## 3 End Effector-Based Devices for Gait and Balance Rehabilitation  
### Advantages of End Effector-Type Rehabilitation Devices  
Among the previously explained rehabilitation devices for gait and balance, end effector-based robots present encouraging results. This is mainly due to their advantages with respect to other robotic devices, such as exoskeletons and treadmills. If we compare them with treadmill-based devices and overground exoskeletons, end effector devices are easier to set-up [35]. In fact, their minimal preparation time and low operating costs have made them attractive for rehabilitation and increased the number of commercially available end effectors for gait training [103]. Since no alignment between patient and robot joints is required in end effectors, they can be easily adjusted to different sizes and patients. However, it is challenging to adapt the exoskeletons to different patients, since the lengths of the robot segments and the joints must be perfectly aligned with the anatomical limbs and joints [103]. Even though exoskeleton-type robots can directly control individual joints (hip, knee, and ankle) and avoid abnormal leg movement patterns, it has been proved that the free motion of the knee and hip joints provided by end effectors promotes \"destabilization training\" [104]. This training can reinforce the neuronal circuit and contribute to postural control and sensory integration [105]. Furthermore, active patient participation in rehabilitation appears to be a promising method of locomotor rehabilitation [106], and according to some studies, it may be considered one of the most important features of any training [107]. In this regard, exoskeletons may produce the opposite effect, that is, make the patient more passive, since they are affixed to the entire lower limb. From a rehabilitation movement standpoint, end effectors are more suitable than treadmill-based robots and exoskeletons when different types of terrain and human gait patterns need to be simulated, like climbing up/down stairs [108]. End effectors can generate movements that are not constrained only to one anatomical plane (i.e., the sagittal plane), and consequently they can increase the patient's influence on the walking trajectory and provide meaningful balance training [46]. In connection with this, end effector robots can provide perturbations in the anterior-posterior or mediolateral directions, as well as a combination thereof. In contrast, the current generation of exoskeletons can only apply perturbation in the sagittal plane, which is a strong limitation, considering that perturbations in everyday life are not restricted to this plane [96]. Despite evidence of encouraging results (e.g., higher rates of independent walking) of end effectors compared to exoskeletons [109], there is still a lack of a clinical consensus on which type of device should be used for patients with neurological disorders [110]. Nonetheless, the number of studies that support and highlight the suitability of end effector-type robots for the improvement of gait and balance in patients with neurological disorders is considerable. As an example, [PERSON] et al. concluded that end effector robot-assisted gait training enhances walking and balance functions in patients with multiple sclerosis [111]. [PERSON] et al. underlined the efficacy of end effector-based training on gait, balance, and coordination in chronic stroke patients [112], and [PERSON] et al. conducted a systematic review that evidenced improvements in overall mobility, gait speed, and functional ambulation with the use of end effector gait trainers in stroke patients also [113]. Several research and review articles have collated information about end effectors already available for gait and balance rehabilitation [36; 46]. However, most of these are part of a classification study that includes other type of rehabilitation devices (exoskeletons, treadmills, stationary trainers, etc.