The 27 eligible subjects were randomly assigned using simple randomization to the RAGT \(\stackrel{-}{\text{c}}\) rHD-tDCS group or RAGT \(\stackrel{-}{\text{c}}\) sHD-tDCS group and received their allocated interventions (Fig. 3). The final analysis included 24 subjects after excluding three subjects who discontinued the intervention for personal reasons: two subjects in the RAGT \(\stackrel{-}{\text{c}}\) rHD-tDCS group and one in the RAGT \(\stackrel{-}{\text{c}}\) sHD-tDCS group.
Figure 4 illustrates the changes in physical functions at Pre, Post, and F/U in the RAGT \(\stackrel{-}{\text{c}}\) rHD-tDCS or RAGT \(\stackrel{-}{\text{c}}\) sHD-tDCS groups analyzed using the Wilcoxon signed rank test and Bonferroni’s post hoc analysis of RM ANOVA. Specific values for physical function in the 10MWT, TUG, FAC, FRT, BBS, DGI, FMA − LE, FMA − TOTAL, and K-MBI outcome measures at the Pre, Post, and F/U time points are presented in Supplementary Table 1 and Supplementary Table 2. The FMA results show the lower extremity (FMA − LE) scores and total upper extremity and lower extremity (FMA − TOTAL) scores for the affected side. The groups did not differ in gait or balance function at the Pre, Post, or F/U time point.
Both groups statistically significantly changed from Pre to all other time points in the TUG (P < 0.05). The 10MWT, FRT, BBS, and DGI scores changed significantly from the Pre to all other time points in the RAGT \(\stackrel{-}{\text{c}}\) rHD-tDCS group (P < 0.05). The FMA − LE and FMA − TOTAL scores improved significantly from the Pre to Post time points in the RAGT \(\stackrel{-}{\text{c}}\) rHD-tDCS group (P < 0.05). The 10MWT and BBS scores in the RAGT \(\stackrel{-}{\text{c}}\) sHD-tDCS group changed from Pre to Post (P < 0.05).
Bonferroni’s post hoc analysis revealed that the 10MWT, TUG, FRT, and BBS scores of the RAGT \(\stackrel{-}{\text{c}}\) rHD-tDCS group changed significantly from Pre to all other time points (P < 0.05), whereas the RAGT \(\stackrel{-}{\text{c}}\) sHD-tDCS group did not show significant improvements from Pre to any other time point. The DGI, FMA − LE, and FMA − TOTAL scores improved significantly from the Pre to Post time points in the RAGT \(\stackrel{-}{\text{c}}\) rHD-tDCS group only (P < 0.05).
In addition, significant time × group interactions were found in FMA − TOTAL with the RAGT \(\stackrel{-}{\text{c}}\) rHD-tDCS group, which demonstrated greater improvements compared with the RAGT \(\stackrel{-}{\text{c}}\) sHD-tDCS group (P < 0.05). The RM ANOVA revealed significant time × group interactions in the K-MBI (P < 0.05), indicating that the RAGT \(\stackrel{-}{\text{c}}\) rHD-tDCS group experienced greater improvement compared with the RAGT \(\stackrel{-}{\text{c}}\) sHD-tDCS group across time points.
Discussion
The purpose of this study was to investigate whether simultaneous application of HD-tDCS could enhance the training effect of RAGT in chronic stroke patients. The findings suggest that adding real HD-tDCS to RAGT has several key advantages in gait, balance, motor function, and ADL performance over RAGT with sham HD-tDCS. Furthermore, the improvements in gait and balance function in the RAGT \(\stackrel{-}{\text{c}}\) rHD-tDCS group were maintained for at least one month after intervention. ADL performance and motor function showed significant time × group interactions.
In stroke patients with gait disorders, the ability to control gait, balance, and motor functions is reduced by hemiparetic abnormalities with asymmetry, increased stride variability, and muscle weakness [30]. Rehabilitation treatment to improve physical function is important because decline of these functions increases the risk of falls during walking among stroke patients, leading to low quality of life [31]. Subacute stroke patients and those who are not able to walk independently may be more likely to benefit from the RAGT intervention [7]. However, evidence for the effect of RAGT in chronic stroke patients is insufficient [32]. Choi et al. (2022) reported improvements in functional gait and balance ability in chronic stroke patients who received RAGT with body weight support five times a week for six weeks [33]. We found significant improvements in gait and balance function (10MWT, TUG, and BBS) in both groups after intervention three times a week for four weeks. The increase in gait and balance function in both groups after the intervention may be due to advantages of RAGT in that a larger number of steps can be practiced per session [34], symmetrical gait can be facilitated [35], and paretic leg step length symmetry is fostered [28].
Although there are few studies using HD-tDCS with RAGT, a clinical improvement in chronic stroke patients was reported in two of three studies that compared real stimulation (anodal on M1) and sham stimulation (supraorbital stimulation) applied with RAGT. Danzl et al. (2013) reported a significant improvement in FAC scores after RAGT with real tDCS intervention compared to those before intervention [36]. Seo et al. (2017) found statistical improvements in the FAC and 6-Minute Walk Test in the real tDCS group at one month F/U after intervention only [37]. Additionally, previous studies investigating the effects of tDCS with RAGT on gait, balance, and motor abilities in subacute and chronic stroke patients found overall improvements in TUG and FAC after intervention, although no significant effects were found in 10MWT, BBS, 6MWT, or FMA − LE [38]. In contrast to previous studies, our results after Bonferroni correction showed significant improvements in gait, balance, and motor functions (10MWT, FRT, BBS, DGI, FMA − LE, and FMA − TOTAL) as well as TUG after intervention with real HD-tDCS in chronic stroke patients. It is thought that combined application of HD-tDCS could help to facilitate cortical excitability of the M1 leg area; therefore, it could enhance the training effect of RAGT to increase its consistency in chronic stroke patients [39].
Stroke affects not only gait, balance, and motor functions but also the ability to perform ADL. Consistent with previous findings, this study found significant time × group interactions in ADL performance and motor function in the RAGT \(\stackrel{-}{\text{c}}\) rHD-tDCS group compared with the RAGT \(\stackrel{-}{\text{c}}\) sHD-tDCS group, which leads us to conclude that the combination of HD-tDCS and RAGT has beneficial effects on motor and ADL function [40, 41]. These results suggest that combining RAGT with HD-tDCS facilitates changes in neuroplasticity that promote physical function recovery and increases cortical activity to enhance subsequently spontaneous activities such as ADL [42].
Furthermore, tDCS has a long-term positive effect on function through learning processes in stroke patients. A study comparing the effects of tDCS and HD-tDCS applied over the M1 on motor learning in a group of children found that both tDCS and HD-tDCS maintained improved motor learning effects not only after training but after six weeks of F/U [43]. The results of a previous study of applying tDCS to RAGT showed that improved gait endurance was maintained for up to one month in chronic stroke patients [37]. In our study, combining HD-tDCS with RAGT maintained the improved gait and balance function after the intervention until the F/U time point, different from a previous study that used tDCS. Therefore, the combination of RAGT and HD-tDCS was confirmed to have long-term effects even on gait and balance function in chronic stroke patients.
This study has some limitations. First, as it was an exploratory clinical trial with a small number of patients, and further studies using a larger stroke population are needed to validate the results. Second, lesion sites such as cortical and subcortical lesions and stroke types were not considered when recruiting study subjects (Supplementary Table 3). The effect of HD-tDCS can be influenced by lesion location and size. Analysis of different effects according to stroke type or lesion location will provide further guidance about more effective methods of combining HD-tDCS and RAGT.