Introduction
One of the most widely consumed ergogenic aids is caffeine (1,3,7-trimethylxanthine). It has been used since the Stone Age (30), and its capacity to enhance muscular work was first identified over 100 years ago (61). It is found naturally in the leaves, fruits, or seeds of various plants and is a constituent of various foods, drinks, and medicinal products. It has been reported (2) that 90% of adults in the United States consume caffeine daily, in the form of coffee, tea, soda, and energy drinks. Caffeine is completely absorbed within the stomach and small intestine 45 minutes after ingestion, and its half-life in the body is approximately 3-4 hours (34). Caffeine stimulates the central nervous system (CNS), resulting in increased alertness and focus. Metabolites of caffeine have been shown to cause vasodilation and increase in urine volume (theobromine), smooth muscle relaxation (theophylline), and stimulation of lipolysis (paraxanthine). Caffeine-mediated increases in oxygen uptake, catecholamine release, and metabolic rate have also been reported (29,34). Ergogenic doses of caffeine range from 2 to 9 mg·kg−1 body weight, with higher doses typically eliciting side effects including anxiety, restlessness, and headaches that may impair performance.
In 140 endurance athletes competing in the 2005 Ironman Triathlon, Desbrow and Leveritt (26) reported that most athletes believe that caffeine improves endurance performance (73%) and concentration (84%). Previously, 2 world-class athletes were stripped of international medals after a positive caffeine test (4), although the status of caffeine as a banned stimulant was lifted by the World Anti-Doping Agency in 2004 due to its widespread use and availability.
There is a large body of literature (12,17,22,23,27,34,44,51) supporting the ingestion of caffeine to augment endurance exercise; yet only a few studies (5,6,8,19,45,46,49,72,76) have examined caffeine's ergogenic properties during short-term exercise which performance is not limited by carbohydrate availability, hyperthermia, or dehydration, but from alternative central and peripheral factors. Data from these studies are equivocal, as there are reports that caffeine improves (3,8,19,67,76), diminishes (38,61), or has no effect (5,7,10,38) on short-term high-intensity activity. Multiple review articles (12,27,34,56,62,68) concerning caffeine and exercise have been published, yet minimal attention was given toward its potential to alter exercise dependent on nonoxidative metabolism, such as resistance training, team sports, and sprint or power-based exercise. The aim of this systematic review is to explore the effect of acute caffeine intake on short-term high-intensity exercise performance, describe proposed mechanisms of caffeine's actions, and identify discrepancies in results across studies. Caffeine-induced changes in performance in athletic populations completing voluntary exercise will be emphasized. Athletes in one study (26) perceived that caffeine had no effect on strength or power during prolonged exercise, and a recent review (66) concluded that the effects of acute caffeine ingestion were equivocal for a high-intensity exercise lasting less than 3 minutes. This review will critically examine the efficacy of caffeine ingestion for short-term high-intensity exercise performance, with the goal of providing clear recommendations for the coach and athlete to follow as they consider caffeine as a means to augment athletic performance.
Methods
A literature search was performed on PubMed and Medline with the key words “caffeine” and “exercise,” “caffeine” and “strength,” “caffeine” and “power,” and “caffeine” and “resistance training.” Reference lists of original research articles and reviews were examined to identify additional studies concerning caffeine and short-term exercise. Studies were also used from the primary author's collection of articles.
Experimental Approach to the Problem
Inclusion Criteria
Inclusion criteria included studies in which changes in short-term high-intensity performance (duration of exercise ≤5 minutes) were examined with intake of caffeine, whereas studies were excluded if any mode of endurance exercise was the dependent variable. The majority of studies used a randomized, double-blind, placebo-controlled design, with caffeine withdrawal implemented at least 24 hours before exercise. Some participants were caffeine consumers, whereas subjects in some studies were naive to caffeine. The mode of caffeine ingestion included anhydrous caffeine alone, caffeine combined with supplements, and as an ingredient in energy drinks. Energy drinks were included in this systematic review as caffeine is typically the primary ingredient in these beverages and is likely responsible for much of their purported ergogenic effects. Furthermore, it is likely that this source of caffeine is more readily available to athletes and other exercisers than anhydrous caffeine. All studies were performed in humans with the majority of subjects being men.
Physiotherapy Evidence Database Scale
This critical review was performed using the Physiotherapy Evidence Database (PEDro) scale. This scale was developed by the Centre for Evidence-Based Physiotherapy (58) and is designed to assess internal validity. Each study was rated on several previously established criteria to yield a maximum score of 10.
Each study was independently rated by 2 reviewers (r = 0.90), and the PEDro score revealed for each article is an average of these scores. The kappa score representing level of agreement between reviewers was equal to 0.96. Studies with PEDro scores below 6 were deemed unacceptable and not used in the analyses.
Results
A review of the literature yielded 27 studies and 2 abstracts that met our inclusion criteria. One study (10) had a PEDro score equal to 5, so was removed from the analysis. Thus, 28 studies were examined in the systematic review. Of these studies, a total of 30 caffeine treatments was employed, as some studies administered more than 1 dose of caffeine (Tables 1 and 2).
Table 1: Summary of studies examining effect of caffeine on power output and sprint- and/or power-based performance.*
Table 2: Summary of studies examining effect of caffeine on resistance training performance.*
The mean PEDro score was 7.76 ± 0.87; only one study (8) received a perfect score equal to 10. The majority of studies failed to satisfy the “baseline similarity” and “concealed allocation” criteria. Baseline similarity was met in one study (8) in which subjects were separated into distinct groups, each of which subsequently received either the placebo or caffeine treatment. This is rarely performed in most studies due to relatively small sample sizes and a greater reliance on convenience sampling. In one study (48), significance (p) values were not reported for the statistical analyses, and in another study (24), data were obtained from less than 85% of subjects who were initially recruited.
Changes in team sports or power/sprint performance were examined in 17 studies (Table 1). Of these, 65% (11 of 17) revealed significant improvements in performance ranging from 1.0 to 20.0%. Mean improvement in these studies was equal to 6.5 ± 5.5%. With the exception of one study (45) in which duration at a workload equal to 100% o2max was increased by 20%, performance enhancements were typically less than 10.0%. In 9 of 11 studies, caffeine was administered in powder or capsule form, whereas in 2 studies in which improved performance was revealed, caffeine was ingested as a gum (9) or as an ingredient in an energy drink (31). Ergogenic doses as small as 100 mg were demonstrated in college athletes (9), although the most widely administered dose (6 mg·kg−1) was predominantly shown to be ergogenic for performance of team sports skills. No caffeine-mediated improvement in performance of laboratory-based tests, such as the Wingate test, was revealed in multiple studies (18,37,38), whereas studies in which exercise bouts completed by athletes simulating the demands of sport, such as swimming (19), sprint cycling (72), and repeated sprinting (65,67), tended to reveal enhanced performance with caffeine ingestion.
Table 2 demonstrates findings from 11 studies in which change in resistance training performance was examined after caffeine supplementation. Six studies (54%) revealed significant caffeine-mediated improvements in performance in the form of enhanced torque (49), number of repetitions performed (42,43,75), or weight lifted (8,76). Caffeine was more likely to increase the number of repetitions completed during resistance training (42,43,75,76) rather than muscular strength as measured by 1 repetition maximum (1RM) (8). Mean improvement across studies was equal to 9.4 ± 5.7%. In 2 (8,42) of 6 studies showing significant effects on performance, low doses of caffeine (110 mg and 2.5 mg·kg−1) were ingested in the form of an energy drink administered pre exercise. In contrast, relatively high doses of pure caffeine (>4 mg·kg−1 or 300 mg) were shown to be ergogenic primarily in studies in which athletes completed testing (48,76).
Discussion
The primary aim of this review was to critically analyze the existing literature to elucidate the potential of caffeine to enhance short-term high-intensity exercise performance. It is well known that acute caffeine ingestion improves endurance performance, yet existing data regarding caffeine's effect on exercise dependent on nonoxidative metabolism are equivocal. The PEDro scale (58) was used to systematically analyze the validity of existing studies. Results revealed that 54-65% of studies demonstrate improved performance as a result of caffeine intake. The mean improvement in performance ranged from 6.5 to 9.4%, although the variability across studies was sizable. This suggests that only some individuals may experience improved performance with acute caffeine intake.
Caffeine ingested in capsule or powder form and as a constituent in energy drinks or supplements was shown to be ergogenic (Tables 1 and 2) for various high-intensity exercise protocols such as resistance training, sprinting, or activities simulating team sports. The lowest ergogenic caffeine doses for short-term high-intensity exercise were ingested as gum (9) (100 mg) or as part of a supplement (8). Nevertheless, the supplement ingested in the study of Beck et al. (8) did not augment 1RM strength in untrained men (7). Unpublished data (13) from a double-blind crossover study in strength-trained men reveal no change in knee extension/flexion torque, total work, or power output when a 2 mg·kg−1 dose of caffeine was ingested 70 minutes before “all-out” isokinetic dynamometry vs. a higher dose (5 mg·kg−1) or placebo. These data corroborate a previous study (48) in untrained men showing no effect of caffeine (300 and 600 mg) on peak torque. Intake of pure caffeine in doses from 3 to 6 mg·kg−1 has been shown to improve performance in team sports (15,65,67), resistance training (43,48,76), and sprint/power-based activity (3,18,60). Overall, lower doses typically ingested via commercially available energy drinks or supplements seem to be as effective as higher doses and may minimize onset of negative symptoms experienced with doses greater than 6 mg·kg−1 that are deleterious to training or athletic performance.
During endurance exercise, Graham et al. (36) revealed that caffeine in capsule form improved performance, whereas coffee or decaffeinated coffee plus caffeine did not. These findings are supported by another study (53). Yet pure caffeine is not widely available to coaches or athletes. In contrast, caffeine-containing energy drinks and supplements are quite available to the consumer. Improved anaerobic performance (1) and muscular endurance (31) after Red Bull ingestion were demonstrated in randomized placebo-controlled studies in recreationally active subjects. There are limited data examining the effectiveness of various products containing caffeine, specifically to enhance short-term exercise performance, so further investigation is merited, especially in competitive athletes.
One potential explanation for the equivocal data regarding caffeine's ability to alter high-intensity exercise may be differences in subjects' training status. Of the studies revealing a significant improvement in short-term high-intensity exercise performance, many (9,19,42,49,72,76) included trained athletes (competitive cyclists, football players, elite athletes, and competitive swimmers) and not untrained, recreationally active, or strength-trained subjects who are typically college students. It is likely that athletes have greater motivation to perform fatiguing exercise and can provide more consistent performance day-to-day (12), which may reduce variability and thus increase statistical power. In many studies (32,49,76), subjects were low-caffeine consumers (<100 mg per day), which may potentiate the ergogenic effect of the drug compared with subjects tolerant to the effects of caffeine (50). However, further study is needed to elucidate this possibility.
An additional explanation is that like creatine, there may be individual “responders” and “nonresponders” to the effects of caffeine during short-term exercise. This may be related to caffeine habituation, although there is evidence (32,34) that caffeine tolerance does not affect subsequent endurance performance. To our knowledge, this has not been tested during short-term high-intensity exercise. In 1 study in 22 strength-trained men (5), 12 men lifted more weight on the 1RM bench press and 11 on the leg press, respectively, with caffeine (6 mg·kg−1), yet 5 and 8 men lifted more weight in the placebo trial. Similar individual variability in performance in response to acute caffeine intake was also observed in men completing resistance training (43). Unpublished data (70) in 14 strength-trained men revealed that 6 of 9 men labeled as “responders” expressed chronic caffeine intakes greater than 225 mg per day, although there was no relationship between caffeine concentration and magnitude of performance improvements. To further explore this discrepancy, scientists should regularly assess caffeine concentration in studies in which changes in short-term exercise performance are examined. However, this measurement is not frequently obtained in the majority of research investigating ergogenic effects of caffeine for short-term high-intensity exercise performance.
Recent data suggest that variations in genotype may alter caffeine metabolism and potentially magnitude of performance in response to caffeine ingestion. Caffeine is metabolized in the liver by cytochrome P450 1A2 (14), which shows marked variability between individuals (40). A single substitution in the gene causes some persons to be slow caffeine metabolizers, whereas those who are homozygous for the allele metabolize caffeine more rapidly (66). A recent report (21) demonstrated that habitual caffeine consumption is related to these genotypes, which may explain the discrepancy in individual responses to caffeine's physiological effects. However, no study has simultaneously examined the effect of differences in genotype on high-intensity exercise performance after caffeine intake.
Purported mechanisms of caffeine's ergogenic effects during high-intensity exercise are demonstrated in Figure 1. Early data (22,44) revealed that caffeine increased lipolysis and spared muscle glycogen during endurance exercise. In contrast, other studies (35,45) revealed that caffeine does not spare glycogen. High-intensity short-term exercise is not limited by carbohydrate availability, so other mechanisms must explain the ergogenic effect of caffeine.
Figure 1: Proposed mechanisms for caffeine's ergogenic effects during short-term high-intensity exercise. Solid lines represent that existing research data tend to support this mechanism; gray, dashed lines represent that existing research data are equivocal or do not support this mechanism.
Although caffeine intake has been shown to increase motor unit recruitment (71), many studies show no effect of acute caffeine intake on neuromuscular activation (electromyographic activity) during the Wingate test (39), submaximal isometric exercise (59), or maximal handgrip exercise (73). Increased intracellular calcium concentration (28) and/or altered excitation-contraction coupling (16) have been proposed as potential mechanisms; however, these effects occur only at supraphysiological caffeine doses that are impractical for humans. Moreover, it is unlikely that altered calcium release causes fatigue during short-term high-intensity exercise (54). In isolated frog myofibrils, physiological doses of caffeine did not alter peak force, time to fatigue, or calcium release (63). Overall, a single mechanism to explain caffeine's ergogenic effects remains elusive, and it is likely located outside the muscle fiber.
An alternative site for caffeine's ergogenic effects may lie in the brain. Davis et al. (25) reported that caffeine delays fatigue via stimulation of the CNS by acting as an adenosine antagonist. Adenosine, a cellular component that increases with muscular contraction, inhibits neuron excitability and synaptic transmission via binding to its receptors (52), leading to decreased arousal and increased sleep (62). In male rats, run time to fatigue was 60% longer with caffeine vs. an adenosine agonist (25). Adenosine receptors exist primarily in type I fibers (37), which may decrease the likelihood of performance gains in activities dependent on type II fibers, such as heavy resistance training or sprinting. Consequently, it is unknown if enhanced short-term high-intensity exercise performance occurs via adenosine antagonism.
Acute caffeine ingestion also modifies perceptual responses that may alter performance. In a meta-analysis (27) of 21 studies using a placebo-controlled double-blind design, data revealed that caffeine decreased rating of perceived exertion (RPE) by 5.6% during prolonged exercise, which explained approximately 33% of improved performance. However, RPE assessment may not apply to the high-intensity intermittent nature of team sports or activities such as sprinting and/or resistance training in which recording exertion during a brief intense exercise bout is impractical. RPE recorded at the completion of knee extension and biceps curls was similar between caffeine and placebo (46). Unpublished data (70) in strength-trained men show no difference in RPE (11) during completion of 40 repetitions of 1-leg knee extension and flexion at 180°·s−1 after ingestion of a low (2 mg·kg−1) (5.36 ± 1.57) or moderate dose (5 mg·kg−1) (5.46 ± 1.51) of caffeine vs. placebo (5.27 ± 1.79). Overall, caffeine does not seem to alter RPE during or at completion of short-term intense exercise, although it may be due to challenges with recording RPE during exercise. Because of the paucity of data, further investigation is merited.
An analgesic effect of caffeine has also been documented. In college-aged men (55) and women (33) completing submaximal cycle ergometry, 5 and 10 mg·kg−1 doses of caffeine significantly reduced leg pain (assessed with a 0-10 scale) (20) vs. placebo. Using a randomized, placebo-controlled, crossover design, unpublished data (70) (Table 3) reveal no effect (p > 0.05) of a low (2 mg·kg−1) or moderate (5 mg·kg−1) caffeine dose on muscle pain during high-intensity exercise. In this preliminary study, 11 strength-trained men completed 2 bouts of 40 repetitions of all-out one-leg knee extension and flexion on an isokinetic dynamometer; muscle pain was assessed at 15 and 35 repetitions. These data suggest that it is unlikely that caffeine attenuates muscle pain during short-term high-intensity exercise, although further study is merited to examine the analgesic effect of caffeine when lower doses are administered and when exercise simulating competitive sports is used.
Table 3: Alterations in muscle pain sensation during 1-leg knee extension and flexion in response to caffeine.*
Previous data (47) in young men and women showed increased (p < 0.05) reaction time and speed of movement when a 300-mg dose of caffeine was ingested, yet no effect was revealed for a 600-mg dose. This may be important in intense exercise including sprinting, yet may be of little importance during resistance training and other laboratory-based protocols.
An effect of caffeine on psychological function is also plausible, as it alters the CNS by promoting serotonin release, increasing sympathetic activity, and decreasing the activity of inhibitory neurons due to adenosine antagonism (69). This may explain the reduced tiredness and improved mood and alertness shown with acute caffeine ingestion (41). An intriguing hypothesis is that compared with placebo, caffeine may reverse the serious withdrawal effects, such as lethargy, irritability, and headaches, reported with 24- to 48-hour caffeine abstention as commonly required in scientific protocols. Data from a recent study (70) revealed improved resistance training performance in 67% of heavy caffeine users (>225 mg per day) vs. men with lower habitual intakes (∼125 mg per day) who typically performed better in the placebo condition. These men commonly reported that they felt “less tired” and had “more energy” in the caffeine trial, although only 28% of subjects correctly identified the caffeine treatment. Further research using psychological scales quantifying mood before and during bouts of high-intensity exercise is warranted to further explore this potential mechanism.
In summary, the exact mechanism explaining ergogenic effects of caffeine for short-term high-intensity exercise is relatively unknown, especially at physiological caffeine concentrations. It is likely multifactorial with central factors such as adenosine antagonism the most probable mechanism, yet enhanced performance may also be related to alterations in perceived exertion, reaction time, cognition, and/or mood.
There is a paucity of research investigating efficacy of caffeine in sports including sprinting, track and field, football, and hockey. Any benefit of chronic caffeine intake for completion of day-to-day training, and subsequent performance at games or meets, also requires further investigation.
Practical Applications
The focus of this review was to critically examine studies investigating the effect of acute caffeine ingestion on performance in activities including sprinting, all-out cycling, team sports, and resistance training that are dependent on nonoxidative metabolism. Data reveal that:
- Doses ranging from 2.5 to 7 mg·kg−1 seem to improve high-intensity exercise performance. Low doses of caffeine (1.0-2.5 mg·kg−1) contained in gum, energy drinks, or supplements are ergogenic, yet higher doses (6 mg·kg−1) seem to be required when caffeine alone is ingested.
- Team sports performance such as repeated sprinting is enhanced when high doses of caffeine (6 mg·kg−1) are ingested pre exercise.
- Performance during repeated high-intensity efforts may be diminished with caffeine intake.
- Trained athletes may be more apt to experience the ergogenic effects of caffeine for high-intensity exercise such as resistance training, tests of peak power, or swimming compared with less active individuals.
- Benefits of caffeine for strength-based exercise, such as 1RM testing, are minimal. Caffeine predominantly enhances resistance training performance in athletes.
- Preliminary data demonstrate that caffeine does not alter RPE or pain sensation during short-term exercise.
- Differences in magnitude of responses to caffeine may be due to discrepancies in rate of caffeine metabolism between individuals.
Acknowledgments
Gratitude is expressed to many participants who completed various studies in the author's laboratory. Furthermore, this work could not have occurred without the assistance of an outstanding collection of undergraduate students. The authors also thank the reviewers for thorough comments that improved the quality of this review. The results of the present study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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