear actuators generate displacements in the aforementioned cables that produce torques, generating angular motion. The initial movement is the supination (pronation) motion when rehabilitating the right (left) hand. Once the desired ROM is achieved, the pronation (supination) motion starts, passing through the neutral position. Figure 5 illustrates the movements that the reported system can generate. Starting at the neutral position (see Figure 5a) where \(\theta=0\), supination (Figure 5b) or pronation (Figure 5c) motion can follow.