# Perspectives and Challenges in Robotic Neurorehabilitation
## 2 An Overview of Robotic Devices for Neurorehabilitation
### Robotic Neurorehabilitation for the Lower Limb
In terms of actuation mechanism, the choice generally falls on electric motors, but an exception does exist when switching to other applications, such as human performance augmentation, as in the case of the BLEEX exoskeleton [109], intended for military use, which is equipped with hydraulic linear actuators. Likewise the upper limb, lower limbs robots use control strategies that take into account motor control paradigms useful for the acquisition or re-learning of motor skills [110; 111]. In this perspective, an interesting example of a control strategy is the \"cooperative patient control\" that implies an active involvement of the patient during walking [112]. In this case, the robot imposes a previously defined physiological trajectory, but, at the same time, it allows the patient to move without temporal constraints. Thus, the subject can perform each step at his own pace while exploring their own motor strategy: This motor command exploration is crucial for the brain to promote motor learning [113]. Therefore, the level of effort and the exploitation of variability [114] are enhanced during the protocol with respect to passive mobilization of the legs, where trajectories are imposed (i.e., position control). The LokoMat supports the cooperative control strategy. Finally, another interesting control strategy in the field of locomotor rehabilitation is to employ surface electromyography to control the level of assistance [115] after the readout of the patient's intention to perform a movement, like knee flexion [116] (implemented in the HAL exosuit). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Exoskeleton** & & & & & **Trigger Event for** & & \\
**Name and** & **Classification Actuators** & **Actuated DOFs** & **Training Modality** & **Movement** & **Type of** \\
**Main Reference** & & & & & **Initiation** & **Disease** \\ \hline \hline \multirow{4}{*}{Lokomat [99]} & \multirow{4}{*}{Treadmill-based} & \multirow{4}{*}{Electric} & \multirow{4}{*}{Hip flex/ext. (\(\circ\)2): Knee flex/ext. (\(\circ\)2)} & \multirow{4}{*}{Passive: Active Assistive} & \multirow{4}{*}{-} & Stroke [117; 18; 118; 19; 120; 121; 122; 123; 124; 125; 126; 127; 128; 129; 130; 131; 132; 133; 134; 135; 136; 137; 138; 139; 140; 141; 142; 143; 144; 145; 146; 147  
As for the upper limb robots, in the following section, we shortly present a case study of an exoskeleton for robotic neurorehabilitation of the lower limbs, developed in our laboratories at the Italian Institute of Technology (IIT) in collaboration with the National Institute for Insurance against Accidents at Work (INAIL). Modular Exoskeleton for Gait Assistance: Twin  
The Twin exoskeleton was born thanks to the collaboration between IIT and INAIL and is currently employed in a clinical trial on spinal cord injury (SCI) subjects [83, 134]. Figure 2 depicts the exoskeleton layout. It is composed by: (i) Three structural elements, including the pelvis (one), femur, and tibia (two per each leg); (ii) by four motors for the hip and knee actuation; (iii) by ergonomic interfaces (braces) of the femur, tibia, and trunk; (iv) by two foot/ankle orthosis; and (v) by a control central unit that contains an inertial measurement unit (IMU), located on the pelvis structural element. Each of the braces, links, and orthosis are available with different sizes in order to fit different subjects' anthropometries. The motherboard installed in the control unit is able to send/receive commands and measurements to/from the actuators and it is located in the back part of the exoskeleton.