Acute and chronic effects of transcranial direct current stimulation (tDCS) on swimming performance and cognitive function of elite swimmers

Abstract

Transcranial direct current stimulation (tDCS) is one of the latest strategies used to improve the performance of elite athletes. The aim of this study was to investigate the effects of short- and long-term unihemispheric concurrent dual-site anodal tDCS of the primary motor cortex (M1) and dorsolateral prefrontal cortex (DLPFC) on swimming performance and maximal strength (1-RM), as well as physiological (blood lactate levels- BL and heart rate- HR), cognitive (reaction times- RTs and distance of perceived fatigue- DPF) and psychological (mental toughness- MT) variables. Nineteen elite male swimmers participated in a randomized and sham-controlled study over 25 days. Freestyle swimming (100 m) test times were recorded in the morning and evening as pre-intervention tests of swimming performance. The swimmers then received acute and multi-session tDCS (2 mA for 20 min; once daily; 3 days per week) in addition to their routine training. Two days after the 10th tDCS session, the participants repeated the swimming performance tests (morning and evening). In addition, 1-RM, RT and MT were assessed as pre-post intervention tests in a rested state the day before and after the swimming tests. After 10 sessions of tDCS, morning and evening swimming performance improved (p < 0.05) and evening BL and DPF, and MT scores were higher than pre-intervention values compared with sham. Mean and best RTs decreased (p < 0.05) in tDCS compared with sham. No significant differences were found for HR and 1-RM scores. We conclude that multi-session tDCS, but not single session, improves swimming performance more than sham-treatment, as well as affects physiological, psychological, and cognitive function variables.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-27803-2.

Introduction

Transcranial direct current stimulation (tDCS) is a noninvasive technique for modulating brain activity by applying a weak electrical current (up to 2 mA) to the scalp (mainly on prefrontal cortex, primary motor cortex, inferior frontal cortex, and temporal cortex)¹. Previous studies (all applying single sessions before the task) have shown beneficial effects of tDCS on athletic performance^2–5. In these studies, researchers stimulated different brain regions such as the primary motor cortex (M1), which is particularly important for the control of voluntary movement in humans⁶. The M1 is the region most strongly associated with athletic performance due to its role in controlling exercising muscles⁷. Studies have reported that increasing the corticospinal excitability of M1 after anodal tDCS (a-tDCS; two sessions before and two sessions during the tasks) has beneficial effects on motor sequence learning⁸, reducing reaction time (RT; single session before the task)⁹, improving endurance performance (review, all single sessions before or during the tasks)¹⁰, and increasing maximal isometric contractions (single session before the task)¹¹. However, as many brain regions are associated with different components of athletic performance (e.g., motor cortex - motor drive; dorsolateral prefrontal cortex -DLPFC - inhibitory control; temporal cortex - ANS control), M1 is not the only active brain area during exercise¹².

DLPFC is involved in executive functions and control of cognitive processes¹³. Studies have shown that a-tDCS of this region can improve endurance performance (single session before the task)^14,15, increase time to exhaustion at maximum peak power (single session before the task)¹⁶, decrease ratings of perceived exertion (RPE) (single session before the task)¹⁷, and reduce the negative effects of mental fatigue (single session before the task)¹⁸. To maximize benefits, some studies have investigated unihemispheric, simultaneous a-tDCS at two sites, specifically M1 and DLPFC^19–21. This dual-site technique has recently been shown to result in higher and longer-lasting levels of M1 corticospinal excitability (single session before the task)¹⁹, improved cognitive performance and endurance (single session before the tasks)²⁰, and increased skill acquisition (three sessions during the tasks)²¹. Some studies have also demonstrated that the administration of tDCS over multiple sessions has higher and longer-lasting beneficial effects, mostly in therapeutic or behavioral studies^22–25 and in a limited number of sports performance studies (single session before the tasks)²⁶. However, most research has investigated only the acute (single session before or during the tasks) effects of tDCS on athletic performance, with some reporting beneficial^2,3,27 and others finding no significant impact^28–30.

In sports such as swimming, athletes often compete in multiple events within a single day. Therefore, consistency and recovery of performance between events or different phases of a competition (heats, semifinals, and finals) are critical to success³¹. The administration of a single session of a-tDCS after mental fatigue in swimming has shown variable results: stimulation of the left DLPFC reduced the negative effects of mental fatigue on 50 m swimming performance in male professional swimmers¹⁸; however, tDCS over the left temporal cortex did not alter the performance or RPE of male master swimmers over 800 m freestyle³². When applied to the primary motor cortex (M1), neither the performance nor the blood lactate or RPE of elite male triathletes in the 800 m freestyle test changed, although their state of mind improved²⁹. Furthermore, when administered via the orbital prefrontal cortex (PFC), tDCS showed better endurance performance in tethered swimming among amateur female swimmers³³.

Sports coaches widely acknowledge that mental preparation plays a crucial role in determining success when competing against opponents with similar physical abilities³⁴. In this context, mental toughness (MT) has been shown to correlate positively with performance levels in various sports³⁵. Athletes with higher MT scores generally participate at a higher level, achieve more, and perform better^36,37. Notably, adolescent swimmers with higher MT achieved better results than their counterparts with lower MT³⁸. Additionally, RT, as an index of information processing speed³⁹, is classified as one of the attentional cognitive functions involved in athletic performance⁴⁰. In swimming specifically, RT has been shown to be a potential determinant in both short and long-distance events and has led to significant improvements in performance at Olympic Games⁴¹. Previous research has demonstrated that tDCS (during single session up to 20 min with 0.5–2 mA) can improve simple RT in older adults¹, in boxing (after single session of tDCS and tsDCS - Transcutaneous Spinal Direct Current Stimulation- for 13 min with 2 mA)⁴², and under time to exhaustion task in hypoxic conditions (after one session of 20 min with 2 mA stimulation)²⁰.

In high-speed sports such as 100 m freestyle swimming, monitoring physiological factors such as blood lactate⁴³ and heart rate⁴⁴ is fundamental for effective training and performance enhancement. Furthermore, maximum strength (1-RM- both lower and upper body) has been identified as one of the most important factors closely related to sprint swimming performance⁴⁵. Based on the existing literature examining the influence of tDCS on strength-based tasks, where Kenville et al. 2020 reported significant improvements in squat performance following cerebellar tDCS (single session)⁴⁶, and considering the critical role of lower-limb strength in swimming performance, we hypothesize that the chronic application of tDCS will enhance lower-limb strength and, consequently, improve swimming performance. Despite the clear importance of these physiological and strength factors, few studies have examined how neuromodulation techniques like tDCS might influence these critical parameters in elite swimmers, representing a significant gap in the current literature. Our mechanistic hypotheses for the potential effects of dual-site tDCS on performance include (i) reduction of fatigue perception⁴⁷, (ii) improvement of limb coordination²⁶, and (iii) enhanced motor unit recruitment⁴⁸, which may contribute to increased lactate production and strength gains.

Given the varying results of previous studies and the potential benefits of dual-site stimulation^19–21, our research aims to expand the current understanding of tDCS applications in elite athletes. Since previous studies have examined (a) the effects of a single tDCS session^{2–5,27–30}, (b) its application at a single site^{8–12,14,16–18}, (c) cognitive function alone⁴⁹, or (d) non-elite athletes^{3,5,11,17,18,42,49,50}, we hypothesize that multiple sessions of dual-site brain stimulation (administered after morning training and before evening training) may be more effective than a single-session, single-site stimulation in enhancing elite swimmers’ performance and related physiological, cognitive and psychological factors. To rigorously test this, a sham stimulation condition was included as a control to account for placebo effects. Accordingly, we investigated the effects of unihemispheric concurrent dual-site anodal-tDCS on: (1) morning and evening performance after multiple sessions of stimulation, (2) morning-to-evening performance recovery (along with heart rate, blood lactate levels, and distance of perceived fatigue) after both single and multiple sessions, and (3) 1-RM, RTs, and MT after multiple sessions of stimulation in elite swimmers.

Methods

Participants

Nineteen elite male swimmers (19 ± 2.9 years, 1.81 ± 0.05 m; 74.7 ± 8.3 kg) volunteered for the study. All participants were among the top three swimmers in the country in different disciplines. They trained regularly at a high level (8.5 ± 1.6 sessions per week) and had participated in most regional and national competitions for 11.3 ± 2.5 years. A sensitivity power analysis was performed using G*Power version 3.1.9.2 software to calculate the detectable effect size with a sample size of 20 participants, an alpha error probability of 0.05 and a power of 0.8 (1-β). The analysis yielded an effect size (f = 0.54) for the between-within interaction based on ANCOVA^2,49,51. The general characteristics of the participants are shown in Table 1. We lost one participant during the protocol due to injury, which prevented him from continuing with regular training and tDCS sessions. All participants were right-handed according to the Edinburgh Handedness Inventory⁵². Participants reported no history of neurological or psychiatric disorders, no history of seizures, no metallic head implants and no current medication use, as assessed by the relevant questionnaire⁵³. These criteria were applied to exclude any contraindications to tDCS. Participants were also assessed for sleep quality, alcohol, tobacco, and caffeine consumption, as well as stimulant use and specific diets or supplements. The athletes or their parents (for those under 18 years of age) were informed about the study and signed the written informed consent form. The study was approved by the Ethics Committee of Shahid Beheshti University (Code: IR.SBU.REC.1402.177), Tehran, Iran and conducted in accordance with the declaration of Helsinki.

Table 1.

General characteristics of all participants. Data are means and SD.

	Sham (n = 9)	tDCS (n = 10)	Total (n = 19)
Age (Years)	19.3 ± 3.5	18.7 ± 2.4	19 ± 2.9
Years of Training	11.4 ± 3.0	11.2 ± 1.9	11.3 ± 2.5
Height (m)	1.82 ± 0.06	1.80 ± 0.05	1.81 ± 0.05
Weight (kg)	75.4 ± 9.7	74.2 ± 7.3	74.7 ± 8.3
Body Mass Index (kg/m 2 )	22.6 ± 1.9	22.8 ± 1.4	22.7 ± 1.6
Total Body Fat (%)	15.9 ± 2.5	16.6 ± 3.9	16.3 ± 3.2
Skeletal Muscle (%)	36.2 ± 1.4	35.9 ± 2.8	36.0 ± 2.2

Experimental procedure

This study was a randomized, sham-controlled trial in which participants were first ranked according to their most recent performance records and then randomly allocated into two groups with comparable records using a computer-generated randomization sequence and were blinded to their group assignment. All protocols were conducted over 25 days (Fig. 1) in a room adjacent to the athletes’ daily training pool, with controlled temperature (22 ± 2 °C) and humidity (45 ± 5%). On the first day, participants were familiarized with the experimental procedure and completed relevant questionnaires (informed consent form, health, handedness, dass21). Their body composition (Bodecoder, model no. CHL-818E), 1-RM (repetition maximum), reaction times (RT) tests and mental toughness (MT) were performed as pre-intervention tests. On the second day, the pre-intervention tests of swimming performance were measured twice a day (at the same times as the morning training sessions, 08:00–10:00, and the evening training sessions, 18:00–20:00) to simulate the real conditions (heats in the morning, finals in the afternoon). Following the swimming tests, heart rate, distance of perceived fatigue (DPF) and blood lactate were measured. One session of tDCS or sham treatment was conducted between the two swim tests. In the following 10 sessions, tDCS or sham treatment was administered once daily (between the morning and evening training) three days per week in randomized order. Forty-eight hours after the last tDCS session, swimming performance was measured in the post-intervention tests as in the pre-intervention test swimming trials (without tDCS or sham treatment in between, and at the same times as the morning and evening training sessions). The day after the swimming post-test, 1RM, RTs and MT tests were performed as post-tests (Fig. 1). The study was conducted during the specific preparation period, prior to the pre-season phase.

Fig. 1.

Schematic presentation of 1 session and 10 sessions of tDCS and testing timetable. RT (Reaction Time), SMTQ (Sports Mental Toughness Questionnaire), 1-RM (one-Repetition Maximum), 100 m (100 m swimming test), DPF (Distance of Perceived Fatigue), HR (Heart Rate), BL (Blood Lactate) and tDCS (Transcranial Direct Current Stimulation).

Cognitive function

Reaction time tests

The Visual Choice RT Apparatus (Model 63035 A, Lafayette Instrument Company, Indiana, USA) was used to measure RT. Three forms of RT tests were presented manually to the participants. One visual RT test, in which colored lights randomly lit up on the reaction panel and participants selected the corresponding button with their index finger, and two auditory simple RT tests, in which participants pressed the button with their index finger immediately after the beep sound. All participants were asked to respond as quickly as possible. The RT for each stimulus was recorded and the best value and the mean of 3 trials for each form of stimulus was calculated. RT tests were performed the day before the pre-intervention test and the day after the post-intervention swimming tests. A standardization protocol was implemented, requiring participants to avoid strenuous activity and abnormal dietary intake prior to testing; Hydration levels and sleep routines were also monitored, and compliance with these requirements was confirmed through self-reports and daily checks. RT is a critical factor in higher cognitive functions, and it depends on the processing speed of the central nervous system and is influenced by the speed of information processing, attention span, language skills and visual-spatial orientation⁵⁴. RT has been shown to be a potential determinant in both short and long-distance events and has led to significant improvements in performance at Olympic Games⁴¹.

Distance of perceived fatigue (DPF)

In this study we used a new sport-specific athlete-reported distance of perceived fatigue (DPF) by asking the swimmers “At what distance did you feel fatigue?“ When completing 100 m swim tests, they reported a distance between 50 and 100 m, at which distance they felt fatigued and just tried to finish the test. Experienced swimmers can feel the distance as they swim. This is the first study to use this assessment, and further research is needed to explore its validity and reliability. However, the relevance of this assessment was endorsed by all elite swimmers and coaches, and experienced coaches reported that fatigue distance matched the onset of swimming technique failure.

Psychological function

Mental toughness (MT)

Mental toughness was assessed using the 14-item Sports Mental Toughness Questionnaire (SMTQ)⁵⁵, which includes an overall score for mental toughness in sport and three subscales: confidence, consistency and control. Participants completed the questionnaire on two occasions as a pre- and post-intervention tests, the day before and the day after the swimming pre- and post-intervention tests. The Sports Mental Toughness Questionnaire (SMTQ) is a valid short questionnaire³⁴.

Performance

1-RM estimate

To determine the participants’ maximum upper body strength, they performed a one-arm preacher curl and their one-repetition maximum (1-RM) was estimated using the Brzycki model⁵⁶.

Where mass is the mass of the dumbbell lifted (kg) and rep is the number of repetitions of the one-arm preacher curl. In this case, the average number of repetitions for the 1RM assessment was 5.53 ± 2.22 in the pre-intervention test and 4.89 ± 2.58 in the post-intervention test. After the specific warm-up and a 15-minute passive recovery period, participants used dumbbells between 10 and 20 kg to determine the estimated 1-RM, with the number of repetitions being less than 9 repetitions according to Brzycki et al.⁵⁶. One-RM tests were performed the day before and the day after the pre- and post-intervention swimming tests. Biceps brachii contributes to stroke efficiency in pull phase force in the water⁵⁷, and assessing bicep strength allows us to capture potential adaptations in the upper limb musculature induced by tDCS.

Swimming performance test

The participants performed 100 m freestyle swimming tests in a 50 m indoor swimming pool (air temperature 29 C and water temperature 26.5 C; Azadi Sports Complex). After a 20-minute warm-up, started 25 min before the test and kept consistent across all athletes and training sessions, participants were divided into pairs according to their most recent records to maintain their fastest possible competitive pace throughout the swimming tests. Participants began with competitive starts and times were recorded by two experienced coaches using digital stopwatches (Casio HS-80, Hubei, China). Swimming tests were performed twice a day (morning and afternoon) within one day, with approximately seven hours between, in pre- and post- intervention tests (Fig. 1).

Physiological function

HR

HR was recorded immediately after each 100-meter freestyle swimming tests (Polar Electro, Kempele, Finland).

Blood lactate

Blood lactate was measured in auricular capillary blood using a portable lactate analyzer (h/p Cosmos Sirius, Germany-precision = ± 3%: 0.2 mmol/L) and lactate scout sensors kit (EKF Diagnostics 92; Cardiff-UK). Since it takes time for lactate produced in the muscles to appear in the blood, and the required time depends on both the duration and intensity of activity, we measured blood lactate in the first five participants after the initial 100 m freestyle swim test at 3, 5, 7, and 10 min. The highest value was observed at the 7th minute, which was then used for the remaining participants.

Transcranial direct current stimulation (tDCS)

To apply tDCS over the brain, a two-channel brain stimulator (NeuroStim 2, Medina Tebgostar, Tehran, Iran) was used during the experimental sessions. The Unihemispheric concurrent dual-site anodal tDCS (a-tDCS-UHCDS) montage was used to stimulate M1 and DLPFC areas simultaneously^19,58,59. According to the 10–20 international electroencephalographic electrode systems, anode electrodes (5 × 4 cm²) were placed over the left DLPFC (F3) and left M1 (C3) and cathode electrodes (9 × 4 cm²) were placed vertically over the contralateral supraorbital region (F4 and Fp2)⁶⁰. All four carbon electrodes were covered with a saline-soaked surface sponge (NaCl 140 mmol dissolved in Milli-Q water). Although multiple brain regions contribute to athletic performance and training, the left M1 and DLPFC are particularly important cortical areas for sports performance²⁷, and simultaneous stimulation of these regions has been reported to produce greater and longer-lasting effects than stimulating either area alone^19–21, possibly by leveraging their complementary roles in motor and cognitive performance. In the tDCS condition, the stimulation current intensity of 2 mA was administered for 20 min with a 30 s ramping up at the start and a 30 s falling at the end of the stimulation. In the sham condition, the current ramped up for 30 s at the start and then fell in 30 s at the end, without 2 mA current until the end of the 20 min to provide an adequate protocol to make the stimulation blind²³. Each participant first received one session of tDCS between the two 100 m swimming pre-intervention tests, and then 10 sessions of tDCS three days a week (30–150 min after the morning training and 150–30 min before the evening training, in a purposeful random order), alongside their routine training (Fig. 1). This tDCS administration schedule was chosen because it was not feasible to have elite swimmers available at 5:00 a.m. (e.g., 150 min before morning training) or 10:30 p.m. (e.g., 150 min after evening training). Their routine training included eight swimming sessions in the pool and two strength training sessions per week in the gym, and we ensured that they maintained this routine without changes throughout the tDCS protocol.

To investigate the tDCS side effects, participants completed a 5-point Likert-scale⁶¹ at the end of each stimulation session regarding different sensations during stimulation. Blinding efficacy was not formally assessed, and this omission should be considered a limitation⁶². The research staff member who applied the electrical stimulation was not blinded to the stimulation condition (i.e., sham vs. tDCS)^20,42,49.

Statistical analysis

After ensuring the normality of the data with the Shapiro-Wilk test, based on our five main objectives: (1) the effects of chronic tDCS on morning performance (and HR, BL and DPF), (2) the effects of chronic tDCS on evening performance (and HR, BL and DPF), (3) the effects of acute tDCS on morning to evening (delta) performance recovery (and HR, BL and DPF), (4) the effects of chronic tDCS on morning to evening (delta) performance recovery (and HR, BL and DPF), and (5) effects of chronic tDCS on 1-RM, RTs and MT, we used an ANCOVA with the pre-intervention test scores as covariates to compare post-intervention test results between the tDCS and sham groups. We used the Statistical Package for Social Sciences (SPSS, version 24) and the results are presented as mean ± standard deviation (SD). Cohen’s d effect size (ES/η2) was analyzed, observed Power (OP) and confidence intervals (CI 95%) were reported (presented in supplementary 1). The statistical significance level was set at P < 0.05 for all analyses.

Results

Performance

ANCOVA results showed significant group effects for the morning performance (F₁,₁₆=4.59, p = 0.048, ES = 0.22) and the evening performance (F1,16 = 6.31, p = 0.023, ES = 0.28) indicating greater improvement after 10 sessions of tDCS compared to the sham group, after adjusting for pre-intervention values. There were no significant group effects for the morning-to-evening performance recovery after 10 sessions (p = 0.76) and one-session of tDCS (p = 0.98) (Fig. 2.A).

Fig. 2.

(A): 100 m Swimming free-style performance, (B): Blood Lactate and (C) following 1 session and 10 sessions of tDCS training, and One Repetition Maximum (1-RM) of upper-body strength following 10 sessions of tDCS training.

Blood lactate

ANCOVA results showed approached significant (p = 0.09) group effects for the morning blood lactate, and significant group effects for the evening blood lactate (F₁,₁₆=6.29, p = 0.023, ES = 0.28) indicating higher values after 10 sessions of tDCS compared to the sham group, after adjusting for pre-intervention values. There were no significant group effects for the morning-to-evening blood lactate recovery after 10 sessions (p = 0.56) and one-session of tDCS (p = 0.81) (Fig. 2.B).

One repetition maximum (1-RM)

ANCOVA results showed approached significant (p = 0.076) group effects for the estimated 1-RM indicating greater improvement after 10 sessions of tDCS compared to the sham group, after adjusting for pre-intervention values (Fig. 2.C).

DPF and HR

ANCOVA results showed significant group effects for the morning DPF (F₁,₁₆=52.9, p = 0.001, ES = 0.76) and the evening DPF (F₁,₁₆=35.2, p = 0.001, ES = 0.68) indicating greater improvement (delayed fatigue) after 10 sessions of tDCS compared to the sham group, after adjusting for pre-intervention values. There were no significant group effects for the morning-to-evening DPF recovery after 10 sessions (p = 0.74) and one-session of tDCS (p = 0.98) (Fig. 3.A).

Fig. 3.

(A): Distance of Perceived Fatigue (DPF) and (B): Heart Rate (HR) of 100 m free-style swimming following 1 session and 10 sessions of tDCS training.

There were no significant group effects for the morning HR (p = 0.92) and evening HR (p = 0.80) over the 100 m freestyle swimming after 10 sessions of tDCS. There were also no significant group effects for the morning-to-evening HR recovery after 10 sessions (p = 0.85) and one-session of tDCS (p = 0.82) (Fig. 3.B).

RT

ANCOVA results showed a significant group effects for the mean value of the pressing RT (F₁,₁₆=9.22, p = 0.008, ES = 0.36) and best value of the pressing RT (F₁,₁₆=12.48, p = 0.003, ES = 0.43) indicating greater improvement after 10 sessions of tDCS compared to the sham group, after adjusting for pre-intervention values (Fig. 4.A).

Fig. 4.

Three types of Reaction Time (RT); (A) Auditorial Pressing (RT-P), (B): Auditorial Releasing (RT-R), and (C): Visual Color (RT-V) following 10 sessions of tDCS training.

Also, ANCOVA results showed a significant group effects for the mean value of the releasing RT (F₁,₁₆=8.47, p = 0.01, ES = 0.34) and best value of the releasing RT (F₁,₁₆=5.43, p = 0.03, ES = 0.25) indicating greater improvement after 10 sessions of tDCS compared to the sham group, after adjusting for pre-intervention values (Fig. 4.B).

ANCOVA results showed a significant group effects for the mean value of the visual RT (F₁,₁₆=25.19, p = 0.001, ES = 0.61) and best value of the visual RT (F₁,₁₆=19.41, p = 0.001, ES = 0.54) indicating greater improvement after 10 sessions of tDCS compared to the sham group, after adjusting for pre-intervention values (Fig. 4.C).

MT

ANCOVA results showed a significant group effects for the total SMTQ14 scores (F₁,₁₆=4.67, p = 0.04, ES = 0.22) and constancy scores showed approached significant (p = 0.078) indicating greater improvement after 10 sessions of tDCS compared to the sham group, after adjusting for pre-intervention values. However, there were no significant group effects for confidence (p = 0.32) and control (p = 0.11) values after 10 sessions of tDCS (Fig. 5.A-D).

Fig. 5.

Scores of Sports Mental Toughness Questionnaire (SMTQ) and subscales; (A) Total SMTQ, (B): Confidence, (C): Constancy, and (D): Control following 10 sessions of tDCS training.

Discussion

In this study, we investigated the acute and chronic effects of unihemispheric concurrent dual-site tDCS on swimming performance and maximal strength, as well as physiological (blood lactate levels- BL and heart rate- HR), cognitive (reaction times- RTs and distance of perceived fatigue- DPF) and psychological (mental toughness- MT) variables in elite swimmers. The main results following one and 10 sessions of tDCS targeting M1 and DLPFC were as follows: (a) Following 10 sessions of tDCS, significant improvements in both morning and evening 100 m swimming times were observed compared to sham treatment, accompanied by a significant increase in DPF and in blood lactate levels in the evening, with no significant changes in heart rate; (b) No significant changes were observed in any of these factors (swimming time, BL, HR and DPF) from morning to evening after either a single session or 10 sessions of tDCS. (c) RT decreased significantly across all three RT test modalities (visual and auditory), (d) the overall MT score showed significant improvement, although increases in the subscales (confidence, constancy, and control) did not reach statistical significance, (e) there were notable but statistically non-significant increases in estimated 1-RM. These results support our initial hypothesis about the benefits of multi-session tDCS protocols for performance enhancement and related physiological, cognitive and psychological factors in elite swimmers.

Based on our findings, 10 sessions of tDCS (between the morning and evening training), but not a single session, significantly affected professional swimmers’ performance. To our knowledge, this is the first study investigating the multi-session effects of dual site tDCS on anaerobic-based athletic performance (specifically 100 m swimming) in elite swimmers. Ma et al. (2022) reported improvements in overall aerobic athletic performance among 12 male rowers after 10 sessions of 1 or 2 mA tDCS applied to M1, though their study lacked a sham control group. They recommended further research with proper control groups and larger sample sizes²⁶. They also did not mention whether tDCS was administered before or after training sessions.

In the therapeutic domain, multiple studies have demonstrated positive outcomes from multi-session tDCS applications. Evidence consistently suggests that multiple tDCS sessions produce greater and more sustained beneficial effects compared to single sessions^{22–25,63,64}. Ma et al. and other researchers using fMRI have documented morphological brain adaptations and neuroplasticity following tDCS administration^23,26,65. Our findings align with this research, suggesting that unihemispheric concurrent dual-site tDCS of the left M1 and DLPFC may effectively enhance athletic performance.

While the precise mechanisms of tDCS remain unclear, its effects appear highly dependent on the specific brain regions stimulated⁴. Neurons in the M1 region encode various motor parameters including direction, velocity, position, and muscle activity⁶⁶. When applied over M1, tDCS enhances functional connectivity within cortico-cortical and cortico-subcortical motor networks⁶⁷. The DLPFC, meanwhile, is recommended to play a crucial role in control of cognitive processes^13,68. Although multiple brain regions contribute to athletic performance and training, with some functional differences between right and left M1 in unimanual versus bimanual tasks, the left M1 and DLPFC are particularly important cortical areas for sports performance^27,69.

Grosprêtre et al. (2021) demonstrated distinct effects based on stimulation site: single session M1 tDCS (before the task with 2 mA, 20 min) improved power performance by increasing spinal excitabilities, while DLPFC stimulation specifically enhanced fine motor control tasks without affecting cognitive performance⁴. Notably, simultaneous stimulation of left M1 and DLPFC (before or during the task, single or three sessions with 0.3, 1 and 2 mA for 20 min) has demonstrated superior efficacy compared to single-site stimulation^19–21. Therefore, our findings support the hypothesis that dual-site tDCS targeting left M1 and DLPFC may effectively enhance both motor performance and cognitive function in elite athletes.

The main finding of this study was the significant improvement in 100 m swimming records after 10 sessions of tDCS compared to sham. As suggested by previous research, tDCS-induced increases in network activity and premotor stimulation contribute to performance enhancement². Evidence indicates that tDCS (single session before the task with 2 mA, 10 min) likely facilitates increased motor unit recruitment⁴⁸, leading to significantly higher lactate production and improved one-repetition maximum. This physiological mechanism is particularly relevant for 100 m swimming, which is fundamentally a lactate-based event where increased lactate production and tolerance play crucial roles in performance enhancement^16,70.

A proposed mechanism for the effects of tDCS on performance is increased cortical excitability in the primary motor cortex and reduced resting potential, resulting in delayed fatigue onset^16,71. The absence of significant effects after a single tDCS session across all measured factors suggests that multiple tDCS sessions, particularly when combined with regular training, are required to induce such excitability changes. Research suggests that maximizing voluntary recruitment of all motoneuron pools at their highest firing rates is essential for peak performance in short, all-out activities⁷². While strength improvements can enhance swimming performance⁷³, and cerebellar tDCS (single session before the task with 2 mA for 20 min over M1) has been shown to improve strength⁴⁶, it is noteworthy that despite the experimentally meaningful estimated 1-RM improvement in the tDCS group (tDCS: 2.3 kg = 0.15% vs. sham: 0.9 kg = 0.05%), greater individual differences during puberty and wider dispersion of results likely contributed to statistically non-significant outcomes.

Notably, both mean and best scores across all auditory and visual RT measures improved significantly following chronic tDCS administration. This suggests that tDCS applied over the left M1 combined with training may increase sensorimotor integration and interlimb coordination²⁶. Since RT is critical for the start phase of 100 m swimming events, the improvements observed across all RT tests—particularly auditory RT—may encourage athletes and coaches to incorporate this non-invasive brain stimulation technique in RT-dependent sports. While most RT improvements typically result from eye-hand coordination⁷⁴, sensorimotor systems like auditory-hand coordination are especially important in swimming performance. Previous tDCS studies have demonstrated that electrical stimulation of the DLPFC (single session) improves information processing speed and reduces the refractory period following initial stimulation⁷⁵. Repeated tDCS sessions (five consecutive sessions concomitant with task on M1 for 20 min with 2 mA over M1), especially when combined with task performance, are more likely to promote synaptic plasticity and long-term potentiation (LTP)⁷⁶. This process resembles motor learning, in which cumulative stimulation (three sessions during task with 2 mA for 20 min over M1) strengthens functional connectivity within motor and cognitive networks, thereby enhancing reaction time⁷⁷.

Although RPE and HR are important metrics in competitive sports, we observed no significant changes in HR or RPE (0–10 points Borg scale, not reported in results) during morning and evening tests after 10 sessions of tDCS. Since RPE is typically assessed during a test, it may serve as a suitable metric in prolonged performance settings; however, when used after the completion of a short sprint event such as the 100 m swim, it is less reliable, as all swimmers are already nearly fully fatigued. However, our newly developed Distance Perception of Fatigue (DPF) measure revealed a substantial improvement of over 10 m in fatigue perception (from 60 m to 72 m) in the tDCS group compared to only 3 m in the sham group. Traditional Borg RPE and HR measurements appear more relevant for endurance events than for sprint events like 100 m swimming^20,78,79. In contrast, the specific perception scale introduced in this study may provide more reliable results in this context, though its validity should be further evaluated in future studies. Interestingly, nearly all swimmers reported better subjective experiences during evening tests compared to morning sessions, confirmed by slight increases in evening DPF despite significant improvements in evening performance records (0.30 s) across both groups. Consistent with this, significant increases in blood lactate levels of the tDCS group in the evening, but not in the morning, were observed. Previous literature indicates that neuromuscular performance, hormonal responses, and perceptual factors can fluctuate across the day, often favoring evening performance due to circadian rhythms in body temperature, metabolic activity, and central nervous system excitability^80–84. In our study, we assessed both morning and evening swimming performance specifically to account for potential diurnal effects. While both groups followed the same testing schedule, we acknowledge that diurnal variation may have influenced absolute performance levels.

Neuroimaging and neurophysiological evidence indicate that tDCS administration over M1 significantly influences fatigue perception⁴⁷ and pain reduction⁸⁵. Given that early central fatigue functions as a mental barrier preventing athletes from accessing their full physiological capacity⁸⁶, tDCS could serve as a valuable complementary strategy to delay unwanted early central fatigue. Significantly increased blood lactate in the evening test and approached significant (p = 0.09) in the morning test after 10 sessions of tDCS administration in our study support this mechanism. While tDCS targeting M1 and/or DLPFC has demonstrated fatigue reduction across various neurological and immune disorders^14,87, the underlying mechanisms of tDCS-induced performance improvements may vary with the nature of the sport (e.g., cyclic sports such as swimming vs. acyclic sports such as football), and further research is needed to clarify how tDCS affects fatigue and performance across different activities.

Success against similarly physically capable opponents often hinges on psychological preparation³⁴. Our study demonstrated a significant increase in total MT scores as a psychological trait, with considerable improvements across all three subscales. Higher MT levels significantly reduce stress, anxiety, and depression, potentially benefiting athletic performance⁸⁸. Mental toughness has been associated with enhanced sport-related well-being^89,90 and represents a prominent psychological correlate of behavioral persistence in discrete physical tasks⁹¹. Therefore, increased MT scores may serve as a psychological mechanism underlying performance improvements in swimmers. The weaker correlation between post-test performance improvement and physiological factors compared to pre-intervention measurements (not reported in detail) further confirms the importance of psychological factors in performance enhancement.

Considering tDCS’s beneficial effects on performance under hypoxic conditions²⁰ and the brain’s pivotal role in both moderate and severe hypoxia^20,92, the positive impact of tDCS on swimming performance (as a moderate hypoxic event) may be particularly significant. However, consistent with previous research^4,50, our study confirmed that acute (single-session) tDCS was insufficient to induce behavioral and cognitive changes across several domains. The efficacy of tDCS on performance may depend on athletes’ professional level, particularly given the typically close physical fitness levels despite varying mental fitness among elite competitors.

There are several limitations to this study. First, the relatively small sample size (n = 19) may limit the generalizability of the results and the statistical power to detect smaller effect sizes. Second, the wide age range of participants included critical maturational stages that may have influenced training adaptations and performance outcomes independent of the tDCS intervention. Third, sleep quality was not controlled or monitored.

Methodological limitations include the use of digital handheld stopwatches instead of automatic timing systems (e.g. swimming touchpads), which may lead to measurement errors in performance assessment. In addition, the Distance of Perceived Fatigue (DPF) scale, while practically useful, is not fully validated and standardized, which could affect the reliability of the results. The application of tDCS using the EEG 10–20 system without cortical mapping using TMS or neuronavigation may have reduced the accuracy of stimulation site localization, although such techniques remain difficult to implement in field-based research settings.

Finally, potential motivational biases may have influenced the results, particularly as the swimmers competed against each other in pairs during the performance tests, which may have introduced further motivational influences in addition to the intended intervention effects.

Conclusion

While several lines of inquiry regarding tDCS strategy remain open, our findings nevertheless recommend athletes to use multi-session tDCS (as a non-invasive, portable, and relatively cheap method), between morning and evening training sessions, as an effective brain-conditioning approach to enhance swimming performance, anaerobic energy production, mental toughness, and reaction times. Improvements in blood lactate, distance of perceived fatigue, and maximum strength, along with enhanced performance, underscore the possibility of improved motor unit recruitment through tDCS. Further studies are required to clarify the unknown aspects of brain stimulation and its effects on the multifactorial nature of athletic performance.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Conceptualization, MK, BA, AF, and RE; Data curation, MK; Formal analysis: MK, BA, TRL, and AF; Investigation, MK, BA, AG, and AF; Methodology, MK, BA, AF, and RE; Writing – original draft, MK; Writing – review & editing, BA, AF, AG, RE and TRL; Supervision, BA. All authors approved the final version of this manuscript.

Funding

This research received no external funding and did not have an external sponsor.

Data availability

The data that support the findings of this study are available on request from the corresponding author.

Declarations

Competing interests

The authors declare no competing interests.

Ethics approval

The study was approved by the Ethics Committee of Shahid Beheshti University (Code: IR.SBU.REC.1402.177), Tehran, Iran and conducted in accordance with the declaration of Helsinki.

Consent to participate

All participants read the patient information sheet regarding the study and signed the informed consent form. In cases where the swimmers were under 18 years of age, the parents signed the consent form.