## 2 Gait and Balance Rehabilitation
### Rehabilitation Approaches and Training Modes  
Nowadays, two main rehabilitation approaches can be distinguished in the literature: bottom-up and top-down approaches. Bottom-up approaches are based on the mechanism of neural plasticity, and the idea is to act on the physical level (bottom) and expect changes at the central neural system level (top). In contrast, top-down approaches consist of defining the rehabilitation therapies based on the state of the brain after a neurological disorder [48]. Functional electrical stimulation (FES), brain-computer interfaces (BCIs), and robotic devices are potentially powerful tools for top-down neurorehabilitation. FES refers to the use of electrical stimulation of lower-motor neurons to cause muscle contraction and to accomplish functional tasks such as standing or walking [49], whereas BCIs are systems that record, decode, and translate measurable neurophysiological signals into an effector action or behavior [50]. Hence, BCI systems can be combined with robotic devices or with FES to promote rehabilitation according to the patient's intention directly decoded from the brain activity. Although many robotic devices have potential for top-down approaches, in this review article we focus on the application of robotic devices for direct bottom-up rehabilitation (see Figure 2). Robotic systems can aid in rehabilitation as assistive robotic devices and as therapy robotic devices [51]. Assistive robotic devices do not generate exercises for the affected limbs, and their objective is to assist the patients in performing ADLs. Wheelchairs, crutches, and walkers are examples of assistive devices. In contrast, therapy robotic devices mimic the task of a physiotherapist and can generate rehabilitation exercises that are useful for regaining essential functionalities such as balance and walking. Therapy robotic devices can be classified based on the training mode and thus according to human-robot interaction control strategies. Different training modes and control strategies have been proposed in the literature for therapy robots [52], and although the terms may vary among different works, the main principle behind them is similar. In this review, we classify therapy robotic devices as active, passive, and hybrid, as in [40]. * **Active robotic** devices provide **passive training**. That is, the device helps the patient to follow rehabilitation trajectories predefined by the physiotherapists. Studies report that passive exercises (from the patient's point of view) with active rehabilitation devices increase the range of motion of lower-limb joints and reduce muscle atrophy [53]. This type of training mode is suitable for early-stage rehabilitation where the patient still does not have enough muscle strength. Figure 2: General classification of the devices used in the rehabilitation field. Categories further considered in this article are represented in bold. **Passive robotic** devices promote **active training**. These devices are used at intermediate-advanced rehabilitation stages when patients can perform their own movements. In this training mode, the participation of the patient is necessary, and the robot can assist the patients to complete the rehabilitation trajectory or provide resistance force to make the exercise more challenging. It has been observed that the active participation of the patients promotes neuroplasticity and improves the rehabilitation results [54, 55]. **Hybrid robotic** devices are a combination of the previous two. They provide **hybrid training**, where the device can interact with the patient during the rehabilitation trajectory or just follow the patient motion. From now on, the terms \"therapy devices\" and \"rehabilitation devices\" are going to be used indistinctly. Overall, lower-limb rehabilitation robots can be divided into two main groups according to the position of the patient during the rehabilitation process: sitting- or lying-type rehabilitation devices, also known as stationary trainers, and standing- or walking-type rehabilitation devices.