ABSTRACT
To investigate the acute effect of caffeine on performance in official soccer matches, a double-blind, balanced crossover design was applied to 13 professional male Brazilian soccer players in the first division of the state championship. Supplementation was carried out of caffeine (400mg, approximately 6mg/kg) in capsule form or a placebo. No effect of supplementation on the total distance covered (p=0.53; CI95%=-370, 796; d=0.22) or the total distance covered at different speeds (all p≥0.45; CI95%=-40.14, 84.57; ŋp2=0.048), acceleration (p=0.38; CI95%=-5.67, 11.8; ŋp2=0.06; d=0.22), number of sprints (p=0.52; CI95%=-1.55, 2.63; d=0.16), HRmean (p=0.52; CI95%=-1.87, 3.49; ŋp2=0.03; d=0.22), HRmax (p=0.10; CI95%=-6.69, 0.77; ŋp2=0.19; d=0.00), RPE (p=0.15; CI95%=-0.258, 1.48; d=0.50) or efficacy index (p=0.48; CI95%=-0.06, 0.12; ŋp2=0.04; d=0.22). Therefore, acute caffeine supplementation not effective to increase the performance of soccer players in official matches.
Keywords: Sports nutrition; Sports performance; Physiology; Biomechanics
RESUMO
Para investigar o efeito agudo da cafeína no desempenho em partidas oficiais de futebol, foi aplicado um delineamento duplo-cego, cruzado balanceado em 13 homens jogadores profissionais de futebol brasileiros da primeira divisão do campeonato estadual. A suplementação foi realizada com cafeína (400mg aproximadamente, 6mg/kg) em capsulas ou placebo. Não houve efeito da suplementação na distância total percorrida (p=0,53; IC95%=-370, 796; d=0,22) ou na distância total percorrida em diferentes velocidades (todos p ≥0,45; IC95%=-40,14, 84,57; ŋp2=0,048), aceleração (p=0,38; IC95%=-5,67, 11,8; ŋp2=0,06; d=0,22), número de sprints (p=0,52; IC95%=-1,55, 2,63; d=0,16), FCmédia (p=0,52; IC95%=-1,87, 3,49; ŋp2=0,03; d=0,22), FCmáx (p=0,10; IC95%=-6,69, 0,77; ŋp2=0,19; d=0,00), RPE (p=0,15; IC95%=-0,258, 1,48; d=0,50) ou índice de eficiência (p=0,48; IC95%=-0,06, 0,12; ŋp2=0,04; d=0,22). Portanto, a suplementação aguda de cafeína não é eficaz para aumentar o desempenho de jogadores de futebol em partidas oficiais.
Palavras-chave: Nutrição esportiva; Desempenho esportivo; Fisiologia; Biomecânica
RESUMEN
Para investigar el efecto agudo de la cafeína en el rendimiento en partidos oficiales de fútbol, se aplicó un diseño cruzado balanceado y doble ciego en 13 jugadores profesionales de fútbol brasileños de la primera división del campeonato estatal. La suplementación se realizó con cafeína (400mg aproximadamente, 6mg/kg) en cápsulas o placebo. No hubo efecto de la suplementación en la distancia total recorrida (p=0,53; IC95%=-370, 796; d=0,22) o en la distancia total recorrida a diferentes velocidades (todos p ≥0,45; IC95%=-40,14, 84,57; ŋp2=0,048), aceleración (p=0,38; IC95%=-5,67, 11,8; ŋp2=0,06; d=0,22), número de sprints (p=0,52; IC95%=-1,55, 2,63; d=0,16), FCmedia (p=0,52; IC95%=-1,87, 3,49; ŋp2=0,03; d=0,22), FCmáx (p=0,10; IC95%=-6,69, 0,77; ŋp2=0,19; d=0,00), RPE (p=0,15; IC95%=-0,258, 1,48; d=0,50) o índice de eficiencia (p=0,48; IC95%=-0,06, 0,12; ŋp2=0,04; d=0,22). Por lo tanto, la suplementación aguda de cafeína no es eficaz para aumentar el rendimiento de los jugadores de fútbol en partidos oficiales.
Palabras-clave: Nutrición deportiva; Rendimiento deportivo; Fisiología; Biomecánica
INTRODUCTION
Considering the physical demands in official matches and the large number of games played during the season, soccer players can present acute and chronic fatigue, causing a decrease in performance (Silva et al., 2018). Hence, the quest for strategies aimed at augmenting performance while mitigating fatigue remains incessant within the realm of high-performance sports (Lowery et al., 2023). One prevalent approach entails the utilization of supplementation (Kerksick et al., 2018).
Amidst the array of nutritional ergogenic aids, caffeine (CAF) stands as a subject of extensive investigation within the scientific literature (Guest et al., 2021). Primarily, this substance exerts its influence by stimulating the central nervous system, thereby augmenting cognitive functions and alertness, due to its action as an adenosine receptor antagonist (Grgic et al., 2020; Barreto et al., 2021). Caffeine and its primary metabolites, paraxanthine, theophylline, and theobromine, act as non-selective competitive antagonists of adenosine receptors, which are distributed throughout the central nervous system. This antagonism prevents adenosine from binding to its receptors, thereby inhibiting its depressant effects on neuronal activity, leading to increased focus and alertness. Additionally, this mechanism increases the release of catecholamines such as adrenaline and noradrenaline, further enhancing alertness and reducing the perception of effort and pain during physical activities (Barreto et al., 2021). This phenomenon potentially elucidates the mitigation of fatigue effects observed among athletes who consume CAF (Souza et al., 2022). CAF can enhance the availability of calcium within the myofibrils, optimize substrate availability during exercise, increase calcium availability and muscle contractility, and increase the oxidation of free fatty acids. All these effects can occur simultaneously, but one is not the cause of the other (Souza et al., 2022). Such effects can prolong exercise tolerance and increase performance (Grgic et al., 2020).
Despite caffeine having been extensively researched in soccer, the majority of studies have focused on its effects on players' physical performance in tests or simulated matches (Mielgo-Ayuso et al., 2019). The scant number of studies that have analyzed simulated soccer matches, which are incapable of fully replicating the demands inherent to official matches, have yielded divergent outcomes (Del Coso et al., 2012; Lara et al., 2014; Pettersen et al., 2014). To the best of our knowledge, only two studies have examined the impact of CAF in simulated matches with significant results (Del Coso et al., 2012; Lara et al., 2014). The results demonstrated increase in running distance, running distance at high intensity, and the count of sprints. In contrast, Pettersen et al. (2014) observed no significant differences between CAF and placebo (PLA) in terms of distance covered while running, high-intensity running, or the number of sprints. It is recognized that physical performance in soccer can be impacted by various uncontrollable factors, particularly during official matches. These factors include environmental conditions (e.g., weather, pitch quality), player-specific conditions (e.g., fatigue, injuries), and situational variables (e.g., referee decisions, tactical strategies used by opponents) (Teixeira et al., 2021). However, no studies investigating the supplementation of CAF in uncontrolled official matches have been identified.
Therefore, it is necessary to investigate the effects of CAF supplementation in official matches. To the best of our knowledge, no studies involving CAF have been conducted with professional soccer players during official soccer matches. Therefore, the aim of the current study was to examine the acute effects of caffeine supplementation on physical performance and psychophysiological variables during official soccer matches. The hypothesis posits that caffeine supplementation will enhance physical performance through optimized physiological and psychological responses.
METHODS
This study was submitted to and approved by the Ethics and Research Committee of the Universidade Federal de Minas Gerais (CAAE: 38162020.4.0000.5149), and followed a double-blind, balanced crossover design. Therefore, thirteen male Brazilian highly trained/national level (McKay et al., 2022) professional soccer players (26 ± 4.5years, 74 ± 6kg, 179 ± 5cm, O2Max: 54 ± 3mL.kg-1·min-1, body fat: 9 ± 3%) participated in two full-fledged competitive matches with an interval one week between them. As these were official matches, the games were held in different locations, with varying environmental conditions and opponents. However, all the analyzed matches were part of the same championship. Six athletes were defenders, four were midfielders, and three were forwards, and they played both analyzed matches in the same position. No a priori sample size calculation was conducted, as the sample consisted of highly trained professional soccer players, making it difficult to conduct interventions during official matches. One subsequent to the consumption of PLA and the other following the intake of CAF 400mg, approximately 6mg/kg, administered in capsules, 60 minutes prior to the matches (Del Coso et al., 2012; Mielgo-Ayuso et al., 2019). The randomization process was conducted by drawing lots, and the capsules, which were identical in appearance, were prepared in advance by an assistant researcher. To maintain blinding, the capsules were indistinguishable and administered in the same manner for both conditions. Blinding effectiveness was not directly assessed through questionnaires or similar methods. It was requested and ensured by the coaching staff that the training load in the days leading up to the matches was the same for all participants in the study. The players received instructions not to alter their diet or consume any nutritional supplementation during the study. They were also advised to abstain from consuming other stimulant substances for up to 12 hours preceding each match. As these were official matches of the state championship, there were no dropouts or missing cases throughout this study.
During the official matches, all players wore a device equipped with a global positioning system featuring a sampling frequency of 10Hz, as well as a 200Hz 3D accelerometer, gyroscope, and magnetometer (Polar Team® Pro System; Polar Electro Oy, Kempele, Finland) (Akyildiz et al., 2020). The performance analysis encompassed distances covered from 3.00 to 6.99km/h (RD1), 7.00 to 14.99km/h (RD2), 15.00km/h to 19.99km/h (RD3), 20.00 to 24.99km/h (RD4) and above 24.99km/h (RD5) (Barnes et al., 2014). The number of accelerations was quantified between 2.00 and 2.99m/s2 (AC1), and above 2.99m/s2 (AC2), and decelerations between -2.00 and -2.99m/s2 (DC1), and less than -2.99m/s2 (DC2) (Silva et al., 2023). The mean (HRmean) and maximum heart rate (HRmax) were also analyzed, with the HRmax obtained through the Yo-Yo intermittent level 1 recovery test (Bangsbo et al., 2006). The Training Impulse (TRIMP) was calculated by dividing the intensity zones relative to the HRmax (zone1: 50 to 60%; zone2: 60 to 70%; zone3: 70 to 80%; zone4: 80 to 90%; zone5: 90 to 100%), and the accumulated time in each zone was be multiplied by its value, with the obtained results subsequently being summed (Edwards, 1993). The physical efficacy index (EI) was also analyzed, which is the ratio of running distance per minute divided by the mean of the percentage of HRmax. The rate of perceived exertion (RPE) was assessed 30min after the match and the assigned score was multiplied by the standard total match time (Morandi et al., 2020). The Morandi scale, adapted from the CR10 scale, ranges from 0 to 10, with specific verbal anchors at levels 1, 3, 5, 7, and 9 to describe varying intensities of exertion and fatigue. This design helps in distinguishing different levels of effort accurately. This scale was chosen because the athletes involved in the study were already familiar with it, ensuring consistent and reliable responses (Morandi et al., 2020).
Statistical analysis
The normality and sphericity of data variances were verified by Shapiro-Wilk and Mauchly test. The Wilcoxon test to verify if there was a difference in the total time the players remained on the field. The paired t-test was used to verify differences in RPE between conditions. For performance and physiological variables over time a two-way repeated measures ANOVA with the following fixed factors analyzed were the experimental condition (placebo vs. caffeine) and match halves (first half vs. second half). The interaction between these factors was also evaluated to determine whether the effects of supplementation varied between the halves of the match. When relevant, BonFerroni's post-hoc was applied. To assess the effect size, Cohen's d (small = 0.2 - 0.3; moderate = 0.5 - 0.8; large > 0.8), and square partial ETA (ŋp2) (small = 0.01; moderate = 0.06; large > 0.14) was used. Results are presented as mean ± standard deviation and confidence interval (CI95%). The significance level adopted was α=0.05 and the SPSS software version 20.0 was used.
RESULTS
The Figure 1 below showed the individual behaviour of variables analyzed in relation to the mean of the Placebo and Caffeine conditions. No differences (p=0.81; d=0.13) in the duration players spent on the field (PLA=92.4 ± 7.8min; CAF=91.3 ± 9.2min). Furthermore, no effect of CAF on the total distance covered (p=0.53; CI95%=-472, 1.027; ŋp2=0.03; d=0.25). However, the total distance covered in the 1st half is greater than in the 2nd half (p=0.01; ŋp2=0.52; d=1.08) both in the PLA (p=0.02; CI95%=98.66, 1225.95; d=0.89) and CAF (p=0.01; IC95%=269.25, 1473.36; d=1.26). When analyzing the total distance covered at different speeds separately, no difference was found between the PLA and CAF (p=0.453; CI95%=-40.14, 84.57; ŋp2=0.048). However, the total distance covered in RD2 was greater in the 1st half (p=0.01; CI95%=265.93, 670.37; ŋp2=0.96; d=1.50). The same occurred for RD3, which was higher in the 1st half (p=0.01; CI95%=40.17, 183.59; ŋp2=0.96; d=0.80).
Individual behavior of performance and physiological variables in relation to the mean of the Placebo (gray bar) and Caffeine (white bar) conditions.
Regarding the accelerations, no supplementation effects was observed for the total number of actions in AC1 (p=0.38; ŋp2=0.06), yet the 1st half exhibited a greater number of actions in AC1 (p=0.01; CI95%=2.91, 13.46; ŋp2=0.48; d=0.89). Similar results were observed for the number of actions in AC2, which exhibited no difference between the conditions (p=0.64; ŋp2=0.01), but were greater in the 1st half (p=0.01; CI95%= 0.84, 4.16; ŋp2=0.45; d=0.66). Comparable results were also identified for the number of actions in DC1, which did not demonstrate changes regardless of supplementation (p=0.01; ŋp2=0.24), but were higher in the 1st half (p=0.01; CI95%=3.36, 16.79; ŋp2=0.47; d=1.08). However, this pattern was not observed for the number of actions in DC2; there were no changes attributed to supplementation (p=0.75; ŋp2=0.01) or match half (p=0.06; ŋp2=0.27; d=0.48).
Regarding the number of sprints, there was no alteration irrespective of supplementation (p=0.52; ŋp2=0.03) or match half’s (p=0.60; ŋp2=0.02). However, the maximum speed attained by the players in the PLA was greater than CAF (p=0.03; CI95%=0.11, 2.71; ŋp2=0.32; d=0.50). Table 1 below presents the results of these variables.
No supplementation effect on absolute values (p=0.52; CI95%=-1.87, 3.49; ŋp2=0.03; d=0.03) or percentages (p=0.48; CI95%=-0.93, 1.85; ŋp2=0.04; d=0.04) of HRmean. Additionally, there was no difference for the absolute values (p=0.76; CI95%=-5.40, 4.09; ŋp2=0.01; d=-0.06) and percentages (p=0.68; CI95%=-2.87, 1.94; ŋp2=0.01; d=-0.07) of HRmax. However, irrespective of supplementation, there was an observed effect of 1st vs 2nd half on HRmean in absolute values (p=0.01; ŋp2=0.97; CI95%=10.17, 12.36; d=0.54) and percentages (p=0.01; CI95%=5.12, 6.41; ŋp2=0.96; d=0.51), with values being higher during the 1st half. Nonetheless, in relation to absolute HRmax (p=0.11; CI95%=-6.69, 0.77; ŋp2=0.19; d=-0.04) and percentage (p=0.10; CI95%=-3.43, 0.34; ŋp2=0.20; d=-0.03), no differences were found. Regarding TRIMP, there was also no change attributed to supplementation (p=0.60; ŋp2=0.02), but this variable was higher during the 1st half (p=0.01; CI95%=11.86, 43.21; ŋp2=0.55; d=0.72). Table 2 below presents these results.
The RPE exhibited no difference (p=0.15; CI95%=-0.26, 1.48; d=0.50) between PLA (7.8 ± 1.0) and CAF (7.2 ± 1.4). As for the EI, there was no change attributed to supplementation (PLA:0.81 ± 0.14; CAF:0.78 ± 0.07; p=0.48; CI95%=-0.06, 0.12; ŋp2=0.04; d=0.22), but it was lower during the 2nd half when compared to the 1st half (p=0.01; CI95%=0.06, 0.29; ŋp2=0.50; d=1.21) in both PLA (1st=1.15 ± 0.13; 2nd=1.01 ± 0.26; d=0.68) and CAF (1st=1.16 ± 0.11; 2nd=0.94 ± 0.22; d=1.27).
DISCUSSION
The aim of the current study was to examine the acute effect of CAF supplementation on performance and psychophysiological variables in official matches. The primary hypothesis posited was that acute CAF would enhance the players' performance. Nevertheless, the outcomes of the current study suggested that there were no differences in any of the performance or psychophysiological variables investigated, except for maximum speed, which showed a significant difference.
We did not find any other studies that investigated CAF in elite official matches. Only three studies were discovered that explored the effects of CAF in simulated matches (Del Coso et al., 2012; Lara et al., 2014; Pettersen et al., 2014). Del Coso et al. (2012) had investigated the effect of CAF in simulated matches and found increased the total running distance, high-intensity running distance and the number of sprints. However, the HRmean and HRmax were not affected. Lara et al. (2014), applied the same experimental design, likewise identified differences between PLA and CAF in total running distance, running distance at speeds, and the count of sprints; nevertheless, the sample comprised female athletes. Corroborating the results of the current study, Pettersen et al. (2014) did not observe differences between PLA and CAF regarding total distance covered during running, high intensity running, sprint distance or acceleration/deceleration counts.
The potential explanation for the discrepant outcomes in these studies might be associated a small difference, though, in blinding (Red Bull® vs. Pepsi and®) the varying forms of supplementation and the consumption of energy drinks that, aside from CAF, encompassed additional substances like sodium bicarbonate and taurine (Del Coso et al., 2012; Lara et al., 2014; Pettersen et al., 2014), whose combined effects are not well known. Furthermore, performance in the soccer can be influenced by physical fitness (Krustrup et al., 2005; Taylor et al., 2017), competitive level (Mohr et al., 2003; Vieira et al., 2019), artificial turf versus natural grass (Andersson et al., 2008; Sanchez-Sanchez et al., 2021), heat stress due to environment temperature (Mohr and Krustrup, 2013; Draper et al., 2023), ball possession, and player/team technical standard (Wang et al., 2022).
The effects of caffeine are quite consistent in controlled situations (Grgic et al., 2020). However, soccer is an unpredictable sport, with many variables that are impossible to control. Competitive sports, in general, are difficult to control due numerous interactions with other players and varying environmental conditions. Therefore, these factors might have influenced the results, considering that the data collection was carried out during official matches. Additional studies need to be conducted in complete and official matches to investigate the specific effects of caffeine on soccer performance. Furthermore, since the effects of caffeine depend on individual responses, there may be both high and low reactions to various doses of this substance (Apostolidis et al., 2019; Ellis et al., 2019). As the mechanisms of caffeine action are linked to fatigue (Guest et al., 2021; Souza et al., 2022) and to increased focus, attention, decision-making, and passing accuracy (Foskett et al., 2009), its effects might become more evident in longer matches or in non-standard environments.
To the best of our knowledge and within the constraints of our study, no investigations were identified that examined the impact of caffeine (CAF) on heart rate (HR) and Training Impulse (TRIMP) based on HR during official matches, which constrains our discussion. However, studies have shown that caffeine can significantly affect HR in controlled environments (Grgic et al., 2020). TRIMP, calculated from HR data, is sensitive to variations in exercise intensity and duration (Foster et al., 2001). Despite the lack of specific research in official matches, our results indicated a decline in both HR and TRIMP during the second half, aligning with existing literature in team sports (McLaren et al., 2018). Further research, including simulated matches and other physical tests, could provide deeper insights into the specific effects of caffeine on these parameters.
Regarding RPE, the present study utilized the scale developed by Morandi et al. (2020). This scale was chosen due to the athletes' familiarity with it. The sole study that addressed RPE in team matches was conducted by Lara et al. (2014) discerned no noteworthy distinction between CAF and PLA. Given the mechanisms of caffeine's effects (Del Coso et al., 2011; Guest et al., 2021), it was anticipated that supplementation would lead to a decrease in RPE. Nevertheless, the systematic review examining this supplementation in team sports (Salinero et al., 2019) predominantly aligns with the findings of the present study.
Regarding the EI, this variable possesses sensitivity in detecting alterations in the training status of soccer players (Suarez-Arrones et al., 2015). This sensitivity arises from the fact that the EI is a metric enabling the concurrent assessment of external and internal loads (Lima-Alves et al., 2022). An illustrative instance was offered by Suarez-Arrones et al. (2015), who scrutinized the EI of elite soccer players during official matches within a European championship. The study's outcomes revealed a diminished EI value in the 2nd half of the match, as compared to the 1st half. Thus, in the event of detecting any dissimilarity in the efficacy index between CAF and PLA, one could speculate either that players exerted reduced physiological exertion to cover a greater distance, thereby escalating the EI, or the converse. Nonetheless, in the present study, CAF did not exhibit the capability to influence the EI.
In summary, this study presents some important strengths and limitations. One of the strengths is the innovation of testing caffeine supplementation during official soccer matches, which had not been explored before. This approach provides a more realistic view of the effects of caffeine under true game conditions, enhancing the practical relevance of the results. However, the unpredictable nature of team sports and the presence of numerous uncontrollable variables, such as interactions with other players and varying environmental conditions, may have influenced the results obtained. Additionally, the sample being restricted to a small number of official matches and a specific group of highly trained players limits the generalization of the findings. Future studies should consider a larger number of matches and a more diverse sample to confirm and expand these results. Furthermore, this clinical trial did not measure any biochemical method to ensure that the tested substance was truly bioavailable. This is occur given the constraints of conducting interventions during live matches, the emphasis was on practical and minimally invasive data collection methods. Furthermore, as a suggestion to future studies, applying the CaffEQ-BR (Mendes et al., 2020) and a questionnaire on habitual caffeine consumption to measure the level of expectancy effects and estimated mean caffeine consumption (Filip et al., 2020) could have been useful.
CONCLUSION
In this experimental design, acute caffeine supplementation does not appear to be sufficiently effective in enhancing the players' physical performance or exerting a positive influence on psychophysiological variables within official matches. Lastly, these findings should be cautiously interpreted due to certain limitations inherent in this study, such as the restricted number of official matches scrutinized, the sample size, and the difficult control of intervening variables. It is noteworthy to emphasize the necessity for additional research on caffeine supplementation during official matches and competitions.
ACKNOWLEDGEMENTS
The authors would like to thank Pró-Reitoria de Pesquisa (PRPq) of the Universidade Federal de Minas (UFMG).
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FUNDING
All authors declare no support from any organization for the submitted work, no financial relationships with any organizations that might have an interest in the submitted work, and no other relationships or activities that could appear to have influenced the submitted work.
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Publication Dates
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Publication in this collection
02 Dec 2024 -
Date of issue
2024
History
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Received
05 Feb 2024 -
Accepted
16 Oct 2024