## 4 Discussion
This aligns with the findings of our study, where the observed gains likely resulted from the use of a selective instability device. This device was not excessively unstable but appeared to provide the right amount of instability to induce effective adaptations. The improvement in ET performance likely reflects the participants' enhanced ability to manage balance challenges induced by the selective instability of the BB device, which could optimize neuromuscular coordination and rapid stabilization during dynamic tasks. However, the lack of additional improvements in the BB + tDCS group compared to in the BB group raises important considerations. In fact, the BB group demonstrated earlier gains in both the YBT and ET (at the second week) that were not observed in the combined group. Given prior evidence suggesting that M1 stimulation can modulate motor function, we expected different results. Nevertheless, our findings did not confirm these expectations. On the other hand, more recent systemic reviews have reported non-significant effects of anodal or cathodal tDCS on postural control and balance in healthy individuals [43]. However, it is important to note that the studies included in their analysis applied tDCS over the cerebellum, whereas our protocol targeted M1. Despite its interesting applications in fields such as pain management, chronic syndromes (e.g., fibromyalgia), or psychological conditions like depression [44], the use of tDCS remains controversial in the context of motor learning. In this sense, several factors may explain this lack of benefits of the tDCS protocol in this study. Firstly, the similarity in outcomes at the fourth week suggests that the balance exercises alone may have reached a ceiling effect, meaning further improvements were limited regardless of the addition of tDCS. It is plausible that if this protocol were applied to a clinical population where balance is much more impaired, such as patients with CAI or post-stroke patients, the differences between the intervention groups could be more pronounced [45; 46]. For instance, [PERSON] et al. found that a combined intervention of 4 weeks of eccentric ankle training and tDCS improved dynamic balance in individuals with CAI [24]. Secondly, it must be emphasized that the duration of the tDCS application was set to 10 min to match the duration of the balance exercise session, as it was designed to be administered simultaneously. By contrast, most clinical trials employing similar configurations and intensity (2 mA) applied tDCS for 20 min, so longer interventions in combination with longer balance training programs might be more effective. However, it is important to note that a longer duration alone may not guarantee better results. For example, [PERSON] et al. applied tDCS for 20 min in combination with foot exercises and observed no significant differences in balance compared to foot exercises alone [38]. Therefore, while extending the duration of tDCS might be beneficial, it may not be the primary limiting factor in this study. Thirdly, another reason that could explain these unexpected results is that the addition of tDCS might have imposed a cognitive overload on participants. Balance exercises demand focused attention and concentration to efficiently engage the musculoskeletal and neurological motor control systems [47]. Therefore, the concurrent application of tDCS may have distracted the brain from efficiently processing the sensory stimuli needed to maintain balance, especially in those individuals with lower working memory capacity [48; 49]. In relation to clinical implications, the findings of this study highlight the efficacy of a balance training program on a selective instability device for improving dynamic balance. Notably, the significant gains observed in the BB group just two weeks after the baseline assessment indicate that the balance program chosen is potentially suitable.