Caffeine supplementation and physical performance, muscle damage and perception of fatigue in soccer players: A systematic review

nutrients Review Caffeine Supplementation and Physical Performance, Muscle Damage and Perception of Fatigue in Soccer Players: A Systematic Review Juan Mielgo-Ayuso 1,* , Julio Calleja-Gonzalez 2, Juan Del Coso 3, Aritz Urdampilleta 4, Patxi León-Guereño 5and Diego Fernández-Lázaro 6 1Department of Biochemistry and Physiology, School of Physical Therapy, University of Valladolid, 42004 Soria, Spain 2Laboratory of Human Performance, Department of Physical Education and Sport, Faculty of Physical Activity and Sport, University of the Basque Country, 01007 Vitoria, Spain; [email protected] 3Exercise Physiology Laboratory, Camilo JoséCela University, 28692 Madrid, Spain; [email protected] 4Elikaesport, Nutrition, Innovation & Sport, 08290 Barcelona, Spain; [email protected] 5Faculty of Psychology and Education, University of Deusto, Campus of Donostia-San Sebastián, 20012 San Sebastián, Guipúzcoa, Spain; [email protected] 6Department of Cellular Biology, Histology and Pharmacology. Faculty of Physical Therapy, University of Valladolid. Campus de Soria, 42004 Soria, Spain; diego.fernandez.lazar[email protected] *Correspondence: [email protected]; Tel.: +34-975-129-187 Received: 21 January 2019; Accepted: 15 February 2019; Published: 20 February 2019   Abstract: Soccer is a complex team sport and success in this discipline depends on different factors such as physical fitness, player technique and team tactics, among others. In the last few years, several studies have described the impact of caffeine intake on soccer physical performance, but the results of these investigations have not been properly reviewed and summarized. The main objective of this review was to evaluate critically the effectiveness of a moderate dose of caffeine on soccer physical performance. A structured search was carried out following the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines in the Medline/PubMed and Web of Science databases from January 2007 to November 2018. The search included studies with a cross-over and randomized experimental design in which the intake of caffeine (either from caffeinated drinks or pills) was compared to an identical placebo situation. There were no filters applied to the soccer players’ level, gender or age. This review included 17 articles that investigated the effects of caffeine on soccer-specific abilities (n= 12) or on muscle damage (n= 5). The review concluded that 5 investigations (100% of the number of investigations on this topic) had found ergogenic effects of caffeine on jump performance, 4 (100%) on repeated sprint ability and 2 (100%) on running distance during a simulated soccer game. However, only 1 investigation (25%) found as an effect of caffeine to increase serum markers of muscle damage, while no investigation reported an effect of caffeine to reduce perceived fatigue after soccer practice. In conclusion, a single and moderate dose of caffeine, ingested 5–60 min before a soccer practice, might produce valuable improvements in certain abilities related to enhanced soccer physical performance. However, caffeine does not seem to cause increased markers of muscle damage or changes in perceived exertion during soccer practice. Keywords: football; RPE; DOMS; sport performance; supplementation; ergogenic aids 1. Introduction Soccer is considered one of the most popular sports worldwide. According to the Fédération Internationale de Football Association (FIFA) Big Count survey, there are 265 million active soccer Nutrients 2019,11, 440; doi:10.3390/nu11020440 www.mdpi.com/journal/nutrients Nutrients 2019,11, 440 2 of 15 players and the number is progressively increasing, especially in women’s football [ 1 ]. In addition, soccer attracts millions of television spectators while the socio-economic impact of elite soccer affects almost every culture worldwide [ 2 ]. Thus, the study of soccer and the variables that affect performance in this complex team sport can have a great impact on sport sciences. Briefly, modern soccer is characterized by the continuous combination of short sprints, rapid accelerations/decelerations and changes of direction interspersed with jumping, kicking, tackling and informal times for recovery [ 3 ]. In addition to these physical fitness variables, players’ techniques and cognitive capacity, team tactics, and psychological factors might also have an impact on overall soccer performance [ 4 , 5 ]. Unlike other team sports, such as basketball or handball, soccer is a low-scoring game and, thus, the margins of victory are close/reduced, particularly at the elite level. In consequence, the study of the effects of ergogenic aids on performance have become an important subject for players, coaches and sport scientists associated with soccer because it has the potential to increase success in the game [6]. Caffeine (1, 3, 7-trimethylxanthine) is one of the most popular supplements among athletes for its potent stimulant effects and due to its easy availability in the market in different commercial forms (energy drinks, caffeinated beverages, pills, pre-workout and thermogenic supplements, etc.). In addition, the ergogenic effects of the acute ingestion of caffeine have been widely reported on different forms of exercise, although most of the classic studies focused on endurance performance [ 7 , 8 ]. In the last few years, several investigations have found that caffeine can also increase anaerobic and sprint performance, although the direct application of these research outcomes to the complexity of soccer is complicated [ 9 – 11 ]. According to the Australian Institute of Sport (AIS), the potential ergogenicity of caffeine reflects level 1 evidence, which allocates it as a safe supplement to use in sport [ 12 ]. In addition, the International Olympic Committee indicates, in its recent consensus statement for dietary supplements, that caffeine intake results in performance gains when ingested before exercise in doses ranging from 3 to 6 mg/kg. Finally, two recent systematic reviews have concluded that caffeine might be ergogenic in team sport athletes [ 6 , 13 ]. With this background, one might suppose that caffeine is also ergogenic in soccer although the information regarding this sport has not been summarized. In the last few years, several studies have investigated the effects of caffeine intake on soccer physical performance [ 14 – 21 ] and in the opinion of the authors, the results of these investigations need to be objectively reviewed and summarized. Therefore, the objective of this systematic review was to critically evaluate the effectiveness of a moderate dose of caffeine on soccer physical performance, muscle damage and perception of fatigue in order to provide more objective and comprehensive information about the positive and negative impact of caffeine on soccer players. 2. Methods 2.1. Search Strategies The present article is a systematic review focusing on the impact of caffeine intake on soccer physical performance and it was conducted following the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines and the PICOS model for the definition of the inclusion criteria: P (Population): “soccer players”, I (Intervention): “impact of caffeine on soccer physical performance, muscle damage and perception of fatigue”, C (Comparators): “same conditions with placebo”, O (Outcome): “soccer-specific abilities, serum markers of muscle damage and perceived fatigue (RPE) and heart rate”, and S (study design): “double-blind and randomized cross-over design” [22]. A structured search was carried out in the Medline (PubMed) database and in the Web of Science (WOS) which includes other databases such as BCI, BIOSIS, CCC, DIIDW, INSPEC, KJD, MEDLINE, RSCI, SCIELO, both high quality databases which guarantee good bibliographic support. The search covered from July 2006, when Hespel et al., [ 23 ] suggested the use of caffeine as an effective supplement for soccer athletic performance, to November 2018. Search terms included a mix of Medical Subject Headings (MeSH) and free-text words for key concepts related to caffeine and soccer Nutrients 2019,11, 440 3 of 15 physical performance, muscle damage or perceived fatigue as follows: (“football”(All Fields) OR “soccer”(All Fields)) AND (“caffeine”(All Fields) OR “energy drink”(All Fields)) AND ((“physical performance”(All Fields) OR performance(All Fields))) OR ((“muscles”(MeSH Terms) OR “muscles” (All Fields) OR “muscle” (All Fields))) OR damage(All Fields) OR (RPE(All Fields) OR “perceived fatigue “(All Fields)). Through this search, relevant articles in the field were obtained applying the snowball strategy. All titles and abstracts from the search were cross-referenced to identify duplicates and any potential missing studies. Titles and abstracts were then screened for a subsequent full-text review. The search for published studies was independently performed by two authors (JMA and JCG) and disagreements about physical parameters were resolved through discussion. 2.2. Inclusion and Exclusion Criteria For the articles obtained in the search, the following inclusion criteria were applied to select studies: articles (1) depicting a well-designed experiment that included the ingestion of an acute dose of caffeine—or a caffeine-containing product—before and/or during exercise in humans; (2) with an identical experimental situation related to the ingestion of a placebo performed on a different day; (3) testing the effects of caffeine on soccer-specific tests and/or real or simulated matches; (4) with a double-blind, and randomized cross-over design; (5) with clear information regarding the administration of caffeine (relative dose of caffeine per kg of body mass and/or absolute dose of caffeine with information about body mass; timing of caffeine intake before the onset of performance measurements, etc.); (6) where caffeine was administered in the form of a beverage, coffee gum or pills; (7) on soccer players with previous training backgrounds in soccer; (8) the languages were restricted to English, German, French, Italian, Spanish and Portuguese. The following exclusion criteria were applied to the experimental protocols of the investigation: (1) the use of caffeine doses below 1 mg/kg or above 9 mg/kg; (2) the absence of a true placebo condition; (3) the absence of pre-experimental standardizations such as elimination of dietary sources of caffeine 24 h before testing; (4) carried out in participants with a previous condition or injury. There were no filters applied to the soccer players’ level, sex or age to increase the power of the analysis. Moreover, the Physiotherapy Evidence Database scale (PEDro), the key factors of which assess eligibility criteria, random allocation, baseline values, success of the blinding procedures, power of the key outcomes, correct statistical analysis and measurement of participants’ distribution of studies, was used to evaluate whether the selected randomized controlled trials were scientifically sound: 9–10 = excellent, 6–8 = good, 4–5 = fair, and <4 = poor) [24]. Papers with a poor PEDro score were excluded (i.e., <4 points). Once the inclusion/exclusion criteria were applied to each study, data on study source (including authors and year of publication), study design, caffeine administration (dose and timing), sample size, characteristics of the participants (level and sex), and final outcomes of the interventions were extracted independently by two authors (JMA and JCG) using a spreadsheet (Microsoft Inc, Seattle, WA, USA). Subsequently, disagreements were resolved through discussion until a consensus was reached. Experiments were clustered by the type of test used to assess the effects of caffeine on soccer physical performance and groups of experiments were created on the effects of caffeine physical performance, muscle damage and perception of fatigue because of its importance to overall soccer performance [4]. 3. Results 3.1. Main Search The literature search provided a total of 135 articles related to the selected descriptors, but only 17 articles met all the inclusion/exclusion criteria (see Figure 1). The number of articles and their exclusion criteria were: 32 papers were removed because they were duplicated; 4 papers were removed because they were performed on a non-human population; another 4 papers were removed because they were narrative or systematic reviews; 13 studies were not carried out during the range of dates Nutrients 2019,11, 440 4 of 15 included in the inclusion criteria. From the remaining 40 articles, another 23 papers were removed because they were unrelated to the effects of caffeine on soccer physical performance. The topics and number of studies that were excluded were: 1 because of lack of information on body mass, 1 because the caffeine content was found in nutritional supplements with other drugs, 1 because it was a suggestion for future research, 1 because the sport investigated was not specified, 4 because they dealt with recovery or sleep processes, 4 because they investigated other team sports (1 rugby, 1 volleyball, 1 tennis, 1 Gaelic football), and the remaining 11 articles because they studied other subjects unrelated to the focus of this systematic review. Thus, the current systematic review includes 17 studies. Nutrients 2019, 11, Firstpage-Lastpage FOR PEER REVIEW 4 of 15 of dates included in the inclusion criteria. From the remaining 40 articles, another 23 papers were removed because they were unrelated to the effects of caffeine on soccer physical performance. The topics and number of studies that were excluded were: 1 because of lack of information on body mass, 1 because the caffeine content was found in nutritional supplements with other drugs, 1 because it was a suggestion for future research, 1 because the sport investigated was not specified, 4 because they dealt with recovery or sleep processes, 4 because they investigated other team sports (1 rugby, 1 volleyball, 1 tennis, 1 Gaelic football), and the remaining 11 articles because they studied other subjects unrelated to the focus of this systematic review. Thus, the current systematic review includes 17 studies. Figure 1. Selection of studies. 3.2. Caffeine Supplementation The participants’ samples included players of both genders (241 males and 33 females), who competed in professional or elite (n = 108), semi-professional (n = 19) and amateur teams (n = 147). In addition, 70 players were adolescents. Out of the 17 investigations, only 2 studies included female soccer players. In 12 out of 17 studies, caffeine was administered based on the soccer player’s body mass, while an absolute dose was provided for all participants in 5 studies. In 2 studies the caffeine dose employed was less than 3 mg/kg, 3 studies used a caffeine dose of around 3 mg/kg, in 2 studies it was 4.5 mg/kg, in 3 studies it was around 5 mg/kg, in 4 studies the dose was 6 mg/kg and 2 studies included a dose above 6 mg/kg (i.e., 7.2 mg/kg). In 1 study, soccer players took different doses (1, 2 Records identified through database searching (n = 135) Screening Included Eligibility Identification Additional records identified through other sources (n = 0) Records after duplicates removed (n = 61) Records screened (n = 61) Records excluded (n = 21) Full-text articles assessed for eligibility (n = 40) Full-text articles excluded, with reasons (n = 23) Unsuitable Outcomes = 1 Unsuitable methodology = 3 Other sports = 4 Subjects unrelated =15 Studies included in qualitative synthesis (n = 17) Figure 1. Selection of studies. 3.2. Caffeine Supplementation The participants’ samples included players of both genders (241 males and 33 females), who competed in professional or elite (n= 108), semi-professional (n= 19) and amateur teams (n= 147). In addition, 70 players were adolescents. Out of the 17 investigations, only 2 studies included female soccer players. In 12 out of 17 studies, caffeine was administered based on the soccer player’s body mass, while an absolute dose was provided for all participants in 5 studies. In 2 studies the caffeine dose employed was less than 3 mg/kg, 3 studies used a caffeine dose of around 3 mg/kg, in 2 studies it was 4.5 mg/kg, in 3 studies it was around 5 mg/kg, in 4 studies the dose was 6 mg/kg and 2 studies included a dose above 6 mg/kg (i.e., 7.2 mg/kg). In 1 study, soccer players took different doses (1, 2 and 3 mg/kg). Regarding the form of administration, 9 investigations used capsules filled with caffeine, 3 investigations used caffeinated energy drinks, 3 investigations used a caffeinated sport Nutrients 2019,11, 440 5 of 15 drink, 1 investigation employed a 20% carbohydrate solution and 1 investigation employed caffeinated chewing gum. Most investigations administered caffeine 30–60 min prior to testing, with the exception of the studies conducted by Andrade-Souza et al. (2015) where the consumption of caffeine was carried out 3 h after a practice session, 4 h after its effects were evaluated in a simulated match [ 25 ]. Also, Guttierres et al. (2013) used a protocol that included the ingestion of caffeine 1 h before the test and every 15 min during the protocol [ 26 ]. Finally, Ranchordas et al. (2018) employed caffeine 5 min before the tests because they used caffeinated gums [ 17 ]. In summary, different studies examined the effect of caffeine on soccer physical performance by using a variety of times of ingestion prior to the testing (5 min–60 min). 3.3. Outcome Measures Tables 1–3include information about author/s and year of publication; the sample investigated, with details of sport level, sex and the number of participants; the study design cites the control group if the study included one; the supplementation protocol that specifies the type of caffeine used, the dose and the time that it was administered; the parameters analyzed or main effects either on sport performance (n= 12; Table 1) and muscle damage (n= 5; Table 2) and finally results or main conclusions. Additionally, some studies also presented data on the effects of caffeine on perceived exertion and heart rate (n= 6; Table 3). Nutrients 2019,11, 440 6 of 15 Table 1. Summary of studies included in the systematic review that investigated the effect of caffeine ingestion as compared to a placebo on soccer-specific abilities. Author/s Population Intervention Outcomes Analyzed Main Conclusion Ellis M. et al. 2018 [21] 15 male elite youth players (16 ±1 years) 1, 2 or 3 mg/kg of caffeine capsules 60 min before the start 20-m sprint Arrowhead agility CMJ Yo-Yo IR1 ↑20-m sprint ↑Arrowhead agility ↑CMJ ↑Yo-Yo IR1 Apostolidis A. et al. 2018 [19] 20 well-trained male players High (n= 11) and low (n= 9) responders (21.5 ±4 years) 6 mg/kg of caffeine capsules 60 min before the start CMJ Reaction time Time to fatigue ↑CMJ †Reaction time ↑Time to fatigue Guerra MA Jr. et al. 2018 [18] 12 male professional players (23 ±5 years) 5 mg/kg of caffeine + 20% carbohydrate solution 60 min before the start CMJ at 1, 3 and 5 min after the conditioning stimulus ↑CMJ Ranchordas et al., 2018 [17] 10 male university-standard players (19 ±1 years) 200 mg (≈2.7 g/kg) of caffeinated gum 5 min before the start 20-m sprint CMJ Yo-Yo IR1 †20-m sprint ↑CMJ ↑Yo-Yo IR1 Andrade Souza, V. et al. 2015 [25] 11 male amateur players (25.4 ±2.3 years) 6 mg/kg of caffeine capsules 3 h after the LIST 30-m Repeated-Sprint test CMJ LSPT †30-m Repeated-sprint test †CMJ †LSPT Jordan, J.et al. 2014 [16] 17 male elite young players (14.1 ±0.5 years) 6 mg/kg of caffeine capsules 60 min before the start Sprint time Reaction time †Sprint time ↑Reaction time on non-dominant leg Lara, B. et al. 2014 [27] 18 female semi-professional players (21 ±2 years) 3 mg/kg of caffeinated energy drinks 60 min before the start Height and power of jump Average speed of running Total distance covered Number of sprints ↑Height and power of jump ↑Average speed of running ↑Total distance covered ↑Number of sprints Astorino, T. et al. 2012 [20] 15 female collegiate players (19.5 ±1.1 years) 255 mL (≈1.3 mg/kg) of caffeinated energy drinks (Redbull) 60 min before the start Sprint time †Sprint time Del Coso, J. et al. 2012 [16] 19 male semi-professional players (21 ±2 years) 3 mg/kg of caffeine in energy drink 60 min before the start Maximum height jump Maximum running speed Distance covered Caffeine concentration in urine ↑Maximum height jump ↑Maximum running speed ↑Distance covered ↑Caffeine concentrations in urine Gant, N. et al. 2010 [28] 15 male first team level players (21.3 ±3 years) 160 mg/L (≈3.7 mg/kg) of caffeinated sport drinks 60 min before the start and every 15 min during the test Sprint times Jump power Test of passes Blood lactate Post-exercise caffeine in urine ↑Sprint times ↑Jump power †Test of passes †Blood lactate ↑Post-exercise caffeine in urine Foskett et al. 2009 [15] 12 male professional players (23.8 ±4.5 years) 6 mg/kg of caffeine capsules 60 min before the start LSPT CMJ ↑LSPT ↑CMJ Guttierres, A. P. et al. 2009 [29] 18 male junior players (16.1 ±0.7 years) 250 mg/L (≈7.2 mg/kg) of caffeinated sport drinks 20 min before and every 15 min during the test Jump height Illinois agility test ↑Jump height †Illinois agility test ↑ : statistically significant increase; † change with no statistical significance; ↓ : statistically significant decrease. CMJ: countermovement jump; LIST: Loughborough Intermittent Shuttle Test; LSPT: Loughborough Soccer Passing Test; Yo-Yo IR1: Yo-Yo intermittent recovery test level-1 Nutrients 2019,11, 440 7 of 15 Table 2. Summary of studies included in the systematic review that investigated the effect of caffeine ingestion as compared to a placebo on serum markers of muscle damage. Author/s Population Intervention Outcomes Analyzed Main Conclusion Guttierres, A. P. et al. 2013 [26] 20 male young players (16.1 ±0.7 years) 7.2 mg/kg of caffeinated sport drinks 20 min before and every 15 min during the test Blood glucose Blood lactate Plasma caffeine Free fatty acids Urine caffeine ↑Blood glucose ↑Blood lactate ↑Plasma caffeine †Free fatty acids †Urine caffeine Machado, M. et al. 2010 [30] 15 male players (18.4 ±0.8 years) 4.5mg/kg of caffeine capsules Immediately before the test CK LDH ALT AST basophils, eosinophils, neutrophils, monocyte lymphocytes †CK †LDH †ALT †AST †basophils, eosinophils, neutrophils, monocyte lymphocytes Machado, M. et al. 2009 [31] 20 male players (18.8 ±1 years) 4.5 mg/kg of caffeine capsules Immediately before the test Basic hemogram CK LDH ALT AST AP γ-GT †Basic hemogram †CK †LDH †ALT †AST †AP †γ-GT Machado, M. et al. 2009 [32] 15 male professional players (19 ±1 years) 5.5 mg/kg of caffeine capsules Immediately before the test CK LDH ALT AST †CK †LDH †ALT †AST Bassini-Cameron, A. et al. 2007 [33] 22 male professional players (26.0 ±1.6 years) 5 mg/kg of caffeine capsules 60 min before the start CK LDH ALT AST ↑CK †LDH ↑ALT †AST ↑ : statistically significant increase; † change with no statistical significance; ↓ : statistically significant decrease. CK: creatine kinase; LDH: lactate dehydrogenase; ALT: alanine aminotransferase; AST: aspartate aminotransferase; AP: alkaline phosphorylase; γ-GT: γ-glutamyl transferase. Nutrients 2019,11, 440 8 of 15 Table 3. Summary of studies included in the systematic review that investigated the effect of caffeine ingestion as compared to a placebo on perceived fatigue and heart rate. Author/s Population Intervention Outcomes Analyzed Main Conclusion Andrade Souza, V. et al. 2015 [25] 11 male amateur players (25.4 ±2.3 years) 6 mg/kg of caffeine capsules 3 h after the LIST Perceived effort †Perceived effort Jordan, J.et al. 2014 [16] 17 male elite young players (14.1 ±0.5 years) 6 mg/kg of caffeine capsules 60 min before the start Heart rate †Heart rate Lara, B. et al. 2014 [27] 18 female semi-professional players (21 ±2 years) 3 mg/kg of caffeinated energy drinks 60 min before the start Heart rate †Heart rate Guttierres, A. P. et al. 2013 [26] 20 male young players (16.1 ±0.7 years) 7.2 mg/kg of caffeinated sport drinks 20 min before and every 15 min during the test Perceived effort †Perceived effort Astorino, T. et al. 2012 [20] 15 female collegiate players (19.5 ±1.1 years) 255 mL (≈1.3 mg/kg) of caffeinated energy drinks (Redbull) 60 min before the start Perceived effort Heart rate †Perceived effort †Heart rate Foskett et al. 2009 [15] 12 male professional players (23.8 ±4.5 years) 6 mg/kg of caffeine capsules 60 min before the start Heart rate †Heart rate ↑: statistically significant increase; †change with no statistical significance; ↓: statistically significant decrease. Nutrients 2019,11, 440 9 of 15 4. Discussion The purpose of this systematic review was to summarize all scientific evidence for the effect of acute caffeine ingestion on variables related to soccer physical performance. Due to the differences of the effects studied among the investigations included in the analysis, the following variables have been clustered for a more comprehensive scrutiny. 4.1. Impact on Sports Performance A total of 12 investigations carried out research protocols that studied the effects of caffeine on one or more variables related to soccer-specific abilities. Overall, these investigations showed an improvement in soccer-related skills with the pre-exercise ingestion of caffeine (Table 1). Specifically, Foskett et al., [ 15 ], with 12 first division football players (age: 23.8 ± 4.5 years), observed that the consumption of 6 mg/kg of caffeine before exercise increased passing accuracy and accrued significantly less penalty time during two validated tests to assess soccer skill performance (intermittent shuttle-running protocol and Loughborough Soccer Passing Test; LSPT). In addition, this investigation also found that caffeine improved the functional power of the leg measured by a vertical jump. In the study conducted by Jordan et al., 17 soccer players from the elite youth category (age: 14.1 ± 0.5 years) performed an agility test (reactive agility test) validated for football [ 34 ]. These authors indicated, based on the results of their investigation, the intake of 6 mg/kg of caffeine 60 min before the test significantly improved the reaction time of the players in their non-dominant leg [ 16 ]. In another study conducted with 15 elite young players (age: 16 ± 1 years) that were administered low doses of caffeine (1, 2 and 3 mg/kg), Ellis et al., [ 21 ] observed that improvements in physical performance depended on the dose and the type of task. Specifically, they concluded that 3 mg/kg of caffeine seems to be the optimal dose to obtain positive effects on soccer-specific tests (20 m sprint, arrowhead agility and CMJ). However, the authors also suggested that even higher doses of caffeine might be required to improve endurance performance, as measured by the Yo-Yo intermittent recovery test level 1 (Yo-Yo IR1). In this line, Apostolidis et al., [ 19 ] showed that 6 mg/kg of caffeine ingested 60 min previous to a battery of tests improved aerobic endurance (time to fatigue) and neuromuscular performance (CMJ) in 20 well-trained soccer players (age: 21.5 ± 4 years). Since these authors did not find any change in substrate oxidation with caffeine, measured by indirect calorimetry during the testing, they commented that performance improvements could only be attributed to positive effects on the central nervous system and/or neuromuscular function, although the precise mechanism of caffeine ergogenicity was not indicated in this investigation. Finally, Guerra et al., [ 18 ] investigated the addition of caffeine (5 mg/kg) to a post-activation potentiation protocol that included plyometrics and sled towing. These authors found that, in a group of 12 male professional soccer players ( age: 23 ±5 years ), caffeine augmented the effects of the post-activation potentiation, as measured by CMJ. These investigations, taken together, suggest that caffeine might be effective to improve performance in players’ abilities and soccer-specific skills (jumps, sprint, agility, aerobic endurance, accuracy of passes and ball control). Caffeinated energy drinks are considered as one of the most common ways to provide caffeine before exercise [ 14 ], and the effect of this type of beverages have been also investigated in soccer players. Del Coso et al., [ 16 ] chose 19 semi-professional players (age: 21 ± 2 years) in order to determine if the caffeine, provided via a commercially-available energy drink (3 mg/kg), improved performance during several soccer-specific tests (single and repeated jump tests and repeated sprint ability test) and during a simulated soccer match. For this investigation, players ingested either an energy drink without sugar but with caffeine (i.e., sugar-free Redbull), or a sugar-free soda (Pepsi diet without caffeine) 60 min prior to testing. The results showed that the consumption of the caffeinated energy drink increased the ability to jump, to repeat sprints, and it affected positively total running distance and the running distance at >13 km/h covered during the simulated game. In another similar study carried out with 18 semi-professional women soccer players (age: 21 ± 2 years), Lara et al., [ 34 ] demonstrated that the consumption of an energy drink containing 3 mg/kg of caffeine improved jump height, the ability to perform sprints, the total running distance and the distance covered at high