# Perspectives and Challenges in Robotic Neurorehabilitation
## 2 An Overview of Robotic Devices for Neurorehabilitation
### Robotic Neurorehabilitation for the Upper Limb  
Various robotic systems for the upper limb have been developed, and protocols based on task-oriented repetitive movements have been proposed to improve ROM [19; 20], muscle strength [21], movement coordination [22], and to promote motor learning [23]. Depending on the type and severity of the motor dysfunction and related impairment, one type of device could be more effective than the other. Specifically, if the residual sensorimotor functionalities of the patient are extremely low, exoskeletons could be more appropriate to apply forces to each joint [24]. Moreover, end-effector devices could be more effective to deliver complex patterns of forces (e.g., based on assistance-as-needed strategies) able to exploit the redundancy of the human body, thus speeding up sensorimotor recovery [25]. One of the first end-effector robots developed for upper limb rehabilitation, the MIT Manus [26], belongs to the laboratories of the Massachusetts Institute of Technology (MIT) and was designed for the shoulder and elbow joints. Other interesting examples are the ARM (Assisted Rehabilitation and Measurement) Guide [27] (a counterbalanced robot that does not load the arm and mechanically assist the reaching movement), the GENTLE/s (Robotic assistance in neuro and motor rehabilitation) [28], the Italian NeReBot (Neurorehabilitation Robot) [29], and the ACT\({}^{\text{3D}}\) (Arm Coordination Training Robot), specifically used to quantitatively measure abnormal joint torque coupling in chronic stroke [30]. Additionally, the Mirror Image Motion Enabler [31] and the Bi-Manu-Track [32] are two examples of upper limb robotic devices designed to implement bimanual training protocols. A summary of the most common upper limb end-effector robots and their main features is provided in Table 1.  
Regarding the use of exoskeletons in the neurorehabilitation of the upper limb, interesting attempts have been proposed: SUEFUL [49], ARMin III [50], CADEN (Cable-Actuated Dextrous Exoskeleton for Neurorehabilitation) [51], and RUPERT (Robotic Upper Extremity Repetitive Trainer) [52]. It is worth mentioning that the exoskeletons currently developed differ in terms of mechanical structure. In detail, regarding the upper limb, most of them do not provide actuation for all the degrees of freedom (see [53] for a complete review), as they are only equipped with motors for the movements of the shoulder (L-Exos [54], the Pneu-Wrex [55]) and elbow joints, while additional actuation for the wrist is not available. On the contrary, the prototype, UL-EXO7 [51], and the commercial exoskeleton, ARMEO Power, developed from the ARMinIII [50], also provide forces on the wrist and forearm. The design of the exoskeleton for the hand is indeed more difficult. Yet, remarkable examples, such as the Manovo Power (eventually integrated in the ARMEO Power (Hocoma, Switzerland)) and the IntelliArm exoskeleton [56], exist. Those devices are actuated to train hand opening and closing in reach and grasp movements or fingers' passive stretching. Table 2 summarizes the exoskeletons for the upper limb. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**End-Effector Name and** & **Classification** & **Actuators** & **Targeted** & **Training** & **Type of** \\  
Another interesting classification of exoskeleton and end-effector devices relates to their actuation system. Available possibilities are actuation by a motor, actuation by pneumatic muscle, and non-motorized actuation (such as hydraulic or springs) [53]. In the first category, i.e., actuated by an electric motor, are the exoskeletons, L-Exos [54], UL-Exo7 [62], GENTLE/G [66], REHAROB [67], and Armeo Power (Hocoma, Switzerland), and the end-effector systems, InMotion ARM, In Motion WRIST (Bionik), MIT-MANUS [33], and Braccio di Ferro [43]. Conversely, the exoskeletons, Pneu-Wrex [55] and BONES [65], are based on pneumatic muscles.