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Article

Transcranial Direct Current Stimulation Improves Bilateral Ankle-Dorsiflexion Force Control in Healthy Young Adults

by
Hajun Lee
1,†,
Beom Jin Choi
1,† and
Nyeonju Kang
1,2,*
1
Department of Human Movement Science, Incheon National University, Incheon 22012, Republic of Korea
2
Division of Sport Science, Sport Science Institute & Health Promotion Center, Incheon National University, Incheon 22012, Republic of Korea
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Appl. Sci. 2025, 15(8), 4391; https://doi.org/10.3390/app15084391
Submission received: 18 March 2025 / Revised: 10 April 2025 / Accepted: 14 April 2025 / Published: 16 April 2025
(This article belongs to the Special Issue Advances in Physiotherapy and Neurorehabilitation)
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Abstract

:
This study examined transient effects of transcranial direct current stimulation (tDCS) on bilateral force control in lower extremities. We recruited 14 healthy young adults and administered bilateral ankle-dorsiflexion force control tasks at 10% of maximal voluntary contraction. Participants were able to use real-time visual information on a targeted force level and forces produced by both feet. While performing bilateral force control, we provided active- and sham-tDCS in a random order. Bilateral tDCS protocol used for this study included anodal and cathodal stimulation targeting left and right leg areas of the primary motor cortex between hemispheres. Bilateral force control capabilities were estimated by calculating force accuracy, variability and regularity. In addition, we determined whether force control patterns differed between feet across active- and sham-tDCS conditions. The findings revealed that force accuracy and variability were significantly improved after applying active-tDCS protocol as compared with those for sham-tDCS condition. However, no differences in force control between feet were observed. These findings suggest that bilateral tDCS protocols may be a viable option for improving motor functions of lower limbs.

1. Introduction

Bilateral motor control in lower limbs is important for performing activities of daily living such as walking, postural control and driving a vehicle [1]. For successful bilateral motor control, abilities to modulate and coordinate forces between feet are required [2,3]. Presumably, executing bilateral force control demands more neural resources in sensorimotor cortical regions [4]. Considering potential contribution of greater motor cortical excitability to improved postural control and locomotion [5,6], facilitating neural excitability may improve bilateral force control capabilities.
Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that may modulate cortical excitability by delivering weak electrical direct current to the scalp via electrodes [7]. Potentially, anodal stimulation may facilitate cortical excitability by depolarizing resting membrane potential of neurons to subthreshold levels, whereas cathodal stimulation suppresses cortical excitability by inducing hyperpolarization [8,9]. Previous studies investigated potential effects of tDCS on motor functions of lower limbs in healthy young adults [10,11,12,13,14]. For example, applying anodal stimulation to the primary 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 anodal 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, Nasseri 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.

2. Materials and Methods

2.1. Participants

Fourteen healthy young adults (9 males and 5 females; mean ± standard deviation of age = 24.9 ± 2.5 years) voluntarily participated in this study. Thirteen participants were right-footed, and one participant was left-footed, as estimated by self-report. All participants had no musculoskeletal and cognitive impairments (i.e., Korean mini-mental state exam score ≥ 25) [30]. Specific demographic characteristics are shown in Table 1. Before the experiment, all participants read and signed an informed consent and study protocol approved by the Institutional Review Board of Incheon National University (No. 7007971-202006-002A).

2.2. Experimental Setup

This study was a randomized, single-blinded, and sham-controlled crossover design (Figure 1). Using the internet-based Research Randomizer program provided by Social Psychology Network [31], all participants received two tDCS protocols in a random order: (a) a session of the active-tDCS condition and (b) a session of the sham-tDCS condition. A washout period between the two tDCS protocols was implemented more than five days to minimize carryover effects by brain stimulation and motor practice effects [32]. All participants were instructed to abstain from vigorous physical activities and alcohol consumption for 24 h, and to refrain from ingesting stimulants such as caffeine or analgesic medications for at least 12 h prior to experimental session [33].

2.3. Bilateral Ankle-Dorsiflexion Force Control Tasks

We used an isometric foot-force measurement system (SEED TECH Co., Ltd., Bucheon, Republic of Korea; Figure 2a) incorporating two platforms with force transducers (Micro Load Cell-CZL635-3135, range = 330 lbs, Phidgets Inc., Calgary, AB, Canada). Moreover, we modified each platform and metal strap based on participant’s foot length and metatarsal height for proper foot stabilization. Participants were seated comfortably at a distance of 80 cm away from a 21.5 inch LCD (1920 × 1080 pixels; 60 Hz refresh rate), and generated isometric force against the straps. We administered them to maintain a standardized posture (Figure 2b): (a) 90–100° of knee flexion, (b) 90–95° of hip flexion and (c) 90° of ankle dorsiflexion [34,35].
All participants conducted the first MVC trial for 5 s, and after 110 s of rest time they completed the second MVC trial for 5 s. For each participant, we determined MVC level by averaging two peak forces obtained from each trial. To perform bilateral force control tasks, a submaximal targeted force level was chosen as 10% of each participant’s MVC because this level reflects the fine motor demands of diverse lower limb activities in daily life (e.g., stair climbing or driving a car) [36]. The goal of a bilateral force control task was to generate and sustain a combined ankle-dorsiflexion force from both feet near the targeted force level for a duration of 20 s. During the task, participants relied on real-time visual feedback displayed on a monitor (Figure 2c): (a) white line indicating a targeted force level and (b) red line indicating the total force of ankle-dorsiflexion generated by both feet. A total of 10 trials for each tDCS condition (i.e., active-tDCS and sham stimulation) were conducted per participant, with a 110 s rest time between each trial to minimize fatigue [37].
All experimental tasks were conducted using custom Microsoft Visual C++ (Microsoft Corp., Redmond, WA, USA). Force signals were recorded at a sampling frequency of 200 Hz using a 16 bit analog-to-digital converter (A/D; ADS1148 16-Bit 2Ksps and a minimum detectable force of 0.0192 N), and subsequently amplified using INA122 with an excitation voltage of 5 V (Texas Instruments Inc., Dallas, TX, USA).

2.4. tDCS Protocols

Using a wireless tDCS device (Starstim 8-Neuroelectronics®, Barcelona, Spain), we administered active-tDCS protocol including anodal stimulation on C1 and cathodal stimulation on C2 while performing force control tasks (Figure 2d). Specific parameters of active-tDCS protocol were (a) stimulation intensity = 2.0 mA, (b) electrode size = 25 cm2 (5 × 5 cm), (c) current density = 0.08 mA/cm2, and (d) session duration = 20 min. For the sham-tDCS protocol, 2.0 mA current was delivered for the initial 30 s and ramped down to 0 mA [38] so that all participants were not aware of the specific tDCS condition.

2.5. Data Analyses

Using a customized Matlab Program version R2024b (Math WorksTM Inc., Natick, MA, USA), force signals were filtered using a bidirectional fourth-order Butterworth filter with a cut-off frequency of 30 Hz. To minimize potential initial-phase adjustment and early termination, the central 14 s of each trial were analyzed by excluding the initial and final 3 s of the recorded force data.
Bilateral ankle-dorsiflexion force control capabilities were assessed by calculating four variables: (a) mean force, (b) force accuracy (root mean square error: RMSE), (c) force variability (standard deviation: SD) and (d) force regularity (sample entropy: SampEn). Regarding force regularity, lower SampEn values (i.e., close to zero) reflect more consistent and predictable force output patterns, while higher values suggest increased irregularity in force production. SampEn can be calculated using the following:
S a m p E n = x , m , r , N = l n C m r C m + 1 r
We used 2 for m and r = 0.2 × SD of force data [39,40]. Before statistical analyses, we confirmed the normality of distribution for all dependent variables using Shapiro–Wilk W test. To estimate potential tDCS effects on bilateral force control capabilities, we used a paired t-test on mean force, RMSE, SD and SampEn, respectively. Furthermore, we performed two-way repeated measures ANOVA (Stimulation Condition × Limb Side; 2 × 2) on mean force, SD and SampEn to investigate effects of bilateral tDCS protocol on each leg side (i.e., left or right). For post hoc analysis, we applied Bonferroni’s pairwise comparisons. All statistical analyses were conducted using the IBM SPSS Statistical 28 (SPSS Inc., Chicago, IL, USA) and we set an alpha level at 0.05.

3. Results

All participants completed all experimental procedures and reported no adverse effects. Specific data for all participants are shown in Table S1. A paired t-test on bilateral ankle-dorsiflexion force control capabilities variables indicated a significant reduction of RMSE (t13 = −2.851 and p = 0.014; Figure 3b) and SD (t13 = −3.055 and p = 0.009; Figure 3d) in the active-tDCS condition compared to the sham-tDCS condition. However, there was no significant difference between two tDCS conditions: (a) mean force (t13 = 0.174 and p = 0.865; Figure 3a) and SampEn (t13 = 0.017 and p = 0.987; Figure 3d). These results indicate that the active-tDCS protocol transiently improved force accuracy and steadiness in the lower extremities.
A two-way repeated measures ANOVA on the SD revealed a significant stimulation condition main effect (F1, 13 = 10.912; p = 0.006; η2 = 0.456; Figure 4b). The post hoc analysis indicated that the SD in the active-tDCS condition was significantly lower than in the sham-tDCS condition (p = 0.006). However, the two-way repeated measures ANOVA showed no significant effects on mean force: (a) stimulation condition × limb side interaction (F1, 13 = 0.001; p = 0.980; η2 < 0.001; Figure 4a), (b) stimulation condition (F1, 13 = 0.030; p = 0.865; η2 = 0.002) and (c) limb side (F1, 13 = 1.207; p = 0.292; η2 = 0.085) and SampEn: (a) stimulation condition × limb side interaction (F1, 13 = 4.283; p = 0.059; η2 = 0.248; Figure 4c), (b) stimulation condition (F1, 13 = 1.012; p = 0.333; η2 = 0.072), and (c) limb side (F1, 13 = 0.307; p = 0.589; η2 = 0.023). These results suggest that active-tDCS protocol did not induce different force control patterns between feet.

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 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]. Further, a computational study reported that applying anodal stimulation on the C1 and cathodal stimulation on the C2 (i.e., the C1–C2 montage) resulted in higher electrical field intensities for both hemispheres, reaching deeper regions of the lower limb motor area [20]. These findings suggested that the C1–C2 montage may be suitable for improving lower limb functions such as walking by balancing interhemispheric activity [50]. In fact, our subsequent analyses that examined potential interlimb force control asymmetry confirmed that no significant differences in mean force, force variability and force regularity between feet were observed across two tDCS conditions. Taken together, our findings provide evidence that the C1–C2 montage would be an additional option for tDCS-based lower limb rehabilitation programs.
Despite positive effects of tDCS on bilateral ankle-dorsiflexion force control, these findings should be cautiously interpreted. First, the current study focused on healthy young adults. Given that neuroplasticity in the motor cortex may be influenced by age or clinical characteristics [51], future studies should investigate effects of bilateral tDCS protocol with the C1–C2 montage on lower limb motor functions in older adults or patients with neurological disease such as Parkinson’s disease and stroke. Second, goniometric data of the lower limbs were not measured in this study despite the potential relationship between ankle-dorsiflexion mobility and functional performance in the lower limbs [52,53]. Given that tDCS on the sensorimotor cortex would improve the range of motion of hip, future studies should examine tDCS effects on lower limb motor functions based on bilateral ankle joint mobility. Third, improved bilateral ankle-dorsiflexion force control performance may not necessarily lead to improvements in gross motor functions of lower extremities. However, considering the significant relationship between force control performances and motor functions in the literature [54,55,56], future studies should investigate effects of tDCS protocol with the C1–C2 montage on gross motor functions in lower extremities such as postural control and locomotion. Finally, we did not measure neurophysiological changes by tDCS protocols while performing bilateral ankle-dorsiflexion force control tasks. Future studies should examine cortical excitatory patterns across crucial brain areas induced by bilateral tDCS protocol with the C1–C2 montage and how these alterations improve bilateral force control in lower extremities.

5. Conclusions

This study revealed that bilateral tDCS protocols transiently improved force control capabilities between feet in healthy young adults. The active-tDCS protocol improved bilateral ankle-dorsiflexion force accuracy and variability at 10% of MVC as compared with those in the sham-tDCS condition. However, the force control performances were not different between feet. These findings suggest that bilateral tDCS protocols stimulating the M1 of both hemispheres may be a viable option for improving motor functions between feet, contributing to the development of effective rehabilitation and training protocol. Future studies should examine whether the multiple sessions of the bilateral tDCS protocol enhance lower limb function for various populations including older adults and neurological patients and explore underlying neurophysiological mechanisms.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app15084391/s1, Table S1: Specific data for all participants.

Author Contributions

Conceptualization, N.K.; data curation, H.L. and B.J.C.; formal analysis, H.L. and B.J.C.; funding acquisition, N.K.; project administration, N.K.; supervision, N.K.; visualization, H.L.; writing—original draft, H.L. and B.J.C.; writing—review and editing, N.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Incheon National University Research Grant in 2024 (2024-0059) to NK.

Institutional Review Board Statement

Consistent with the guidelines of Declaration of Helsinki, the study was approved by the Institutional Review Board of Incheon National University on 12 August 2020 (No. 7007971-202006-002A).

Informed Consent Statement

Before the experiment, all participants read and signed an informed consent and study protocol.

Data Availability Statement

Data is contained within the Supplementary Table S1.

Acknowledgments

The authors sincerely thank the study participants.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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Figure 1. Flowchart of the study. MVC = Maximal voluntary contraction; tDCS = transcranial direct current stimulation.
Figure 1. Flowchart of the study. MVC = Maximal voluntary contraction; tDCS = transcranial direct current stimulation.
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Figure 2. Bilateral ankle-dorsiflexion force control task and tDCS protocol. (a) Isometric foot-force measurement system; (b) ankle-dorsiflexion position; (c) visual feedback condition; (d) bilateral tDCS protocol. tDCS = Transcranial direct current stimulation.
Figure 2. Bilateral ankle-dorsiflexion force control task and tDCS protocol. (a) Isometric foot-force measurement system; (b) ankle-dorsiflexion position; (c) visual feedback condition; (d) bilateral tDCS protocol. tDCS = Transcranial direct current stimulation.
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Figure 3. Bilateral ankle-dorsiflexion force control capabilities. (a) Mean force; (b) RMSE; (c) SD; (d) 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 × IQR (maximum value) and Q1 − 1.5 × IQR (minimum value). Asterisk (*) denotes a significant difference between the active- and sham-tDCS conditions (p < 0.05). RMSE = Root mean square error; SD = Standard deviation; SampEn = Sample entropy.
Figure 3. Bilateral ankle-dorsiflexion force control capabilities. (a) Mean force; (b) RMSE; (c) SD; (d) 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 × IQR (maximum value) and Q1 − 1.5 × IQR (minimum value). Asterisk (*) denotes a significant difference between the active- and sham-tDCS conditions (p < 0.05). RMSE = Root mean square error; SD = Standard deviation; SampEn = Sample entropy.
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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 × IQR (maximum value) and Q1 − 1.5 × IQR (minimum value). SD = Standard deviation; SampEn = Sample entropy.
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 × IQR (maximum value) and Q1 − 1.5 × IQR (minimum value). SD = Standard deviation; SampEn = Sample entropy.
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Table 1. Participants characteristics at baseline.
Table 1. Participants characteristics at baseline.
CharacteristicsHealthy Young Adults
Sample size 14
Gender (female/male)6/8
Dominant leg (left/right)1/13
Age (years)24.9 ± 2.5 (22–30)
Height (cm)168.8 ± 8.6 (155.6–183.0)
Weight (kg)67.8 ± 13.4 (46.6–85.7)
Skeletal muscle mass (kg)30.4 ± 8.4 (17.6–42.6)
Body fat mass (kg)13.4 ± 2.0 (11.1–18.4)
BMI (%)23.3 ± 2.6 (18.5–26.5)
K-MMSE29.1 ± 1.6 (25–30)
Data are represented as mean ± SD (range). Note: K-MMSE = Korean mini-mental state exam; BMI = body mass index.
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MDPI and ACS Style

Lee, H.; Choi, B.J.; Kang, N. Transcranial Direct Current Stimulation Improves Bilateral Ankle-Dorsiflexion Force Control in Healthy Young Adults. Appl. Sci. 2025, 15, 4391. https://doi.org/10.3390/app15084391

AMA Style

Lee H, Choi BJ, Kang N. Transcranial Direct Current Stimulation Improves Bilateral Ankle-Dorsiflexion Force Control in Healthy Young Adults. Applied Sciences. 2025; 15(8):4391. https://doi.org/10.3390/app15084391

Chicago/Turabian Style

Lee, Hajun, Beom Jin Choi, and Nyeonju Kang. 2025. "Transcranial Direct Current Stimulation Improves Bilateral Ankle-Dorsiflexion Force Control in Healthy Young Adults" Applied Sciences 15, no. 8: 4391. https://doi.org/10.3390/app15084391

APA Style

Lee, H., Choi, B. J., & Kang, N. (2025). Transcranial Direct Current Stimulation Improves Bilateral Ankle-Dorsiflexion Force Control in Healthy Young Adults. Applied Sciences, 15(8), 4391. https://doi.org/10.3390/app15084391

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