## 4 Discussion  
This study investigated the effects of tDCS protocols on bilateral ankle-dorsiflexion force control performances in healthy young adults. Force accuracy and variability at 10% of MVC were significantly improved in the active-tDCS condition as compared with those in the sham-tDCS condition. However, no force control differences between feet were observed after active-tDCS and sham-tDCS conditions. Providing a single session of bilateral tDCS protocols transiently improved bilateral ankle-dorsiflexion force control performances at 10% of MVC, as indicated by decreases in force error and variability. Previous tDCS studies reported positive effects of tDCS on unilateral force control performances, as indicated by lower ankle force error [15; 21; 22; 23]. The current findings expanded these prior results by showing significant effects of tDCS protocols on bilateral force control improvements in lower limb. Although we did not administer multiple sessions of bilateral tDCS protocols, several studies argued that cumulative effects of tDCS on motor improvements including short-term and long-lasting treatment effects may appear in the motor system [41; 42]. Given that fundamental motor performances of lower limbs consist of bilateral actions such as walking, running and  
Figure 4: Bilateral force control performance for each foot. (**a**) Mean force; (**b**) SD; (**c**) SampEn. Box plots display: (**a**) individual data points as circles, (**b**) mean value as X sign inside box and (**c**) median value as black horizontal line inside box. The top and bottom of each box indicate the interquartile range (IQR = Q3–Q1), with whiskers extending to Q3 + 1.5 \(\times\) IQR (maximum value) and Q1 \(-\) 1.5 \(\times\) IQR (minimum value). SD = Standard deviation; SampEn = Sample entropy. climbing stairs, providing multiple sessions of tDCS protocols may be a viable option for functional recovery of lower limbs in various populations (e.g., injured athletes, older adults and patients with neurological diseases who have locomotion deficits) [14; 28]. Bilateral force control improvements in lower limbs may be related to altered cortical excitability across key brain areas by tDCS protocols. Modulating bilateral motor actions normally require greater neural resources across sensorimotor areas between hemispheres for simultaneously integrating sensory information and coordinating necessary muscles [4]. Previous findings posited that applying anodal stimulation on the M1 of unilateral hemisphere may increase cortical excitability across sensorimotor areas including supplementary motor area and somatosensory cortex [20; 43]. Specifically, anodal tDCS on sensorimotor cortex of dominant hemisphere improved somatosensory perception in both feet contributing to enhanced gait function [44]. Further, bilateral tDCS protocols including anodal and cathodal stimulation that targeted left and right M1 areas elevated cortical excitability of M1 and primary somatosensory cortex between hemispheres facilitating motor learning progress [45]. Taken together, cortical excitability across sensorimotor areas between hemispheres facilitated by bilateral tDCS protocols may contribute to successful bilateral motor control in lower limbs [46]. Interestingly, bilateral tDCS protocols used for the current study targeted C1 with anodal stimulation and C2 with cathodal stimulation. Normally, tDCS protocols used cathodal stimulation for either a reference or suppressing cortical excitability of specific regions. Moreover, previous studies that used unilateral tDCS protocols (i.e., anodal stimulation on Cz and cathodal stimulation on the suborbital region) reported controversial results for improving lower limb function [10; 16; 47]. However, several studies raised the possibility that bipolar montages with co-stimulation of neighboring regions may induce more diffusion of electric currents facilitating aftereffects of stimulation [18; 19; 48; 49].