## 2 Gait and Balance Rehabilitation
### Overground Type Devices  
Regarding overground type robots, three different family of devices can be distinguished: perturbation platforms, overground exoskeletons, and patient-guided systems. Perturbation platforms are devices mainly focused on balance rehabilitation. The Balance Training Assist developed by Toyota Motor Corporation is an example of this type of device. It consists of a standing platform mounted on two wheels, with an upright handgrip, that can self-balance [92]. It can provide sagittal perturbations to the patient, to challenge standing balance, by rotating or by moving forward/backward the standing platform. The perturbation platform proposed by [PERSON] et al. [93] and the SLIP-FALL movable ground platform designed by [PERSON] et al. [94] are other examples of this type of devices that challenge the balance function during walking (i.e., gait stability). In particular, the 4 degrees-of-freedom perturbation platform presented by [PERSON] et al. was designed to check the balance of the patients by applying multidirectional perturbations during the stance phase of walking, whereas the SLIP-FALL movable platform can simulate slippery ground and thus provide controllable slip-like perturbations to the patient. In contrast to the Balance Training Assist platform developed by Toyota, these two perturbation platforms are located at ground level. Exoskeletons  
In last decades, various overground exoskeletons have been developed and commercialized for lower limb rehabilitation. These robotic devices actuate over one or more leg joints, and their main objective is to rehabilitate locomotor function [95]. However, they can also be used to assess the balance function of the patients in the anterior-posterior direction [96]. It is possible to distinguish two types of overground exoskeletons depending on who (the patient or the robotic system) has to carry the weight of the exoskeleton. There exist fully wearable exoskeletons, such as the currently commercial Hybrid Assistive Limb (HAL) [97], ReWalk(tm) (ReWalk Robotics Inc., Marlborough, MA, USA) and EksoNR (Ekso Bionics, San Rafael, CA, USA) exoskeletons, in which the patient supports the weight of the exoskeleton. There are also mobile-platform mounted exoskeletons, such as Gable CORETM (Gable Systems BV, Hengelo, The Netherlands) and NaTUre-gauts [98], where the exoskeleton is mounted on a mobile wheeled platform that supports the weight of the device. Patient-guided systems are the last type of overground devices. From a biomechanical standpoint, patients using these devices rehabilitate in an environment that is very close to the one in ADLs and thus they reach a higher level of independent walking compared to BWS treadmill training [45]. Nonetheless, they are mainly designed for the rehabilitation of partially recovered patients who have regained sufficient strength to walk overground and thus, for patients in their intermediate-advanced rehabilitation stage. Two types of patient-guided robotic systems can be distinguished in the literature: mobile robotic platforms and suspension robotic systems. Mobile robotic platforms are robots especially designed for gait training. They consist of an overground omnidirectional wheeled frame, which is inherently stable, and a BWS (harness or a trunk/pelvic support) connected to it, that provides the user with safety and freedom to initiate and carry out the walking movement [99]. The robotic components can be the support system (active BWS) and/or the wheeled frame, which provides automated navigation or actively follows the patients by measuring their intention [96]. KineAssist [100], Andago(Hocoma, Volketswil, Switzerland), WHERE-I and WHERE-II [101] are examples of mobile robotic platforms. Besides these examples, [PERSON] et al. proposed a novel robot, which combines the balance assessment robot (BAR) developed by University Rehabilitation Institute of Ljubljana (Slovenia) and a mobile platform.