During the competition season, athletes need to seek to improve their performance. In this sense, training loads (i.e., frequency, duration, and intensity) become high (1). Therefore, athletes may experience symptoms of fatigue that decrease muscle capacity for performance (1, 2). Consequently, controlling the training load throughout the season is essential to evaluate performance and avoid fatigue-related problems, such as non-functional overreaching (fatigue lasting from weeks to months), injuries, and illnesses (1).

One of the methods to monitor training load and recovery is the countermovement jump (CMJ) (1, 3, 4). The CMJ test assesses neuromuscular function through lower body power, quantified through jump height (i.e., power). During CMJ, power can be measured using various tools such as contact mats, force platforms, infrared platforms, accelerometers, linear position transducers, and/or video analysis (5, 6). Thus, the participant is asked to squat quickly for a self-selected downward action, followed by a reciprocal upward action, jumping as high as possible (5, 7). The CMJ assessment may indicate a decrease or increase in performance since the rate of strength development analyzed is related to muscle strength. Therefore, the greater the height of the CMJ, the more muscle force is used being an indication of low muscle fatigue (8).

Exercise induced muscle fatigue involves processes at various level of the motor pathway, from the brain to the muscle (9). These processes involve reductions in motor cortex excitability (10, 11), spinal excitability (12, 13), and in the contractile capacity of the recruited muscle fibers (14).

Currently, several ergogenic strategies are used to optimize sports performance. These strategies are beneficial ergogenic measures for recovery and performance, such as rest, adequate sleep, hydration, physiotherapeutic resources, nutrition, and neurostimulation techniques (15, 16). In addition, ergogenic measures that aim to increase supraspinal excitability can lead to a more efficient motor command that, ultimately, could increase the time in which a fixed output could be maintained (for example: muscle power benefit). This hypothesis is already being tested in some sports and several studies have shown improvement in a neuromodulatory technique called transcranial direct current stimulation (tDCS) (17–20). Neurostimulation techniques have been proposed to improve athletes cognitive and psychomotor performance. Among them, non-invasive brain stimulation, such as transcranial direct current stimulation (tDCS), is becoming famous for improving sports performance. The reason is the safety, low-cost, and easy-to-apply technique (19). The use of tDCS consists of applying two electrodes with a low-intensity continuous current (1–2 mA) in specific areas of the brain for a particular time (7–25 min) (21). This stimulation appears to induce changes in cortical excitability lasting from minutes to several hours after its use (22).

Among the areas that can be stimulated by tDCS, the primary motor cortex (M1) stands out as the brain region most related to sports performance due to its role in driving the exercised muscles (19). Research has shown that core fatigue can impact overall exercise performance. In this context, the decrease in motor neuron excitability and the limited ability of M1 and other supraspinal areas to maintain or increase the neural impulse may decrease the muscular capacity to produce force/power, thus leading to fatigue. It is known a single anodal tDCS session in M1 can lead to performance enhancement in athletes in sport-specific motor tasks (17).

However, the literature indicates that results about the effects of tDCS on jumping performance are controversial. For example, Lattari et al. (23) observed an increase in the vertical jump performance of young men with experience in strength training from 20 min of tDCS in the primary motor cortex. On the other hand, Arenas et al. (24), found no significant improvement in vertical jump performance with 15 min of tDCS in the left dorsum lateral prefrontal cortex of healthy young non-athletes. These results may have occurred due to differences in the study methods, such as voltage, stimulation time, target brain area stimulated, or studied population.

Also, as indicated in the literature, studies investigating the effect of tDCS on the performance of soccer players are scarce and necessary (25, 26). These surveys, primarily carried out during the competition period, can help coaches and soccer players control the training and recovery load. In this context, the present study aimed to evaluate the effect of 15 min of anodic tDCS at M1 on vertical jump performance in young soccer players. Based on the available evidence regarding the role of tDCS on performance in athletes, the hypothesis of this study is that tDCS will increase vertical jump performance and improve perceived exertion and recovery compared to sham tDCS.

Participants made four visits, one preliminary and three experimental sessions. All experimental sessions were separated by 24 h. Visits were simultaneously on the experimental days in a temperature-controlled room (−24°C). Participants were instructed to keep the same diet except for drinking coffee or energy drinks during the test week. On the first visit, they were familiarized with all procedures of all three sessions, which included, respectively, (a) the assessment of the Subjective Recovery Scale (SRS) and Visual Analogue Scale (VAS), (b) after 5 min seated the measurement of Rest Heart Rate (RHR); (c) the warmup which includes 1× with 10 s of interval of the following exercises: reverse lunge (20 reps), walking holding the knee unilaterally (20 reps for 3 m), isometric squat (for 15 s) with arms extended in front, low and medium skipping (20 reps). Warmup estimated time (3 min); (d) 3reps of CMJ followed by the Rating of Perceived Exertion (RPE) scale (CR-10) and the Rate Heart (HR) assessment; (e) After this, they did the Intervention, which means the adjustment of the tDCS apparatus and the stimulus for each group [for the active group (15 min of stimulus), sham (30 s of stimulus) and control (rest for 15 min, with no stimulus and without device)]; (f) The SRS, VAS and RHR; (g) 3 reps of CMJ; (h) RPE, and HR assessment. All data included all participants who completed the four sessions' experimental procedures (Figure 1). During the procedures, unique in the CMJ, all participants were stimulated to do their best. The study design is a triple-blinded, controlled, and randomized fashion.

The results of the Rest Heart Rate (RHR) are described in Table 1. The MANOVA results [Wilks' Lambda (W) = 0.446, F (2.24) = 0.537, p = 0.934, effect size (ηp²) = 0.331] did not detect differences between groups for RHR before the first CMJ. The absence of difference persists at the univariate ANOVA in all three days within the groups, respectively RHR day 1 (RHRD1) [F (2.24) = 0.074, p = 0.929, ηp² = 0.006], RHRD2 [F (2.24) = 0.295, p = 0.747, ηp² = 0.024], and RHRD3 [F (2.24) = 0.230, p = 0.796, ηp² = 0.019].

The MANOVA test did not detect differences between groups for the RHR after 15 min of Intervention between all groups [W = 0.446, F (2.24) = 0.537, p = 0.934, ηp² = 0.331]. For this variable, in this moment, univariate ANOVA results for RHR post-day 1 (RHRP1) [F (2.24) = 0.103, p = 0.903, ηp² = 0.008], RHRP2 [F (2.24) = 0.152, p = 0.860, ηp² = 0.012] and RHRP3 [F (2.24) = 0.131, p = 0.878, ηp² = 0.011].

Differences were not detected for the HR after the first CMJ (HR-CMJ) according to MANOVA [W = 0.446, F (2.24) = 0.537, p = 0.934, ηp² = 0.331]. The results of the univariate ANOVA also did not show significant differences according to the following results of HR-CMJ Day 1 (HR-CMJD1) [F (2.24) = 0.020, p = 0.980, ηp² = 0.002], HR-CMJD2 [F (2.24) = 0.040, p = 0.960, ηp² = 0.003], HR-CMJD3 [F (2.24) = 0.108, p = 0.898, ηp² = 0.009].

For the HR assessed after the Intervention (HR-CMJI), the MANOVA [W = 0.446, F (2.24) = 0.537, p = 0.934, ηp² = 0.331] did not detect differences between groups. For the same variable and moment, univariate ANOVA was conducted, HR-CMJI Day 1 (HR-CMJID1) [F (2.24) = 0.266, p = 0.769, ηp² = 0.022], HR-CMJID2 [F (2.24) = 0.323, p = 0.727, ηp² = 0.026], HR-CMJID3 [F (2.24) = 0.027, p = 0.973, ηp² = 0.002], no difference was detected.

The Visual Analogic Scale (VAS) results are described in Table 2. The MANOVA [W = 0.027, F (2.24) = 1.991, p = 0.084, ηp² = 0.837] did not detect any differences between the groups. As well as the univariate ANOVA at rest on Day 1 (RVASD1) [F (2.24) = 0.749, p =, ηp² = 0.059]; RVASD2 [F (2.24) = 2.648, p = 0.091, ηp² = 0.181]; and RVASD3 (RVAS-day 3) [F (2.24) = 2.615, p = 0.094, ηp² = 0.179]. The same situation occurred after the Intervention (PVAS), as seen on Day 1 (PVASD1) [F (2.24) = 0.106, p = 0.900, ηp²² = 0.009]; Day 2 PVASD2 [F (2.24) = 1.329, p = 0.283, ηp² = 0.100]; and PVASD3 [F (2.24) = 0.564, p = 0.576, ηp² = 0.045].

For the Subjective Recovery Scale (SRS), the MANOVA results showed no differences [W = 0.027, F (2.24) = 1.991, p = 0.084, ηp² = 0.837], as well as the univariate ANOVA in the pre-day 1 (SRSPreD1) [F (2.24) = 0.035, p = 0.965, ηp² = 0.003], SRSPreD2 [F (2.24) = 0.000, p = 1.000, ηp² = 0.000], SRSPreD3 [F (2.24) = 2.452, p = 0.107, ηp² = 0.170], and SRS post-day 1 (SRSPostD1) [F (2.24) = 0.304, p = 0.740, ηp² = 0.025], SRSPostD2 [F (2.24) = 0.041, p = 0.960, ηp² = 0.003], and SRSPostD3 [F (2.24) = 1.532, p = 0.237, ηp² = 0.113] the similarity was maintained (Figure 4).

The one-way ANOVA did not detect variation between all groups at the anthropometric variables. Respectively to the Weight, Height, BMI, and Fat Percentual, the statistical procedure shows the following results [F (2.24) = 2.640; p = 0.092]; [F (2.24) = 1.187; p = 0.322]; [F (2.24) = 1.156; p = 0.332] and [F (2.24) = 3.132; p = 0.062].

Thus, for the performance variables, the Jump Height (CMJ-JH) was submitted by the MANOVA test. The result did not detect any significance [W = 0.862, F (2.24) = 0.244, p = 0.994, ηp² = 0.071]. Additionally, the one-way ANOVA procedures did not reveal any differences at any time, which was confirmed by the post hoc analysis (Figure 5). As seen in the moments Pre Day 1 (CMJ-JHPreD1) [F (2.24) = 0.051, p = 0.950, ηp² = 0.004], Post Day 1 (CMJ-JHPostD1) [F (2.24) = 0.155, p = 0.857, ηp² = 0.013], CMJ-JHPreD2 [F (2.24) = 0.161, p = 0.852, ηp² = 0.013], CMJ-JHPostD2 [F (2.24) = 0.294, p = 0.748, ηp² = 0.024], CMJ-JHPreD3 [F (2.24) = 0.124, p = 0.884, ηp² = 0.010], and CMJ-JHPostD3 [F (2.24) = 0.358, p = 0.703, ηp² = 0.029]. The Table 3 contains results in interactions (time × groups) and time (groups throughout the protocol) according to study variables. The Table 4 contains individual pre and post changes in vertical jump height between groups (Intervention/Sham/Control).

For the Vertical Jump Power (CMJ-PO), the MANOVA results did not detect any differences [W = 0.629, F (2.24) = 0.827, p = 0.623, ηp² = 0.207] according to Figure 6. When examining the data in the univariate ANOVA, no differences were observed. These moments are as follows: CMJ-PO Pre Day 1 (CMJ-POPreD1) [F (2.24) = 0.435, p = 0.652, ηp² = 0.035], CMJ-PO Post Day 1 (CMJ-POPostD1) [F (2.24) = 0.407, p = 0.670, ηp² = 0.033], CMJ-POPreD2 [F (2.24) = 0.726, p = 0.494, ηp² = 0.057], CMJ-POPostD2 [F (2.24) = 0.310, p = 0.736, ηp² = 0.025], CMJ-POPreD3 [F (2.24) = 0.285, p = 0.754, ηp² = 0.023], and CMJ-POPostD3 [F (2.24) = 1.627, p = 0.217, ηp² = 0.119].

The MANOVA did not detect any differences in Rating of Perceived Exertion (RPE) throughout the protocol [Wilks' Lambda = 0.027, F (2.24) = 1.991, p = 0.084, ηp² = 0.837] (Figure 7). When examining the data in a univariate manner (univariate ANOVA), no differences were observed. These moments are as follows: moment Pre Day 1 (RPEPreD1) [F (2.24) = 0.225, p = 0.801, ηp² = 0.018], Post Day 1 (RPEPostD1) [F (2.24) = 0.695, p = 0.509, ηp² = 0.055], RPEPreD2 [F (2.24) = 0.626, p = 0.543, ηp² = 0.050], and at RPEPostD2 [F (2.24) = 1.109, p = 0.346, ηp² = 0.085], RPEPreD3 [F (2.24) = 0.960, p = 0.397, ηp² = 0.074], and RPEPostD3 [F (2.24) = 0.304, p = 0.740, ηp² = 0.025], the similarity was maintained.

The present study investigated the effect of transcranial direct current stimulation on CMJ in soccer players. Some studies show that the tDCS technique can influence physical/sports performance (17, 26). However, according to the results presented related to the tDCS montage contrary to our hypothesis, tDCS did not influence performance on the CMJ and perception of effort and recovery. These results are in line with previous studies that did not report significant improvements in performance (46, 47). However, other studies showed positive effects of tDCS on performance (48, 49).

The tDCS has been considered a technique influencing different physical/sports performance aspects. However, studies conducted with athletes involving specific tasks are scarce, and the heterogeneity of the adopted protocols does not allow for any conclusions about its ergogenic effects (19).

In terms of performance variables, several ergogenic measures aimed at optimizing physical performance, attenuating fatigue mechanisms, and facilitating post-physical exertion recovery have been widely studied (50–52). For example, caffeine (53) has shown improvements in vertical jump performance in soccer players, while in another study (54), caffeine ingestion was found to improve tactical performance in soccer athletes. As for tDCS used as an ergogenic measure to improve performance, Lattari et al. (23) demonstrated that anodal tDCS (2 mA for 20 min, with the anode positioned over the Cz point and the cathode over the right supraorbital area) led to improved jump performance in individuals considered advanced in strength training. However, unlike the individuals Lattari et al. (23) investigated, the individuals included in the present study are frequently exposed to CMJ as part of training load control. Therefore, they are likely better trained in the task, presenting a lower potential for improvement.

Regarding the subjective variables, a study by Moreira et al. (26) demonstrated improvement in the Well-Being Questionnaire of soccer players after 20 min of tDCS was applied in the Left Dorsolateral Prefrontal Cortex (DLPC) on the day following an official match. In another study by Valenzuela et al. (32) with swimmers, an improvement in the RPE was observed. In contrast to the present study's hypothesis, anodic tDCS applied to the M1 did not influence the responses of subjective scales in soccer players. It is worth noting that the stimulated area in the cited articles differs from the present study. Although it is not clear how the DLPFC and M1 are connected, a recent study reported that their excitabilities are linked (55). These findings agree with previous studies that also failed to show a reduction in RPE or an increase in performance after the application of tDCS over (20, 56).

Subjective assessments are recommended to monitor the psychometric recovery status of football players to detect early signs of fatigue and optimize high-level training performance (57). The decrease in these measurements are used to express the lesser subjective state of fatigue, effort and recovery of a player or team during training or competition. Improvements in these scores are significant predictors of performance in football athletes (58, 59). Likewise, insufficient recovery, poor quality of sleep from the previous night and increased levels of stress, fatigue (60, 61) during training can negatively impact athletic performance (62, 63). Thus, monitoring these psychometric measures (i.e., through subjective scales) in football players can assist coaches in programming and appropriately adapting training loads in order to maximize performance and reduce the risk of injuries, overtraining and overreaching functional (64–66).

Machado et al. (19) demonstrated that this topic still has limitations, mainly related to the type of montage, stimulation duration, the task performed, and athlete's level.

According to the established montage, performance was sometimes even better for the SHAM group. Therefore, the proposed montage in this study (CZ + Inion) did not seem to be effective for this Intervention, unlike classic montages such as the motor cortex and LDPC (M1 + Fp2) (67) or only the M1 (C3, and C4) (68), among other montages presented by Alix-Fages (20) and Machado (19).

However, this study was conducted in only three sessions, representing acute effects. Therefore, conducting further studies with this montage in a more prolonged intervention approach (chronic effect) would be interesting.

Nevertheless, this is the first study to evaluate the CMJ performance in soccer players throughout the competition period in soccer players. It also is the first study with this tDCS montage in the sports literature. Therefore, this article opens new possibilities for studying the essential effects of various ergogenic measures related to tDCS or other techniques, such as transcranial magnetic stimulation (TMS), as well as different montage configurations and performance variables in soccer or other specificities.

The results the present study showed that transcranial direct current stimulation (tDCS) in the CZ area at the M1 cortex for 15 min, in three sessions, did not improve vertical jump in the CMJ performance and did not improve the results at the subjective scales in soccer players. Future research should explore the effects of tDCS on the performance of professional football athletes. Furthermore, different tDCS configurations such as intensity, sessions, areas (e.i) should be explored in order to find optimal tDCS protocols to improve performance in different physical conditions. The application of tDCS in the setting of this study is not recommended to increase vertical jump performance or reduce perceived exertion and recovery.

There are limitations in the present study besides the prolonged stimulation mentioned above. First, this study should have included measurements to explain the neurophysiological effects of tDCS. Future studies should employ measures to assess neurophysiological effects, such as electromyography and electroencephalography, among others, at the target muscle areas. Additionally, the players were not separated by position, which may result in differences in CMJ performance, as specific position-specific requirements may involve more jumping, such as goalkeepers or defenders.