Transcranial Direct Current Stimulation Improves Bilateral Ankle-Dorsiflexion Force Control in Healthy Young Adults  
[PERSON]  
1 Department of Human Movement Science, Incheon National University, Incheon 22012, Republic of Korea; mrf.hj@imu.  
motor cortex (M1) improved dynamic balance performances, as indicated by reduced gait asymmetry, a longer duration of maintaining a horizontal position and less deviation from the horizontal position [13,14]. Indeed, a recent study revealed that modal stimulation that targeted primary sensorimotor cortex effectively increased cortical excitability across targeted cortical regions contributing to lower motor function [15]. These findings suggested that tDCS protocols targeting motor cortical regions may improve motor functions of lower extremities by modulating cortical excitability. Previous tDCS studies that examined beneficial effects on functional performances in lower extremities such as postural control and jump mainly used unilateral stimulation protocols with anodal stimulation on leg areas of M1 (i.e., the vertex; Cz in the 10-20 system for EEG) [10,16]. Based on the location of cathodal stimulation, [PERSON] and colleagues categorized the unilateral protocols into (a) midline monopolar (anodal stimulation on the Cz and cathodal stimulation on the arm) and (b) midline bipolar non-balanced (anodal stimulation on the Cz and cathodal stimulation on the suborbital region) [17]. Considering a proposition that the distance between anodal and cathodal electrodes may alter the current flow consequently influencing aftereffects of stimulation [18,19], bilateral stimulation protocols can increase improvements in lower limb functions. Further, a recent study argued that anodal stimulation on the C1 and cathodal stimulation on the C2 revealed higher electrical field intensities reaching deeper leg regions of the M1 [20]. Thus, applying bilateral stimulation protocol including anodal stimulation on the C1 and cathodal stimulation on the C2 would be a viable option for improving motor functions in lower extremities.  
For lower limb force control capabilities of healthy young adults, the limited number of studies investigated effects of tDCS protocols by focusing on unilateral force control performances. Applying 2 mA anodal tDCS on the vertex of sensorimotor cortex for 20 min significantly increased force accuracy during dominant ankle-plantarflexion force control task [15,21], and further ankle tracking practices while receiving 1 mA anodal tDCS on the contralateral M1 for 15 min improved tracking accuracy of the non-dominant ankle as compared with those for sham condition [22,23]. In addition to unilateral force control improvements, tDCS protocols may improve bilateral force control in lower limbs because previous findings suggested positive effects of tDCS on interhemispheric connectivity contributing to interlimb motor coordination [24, 25, 26, 27]. Despite these potential effects of unilateral tDCS protocols on lower limbs, whether bilateral tDCS protocol is beneficial for advancing bilateral force control capabilities in lower limbs are still unclear. Thus, we examined the effects of tDCS protocols on bilateral force control capabilities of lower limbs in healthy young adults to serve as applicable reference for future research. Given that functionality of foot and ankle is crucial for locomotor control [28], we administered bilateral ankle-dorsiflexion force control tasks at 10% of maximal voluntary contraction (MVC). During the force control task, participants received tDCS protocols on leg areas of the M1. Based on prior findings that tDCS on hand areas of the M1 improved bimanual force control performances [29], we hypothesized that tDCS protocols would improve bilateral ankle-dorsiflexion force control capabilities in healthy young adults.