## 3 End Effector-Based Devices for Gait and Balance Rehabilitation
### Future Research Directions  
Based on the results of this review, fewer than one third of the end effectors for gait and balance rehabilitation are commercially available. This small number of commercialized devices compared to the number of prototypes limited to research purposes makes it evident that the development of new devices has been surpassing the introduction of currently existing prototypes into the clinical setting. Although prototypes are important tools for preliminary evidence of usefulness in the rehabilitation process, they should be further validated through clinical trials. In agreement with [PERSON] et al. [37], we encourage the authors of the end effectors identified to undergo the process of obtaining FDA approval or a CE mark to transfer their inventions from applied research to the market, which would ensure the effectiveness of these designs in the rehabilitation of gait and balance. We found that the number of DOFs in end effector-based devices for gait rehabilitation ranges from 1 to 15. More DOFs means more flexibility in designing rehabilitation exercises, but usually at the expense of increasing the device cost. Therefore, a trade-off between the number of DOFs and the ability to simulate different gait rehabilitation movements is necessary. Considering that during human gait, the lower limbs are mainly contained in the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Authors** & **Ref** & **DOFs** & **Objective** & **Training Mode** \\ \hline
2005, [PERSON] et al. & [88] & (2T1R) \(\times\) 2 & Gait & Passive \\
2010, [PERSON] et al. & [155] & (2T) \(\times\) 2 & Gait & Hybrid \\
2015, [PERSON] et al. & [156] & (2T) \(\times\) 2 & Gait & Passive \\
2018, [PERSON] et al. & [157] & (1T) \(\times\) 2 & Gait & Hybrid \\
2021, [PERSON] et al. & [158] & (2T1R) \(\times\) 2 & Gait & Hybrid \\
2009, [PERSON] et al. & [159] & (1T) \(\times\) 2 & Gait & Hybrid \\
2015, [PERSON] et al. & [160] & (2T) \(\times\) 2 & Gait & Hybrid \\
2016, [PERSON] & [87] & (1T) \(\times\) 2 & Gait & Hybrid \\
2020, [PERSON] & [86] & 1T & Gait & Hybrid \\
2014, [PERSON] et al. & [161] & 2R & Balance & Hybrid \\
2015, [PERSON] et al. & [162] & 1T3R & Balance & Passive \\
2010, [PERSON] et al. & [163] & 2R & Balance & Hybrid \\
2016, [PERSON] et al. & [164] & 2R & Balance & Active \\
2018, [PERSON] et al. & [165] & 1R & Balance & Hybrid \\
2007, [PERSON] et al. & [166] & 2T & Balance & Passive \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Non-PKM-driven end effectors for gait and balance rehabilitation. sagittal plane, at least 3 DOFs should be provided for each footplate. However, given that an extra DOF outside the sagittal plane allows the simulation of perturbations that arise during walking on uneven and irregular terrains, it seems reasonable to use end effectors with 4 DOFs for gait rehabilitation. Hence, we consider end effector-based devices with at least 6 DOFs (two 3-DOF footplates) and a maximum of 8 DOFs (two 4-DOF footplates) sufficiently suitable for cost-effective gait rehabilitation. Regarding the end effectors identified for direct balance rehabilitation, the DOFs in these devices ranges from 1 to 7. From the authors' point of view, an end effector for balance rehabilitation should be able to introduce at least two rotational perturbations in the mediolateral and anterior-posterior directions, which are the most common rotations during ADLs. On the other hand, prototype end effectors for direct balance rehabilitation are fewer than those for gait rehabilitation. The fact that some studies [63] considered balance rehabilitation as a side effect of gait training (indirect balance rehabilitation) may be one of the reasons for this lower number. Nonetheless, for patients that are in an earlier rehabilitation stage and are not able to stand during gait-like movements, direct balance rehabilitation devices are necessary. Among the currently available devices, only CAREN and DORIS can provide independent and simultaneous gait and direct balance rehabilitation.