## 2 Gait and Balance Rehabilitation
### Rehabilitation Approaches and Training Modes
#### 2.2.2 Standing/Walking-Type Rehabilitation Devices  
Unlike the stationary trainers, the main aim of standing/walking rehabilitation devices is to correct the gait pattern and the balance function in similar conditions to ADLs. The patients rehabilitate in a standing position, and even though these types of devices are usually provided with a BWS system, patients must have a certain level of physical ability to maintain the vertical position [74]. Hence, these systems are feasible for intermediate-to-advanced stages of rehabilitation. The currently available fixed-frame devices for gait and balance rehabilitation can be subclassified into two categories: treadmill-based and end effector-based robotic devices. Treadmills  
Treadmills systems are suitable for gait and balance rehabilitation, being one of the most commonly used systems for conventional therapy [43]. The currently available treadmill-based devices for gait rehabilitation can be divided into two types: treadmills with BWS system, and treadmills-based exoskeletons. Treadmills with BWS consist of a unidirectional treadmill combined with a suspension system that supports part of the patient's weight. While the patient is walking on the treadmill, two to three therapists manually assist the legs and hip to correct the gait patterns [43]. RehaWalk(r) (zebris Medical GmbH, Isny, Germany), C-Mill (Motek Medical, Houten, The Netherlands) and the LokoStation (Woodway Inc., Waukesha, WI, USA) are commercial examples of this gait rehabilitation system. One major limitation of the treadmills with BWS is the number of therapists that are needed and their labor intensiveness. To overcome this problem, treadmills-based exoskeletons were proposed. This type of treadmills consists of a BWS and a bilateral lower limb exoskeleton (or active leg orthoses) attached to the patient's leg while he/she is walking on a treadmill frame [53]. The exoskeletons are mechatronic devices, usually serial robot architectures, that move in parallel to the skeleton of the patient and they adjust the motion of the lower limb joints through a preprogrammed physiological gait pattern. The Lokomat(r) (Hocoma, Volketswil, Switzerland) was one of the first treadmill-based exoskeleton to be commercialized [76], being ALEX [77], LOPES [78] and the ReoAmbulator(tm) (Motorika Medical Ltd., Caesarea, Israel) other well-known devices. With respect to direct balance rehabilitation, some treadmills can act as a balance perturbation source for the patient. The MIT-Skywalker is an example of this [79]. It consists of a double treadmill (one for each foot) that besides rehabilitate gait, through a rhythmic and discrete training, it has the capacity to tilt the surface of the treadmills in the medio-lateral direction and thus to provide balance training. There also exist bidirectional treadmills [80] and treadmills mounted on movable platforms [81] that can generate slip and trip-like perturbations by changing the belt acceleration (e.g., sudden starts), by reversing the rotation and by lateral shifts. The BalanceTutor (MediTouch Ltd., Tnuvot, Israel) is a commercialized example of these perturbation systems. There are also devices that combine treadmills with push/pull mechanisms to assess how patients maintain their balance during perturbed walking, that is, walking in the presence of perturbations with different magnitudes and directions [82; 83]. End effectors  
End effector devices are robotic systems that consist of one or two footplates on which the patients place their feet. In contrast to systems with exoskeletons, where the human-machine mechanical interface involves the entire limb, in end effector systems the human-robot interaction occurs at the distal segments (feet) and thus no alignment between the patient and the robot joints is required. Hence, leg movements are controlled just by the trajectory of a unique footplate or bilaterally placed footplates. These types of devices are suitable for both gait and balance rehabilitation.