Electroencephalographic recordings have shown a functional role of brain oscillations in motor execution of the body’s movements. In particular, in the motor cortex (M1), beta oscillations (13–30 Hz) seem to be prevalent during rest, while gamma rhythm (>30 Hz) synchronizes when subjects focus attention (Jensen et al., 2007) and during voluntary movement preparation (Murthy and Fetz, 2006; Sanes and Donoghue, 2006) and execution (Pfurtscheller et al., 2003). Studies using invasive recordings have provided the general idea that in the central brain regions, gamma band responses are related to externally paced (Pfurtscheller and Lopes da Silva, 1999; Ohara et al., 2000) and self-paced hand and arm movements (Pfurtscheller et al., 2003; Szurhaj et al., 2005; Miller et al., 2007; Ball et al., 2008). The increase in gamma synchronization reflects enhanced cortical excitability (Schroeder and Lakatos, 2009) as well as neural computation of movements’ details (Rickert, 2005). The growing number of studies reporting gamma synchronization during movement execution (Pfurtscheller et al., 2003; Brovelli et al., 2005) suggested that it might be the elective rhythm involved in voluntary movement and that its synchronization may facilitate motor processing.

In the past decades, the functional role of brain oscillations in motor performance has been further investigated using non-invasive brain stimulation (NIBS) techniques. Transcranial magnetic stimulation (TMS) set at 50 Hz has been used to induce changes in cortical excitability, as measured with motor-evoked potentials (MEP) as well as in motor performance (Chen et al., 1997; Oliveri et al., 2005). Among electric stimulation techniques (tES), transcranial direct current stimulation (tDCS) (Paulus, 2011) has been employed to improve performance in several athletes such as cyclists (Okano et al., 2015) and skiers (Reardon, 2016). In particular, after 20 min of anodic tDCS over the left temporal cortex, a significant enhancement of peak power and a reduced effort perception has been found in cyclists during an incremental cycling test (Okano et al., 2015). Such changes have been investigated also in genetic studies (Brunoni et al., 2013; Puri et al., 2015) that have shown an effect of the polymorphism Val66Met in the brain-derived neurotrophic factor (BDNF) gene in the tDCS-induced plasticity in older adults (Puri et al., 2015).

A new electrical stimulation technique, known as transcranial alternating current stimulation (tACS), has been introduced as a promising tool, able to enhance brain oscillations and modulate cognitive (Feurra et al., 2012), perceptual (Kanai et al., 2008, 2010), and motor performances (Pogosyan et al., 2009; Feurra et al., 2011; Giustiniani et al., 2021). The tACS protocols include the application of electrical alternating currents to the scalp through electrodes placed over a target brain region. Such sinusoidal stimulation has been shown to modulate neuronal membrane potential (Ali et al., 2013). tACS is usually delivered at weak intensities (typically between 1 and 2 mA), and depending on the stimulation setting, tACS effects have been shown to outlast the end of the stimulation (i.e., after-effect; Kasten et al., 2016). Previous studies have shown that when applied over M1 at 50 Hz, tACS improves velocity and acceleration of visually triggered movements compared with beta or sham-tACS (Moisa et al., 2016). Interestingly, driving gamma oscillation at 50 Hz in the motor cortex leads to a significant duration-dependent modulation of local GABA_A inhibition. Such changes in GABAA during tACS correlate with motor behavior (Nowak et al., 2017) and might be a link between tACS effects and sport performances (Schutter and Hortensius, 2011; Sakashita et al., 2019). Accordingly, in a recent study, we found that tACS at 50 Hz applied over M1 affects motor performance and induce a long-lasting modulation of M1 excitability levels (Giustiniani et al., 2021), similar results are obtained for motor performance when tACS electrodes are placed over the cerebellum (Giustiniani et al., 2021).

As suggested by previous studies, tACS effects may outlast the end of the stimulation (i.e., after-effect; Zaehle et al., 2010; Helfrich et al., 2014). While tACS online effects mostly have been described in terms of entrainment (Helfrich et al., 2014), tACS after-effects, lasting up to 70 min after stimulation’s end (Kasten et al., 2016), have been shown to rely on other mechanisms not strictly related to the entrainment (Moliadze et al., 2010; Vossen et al., 2015; Antal and Herrmann, 2016; Brighina et al., 2019; Giustiniani et al., 2019). In particular, the offline effect of tACS might be dependent on synaptic plasticity. Namely, the BDNF gene mediates the specific effect of tACS in neuronal oscillations (Riddle et al., 2020). Of note, the presence of the Val66Met, plays a role also in mediating the efficacy of NIBS. Indeed, the presence of the Val66Met polymorphism seems to predict reduced efficacy for neuromodulation. When the Val66Met polymorphism is present, a reduced neural response to brain stimulation would be caused by a decreased NMDA-dependent long-term potentiation of synaptic transmission (Antal et al., 2010; Fritsch et al., 2010; Podda et al., 2016). Interestingly, the presence of this polymorphism has been shown to reduce also the beneficial effect of physical activity (Erickson et al., 2013). Overall, this evidence suggests that entrainment and long-lasting tACS-related effects can improve motor skills, such as motor learning and muscular strength inducing optimal performance during sports through the enhancement of synaptic plasticity (Fritsch et al., 2010). Therefore, the genetic background should be taken into account to estimate tACS-induced effect, especially in the context of sports. Unfortunately, to date, no studies have investigated whether tACS effects might be transferred to the real sport performance and whether the genetic background might influence the response to tACS in athletes. Only recently, some authors have coined the term “neurodoping” to indicate the use of these emerging techniques to enhance physical and mental performance in sport (Jenkinson and Brown, 2011; Reardon, 2016; Colzato et al., 2017). While, on one hand, tDCS effects have been investigated during sports performances (Okano et al., 2015; Reardon, 2016), so far, no studies have been conducted using tACS. In the sports domain, to transfer the stimulation effects from a single movement or motor behavior to a motor performance poses an important and ambitious challenge to researchers, and it constitutes a critical aspect in athletes’ technical–tactical acts (Podda et al., 2016).

Therefore, the aim of the present pilot study was to investigate whether modulating gamma rhythm over M1 could improve physical performance in sports subjects when applied prior to sports performance. To this end, we applied 50-Hz tACS or sham-tACS in sports subjects immediately before the execution of a set of physical tests. As gamma oscillations have been found to be involved in force production (Muthukumaraswamy, 2010; Naro et al., 2017), we applied tACS before a set of physical tasks traditionally employed to measure force levels (Glatthorn et al., 2011; Proia et al., 2019; Bonaventura et al., 2020). Additionally, we investigated whether the presence of the angiotensin-converting enzyme (ACE) polymorphism, traditionally related to the force level, mediated the effect of tACS in tasks requiring grip execution (Costa et al., 2009).

Previous studies have shown that female and male athletes showed significant differences in sports performances and physical tests, attributable to sex-specific differences in neuromuscular control and in body mass (Battaglia et al., 2014; McFarland et al., 2016; Holden et al., 2019). Therefore, it might have important implications to explore separately tACS-related sex-specific responses in sports people. Additionally, as it has been shown that genetic background affects individual response to tES (Brunoni et al., 2013; Puri et al., 2015), we evaluated whether the brain-derived neurothrophic factor (BDNF) Val66Met polymorphisms was associated with tACS physical response.

The aim of this pilot study was to investigate the effect of tACS on physical performance in a group of sport subjects. The preliminary results showed that tACS does not modulate sport performance when applied immediately before it. Although they should be interpreted cautiously due to the small sample size, our results suggest that the lack of tACS effects does not depend on the genetic background of the subjects. To the best of our knowledge, this is the first study investigating the role of tACS in athlete subjects taking into account how genetic background may affect individual response to stimulation. In particular, the novelties of this pilot study were to investigate whether genetic background may influence response to tACS in physical exercise and to apply tACS on motor performance in real-life context.

To date, there has been a small amount of research looking at the motor effects of 50-Hz tACS. All of these studies have been conducted in a laboratory context. The lack of evidence reporting the real-life effect of tACS allows only a partial comparison of our results with previous studies. Despite some studies having reported no changes in motor performance during and after gamma-tACS (Miyaguchi et al., 2018), other studies have found an after-effect in motor behavior after gamma tACS (Joundi et al., 2012; Pollok et al., 2015). In particular, Joundi et al. (2012) reported an improvement of grip force during stimulation. Differences in the results might be due to the mismatch in the applied frequencies (70 Hz in the study of Joundi vs. 50 Hz in our study) as well as in the electrode montage (left M1 and the ipsilateral shoulder vs. left and right M1) and electrode size (5 × 7 cm² and 5 × 10 cm² vs. 5 × 5 cm²). Divergent results might also be explained by the cortical network state dependence of tACS effects (Kutchko and Fröhlich, 2013; Nowak et al., 2017). However, the main difference between previous findings and our preliminary results lays in the application of tACS immediately before motor performance rather than during it. Since it has been shown that different mechanisms might account for tACS online and offline effects, stimulating before the performance might have led to a lack of effect. Overall, these findings suggest the need for well-controlled tACS protocols, which should be ideally based on accurate electric field predictions (Ali et al., 2013). Indeed, when applying tACS, finding online effects does not strictly imply that these effects will outlast the end of the stimulation. At a first glance, the lack of offline effects suggests that 50-Hz tACS applied with this interhemispheric montage does not induce any long-term effect on motor performance. However, it might well be that tACS has induced after-effects that we were not able to detect with our experimental paradigm (i.e., changes in MEP amplitudes). Indeed, as pilot investigation, we applied tACS for the first time in daily living complex tasks rather than in an experimental task that is able to isolate a single variable of interest.

Additionally, since we applied 50-Hz tACS at rest, it is possible to hypothesize that neural networks did not oscillate at this frequency band during rest but only during movement’s execution. Therefore, another possible explanation is that entrainment of brain oscillations did not occur, simply because groups of neurons oscillated at higher or lower frequency bands (Helfrich et al., 2014). In other words, applying gamma tACS while subjects were resting might have caused a failure of entrainment. In this line of reasoning, it is conceivable to hypothesize that applying tACS during the task might have a different effect on sports performance. Future studies are needed to understand the effect of tACS applied during real-life physical performance.

However, we should acknowledge that some methodological points might account for the null results. Indeed, to modulate gamma activity over the left and the right motor cortex, we used an inter-hemispheric montage that is not the most commonly used montage to affect motor cortex. The choice of this montage was made in order to affect both C3 and C4 because the performed tasks, executed using both hands, required a bilateral activation. Although a previous study reported an effect in terms of changes in excitability levels after the application of tACS with this montage (Schutter and Hortensius, 2011), we cannot exclude that applying the reference electrode over an extracephalic region or over the contralateral frontopolar cortex (above the eyebrow) might have allowed a better targeting of M1. However, the simulation of the electric field suggested a reliable modulation induced over the left and right M1. On the other hand, we found a significant effect of age in the performance at some physical tests. This result is in line with previous studies showing that age is associated with slower performance and altered recruitment of specific brain regions, such as the prefrontal cortices during visuo-motor tasks (Berchicci et al., 2014; Battaglia et al., 2020; Giustino et al., 2020). As easily conceivable, age might affect the physical performance so that the older the subjects, the worst the performance. However, further studies would be needed targeting the effect of age in physical tasks.

Regarding the genetic analyses, our preliminary results did not detect any relationship between the BDNF Val66Met polymorphism and the response to tACS. This result is apparently in contrast with a previous study reporting a greater increase in the power of alpha oscillations after alpha tACS in subjects with the Val66Val rather than in the Val66Met carriers (Riddle et al., 2020). This difference in the results might be due to the different stimulation frequency; indeed, given its role in shaping state-dependent neural excitability, the modulation of alpha oscillations might be more dependent on synaptic plasticity and consequently on BDNF polymorphism (Riddle et al., 2020). Therefore, we cannot exclude that that gamma tACS after-effect might be less dependent on the BDNF polymorphism than the effect of alpha tACS. However, this interpretation should be taken cautiously due to the small sample size of our study; indeed, analysis of genetic polymorphisms in such a small sample may not be representative of the general population (Neuling et al., 2013). On the other hand, studies using tDCS have reported no impact of BDNF polymorphism on response to tDCS (Brunoni et al., 2013). Our results are only partially comparable with the previously reported evidence. Indeed, different forms of current (i.e., direct vs. alternating) might exert different effects at cellular level and therefore might be selectively influenced by a given gene. Additionally, in our sample, genotypes were not equally distributed in the experimental groups, so that the AA genotype was found in one subject that was in the sham group, while GG genotype was present in four subjects of the real-tACS group and in one subject of the sham group. The AG genotype was present in seven subjects of the real-tACS group and in four subjects of the sham-tACS group. In other words, being the subject with the AA genotype in the sham group, we cannot state whether this genotype might have an effect on the response to tACS. Similarly, the unequal and low number of the other genotypes distributed among the two experimental groups allow us only to speculate that such genotypes do not affect response to tACS in physical performance. Further investigations are needed to understand the nature and interaction between genetic background and NIBS protocol at a cellular and, mostly important, at a behavioral level. A further limitation of the present study is that we cannot draw conclusion on the relationship between the lack of behavioral effects and the excitability levels, since we did not assess motor cortex excitability. Additionally, larger samples or changes in the stimulation setting and in the applied frequency bands are needed to further investigate the effects of tACS in physical performance. Further studies might investigate online effect of tACS applied during physical performance rather than before it. In other words, an ambitious challenge is to assess the efficacy of tACS in real-world sports performance in order to solve the doubt whether tACS supplementation fits into the regulatory framework at the competitive level. In the meantime, it seems likely that tACS will continue to be explored by elite athletes looking for that elusive edge.

In modern sports, there is huge pressure to improve performance rapidly, both in amateur and professional circles. Therefore, understanding whether neuromodulation through tACS may improve physical performance is a key point of modern sports discipline. This work offers a preliminary and novel first step to study the influence of non-invasive approach with tACS on performance in sports people. Our preliminary results suggest that the modulation of real-life sports performance probably requires further investigation to identify the optimal parameters and stimulation setting. It seems that these settings differ enough from parameters traditionally applied in laboratory context. Further studies are needed to clarify whether tACS might improve real athletes’ performances.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by the ethic committee of Palermo 1 (4/2020). The patients/participants provided their written informed consent to participate in this study.

AG, PP and GB made substantial contributions to conception and design. HM, AI and GM contributed to acquisition of data. SG performed statistical analysis and interpretation of data. AG, GB and PP drafted the article with the help of GM, AI and HM. MO and AP revised the article critically for important intellectual content. All authors approved the final version to be published and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy and integrity of any part of the work are appropriately investigated and resolved. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.