There is growing interest in the efficacy of acute caffeine intake to alter performance during short-term high-intensity exercise reliant on oxygen-independent metabolism. Recent reviews by Davis and Green (10) and Astorino and Roberson (2) comprehensively summarized current knowledge in this area. These data indicate that caffeine enhances performance in high-intensity activities such as swimming (8), sprint cycling (30), team sports performance (28,29), and exercise to exhaustion (5,12), yet it has little effect on Wingate-derived peak power (17,18), with the exception of one study (31) in elite athletes.
An additional focus of previous work pertains to caffeine's effects on resistance training performance, such as isokinetic exercise and traditional strength training. Yet, effects on these exercise modes are not well understood. Early studies by Jacobson and Edwards (23) and Bond et al. (6) revealed no effect of caffeine intake on knee extension/flexion exercise, yet a subsequent study (24) in football players using a similar isokinetic protocol revealed a significant effect of a 7-mg·kg−1 dose of caffeine on muscular performance. A recent systematic review (2) concluded that approximately 60% of studies revealed a significant effect of caffeine on high-intensity exercise performance.
Most often, in these studies, only one caffeine dose is administered to subjects, typically 5-6 mg·kg−1 body weight. This approximates four cups of brewed coffee for a 70-kg individual, a relatively high dose that is twofold greater than the typical caffeine intake reported by adults in the United States (20). To our knowledge, only one study (23) has investigated the effects of multiple doses of caffeine on short-term exercise performance, and relatively high doses (>4 mg·kg−1) were used. In this study in which recreationally active men and women were separated into three groups (placebo and 300- and 600-mg doses of caffeine), data revealed no effect of caffeine on performance. Beck et al. (3) reported a small but significant improvement (+2.1%) in muscular strength when a caffeine-containing (2.5 mg·kg−1) supplement was ingested before exercise. Yet, it is unknown if a low dose of pure caffeine enhances high-intensity performance similar to studies revealing significant increases in resistance training performance when higher doses are ingested (21,31). Determining the ergogenic potential of a low dose of caffeine is important for the exerciser because it can be easily ingested in many widely available products.
The primary aim of the study was to test the ergogenic effects of a low (2 mg·kg−1) and a relatively high dose of caffeine (5 mg·kg−1) on peak torque and various measures of muscular function during fatiguing knee extension/flexion exercise. It was hypothesized that acute caffeine intake would not alter muscular performance versus placebo.
Fifteen active men participated in the study; their descriptive data are shown in Table 1. They met the following inclusion criteria: 1) participation in at least 4 h·wk−1 of exercise, including recreational sports, aerobic, and/or resistance training for at least 1 yr; 2) nonsmoker; 3) current caffeine intake >100 mg·d−1; 4) absence of knee ailments or existing knee pain; and 5) nonobese defined by a body mass index <30 kg·m−2. Written informed consent was obtained from all subjects, and the study procedures were approved by the university's institutional review board.
Subjects initially arrived at the Human Performance Laboratory in shorts and T-shirt, having refrained from lower-body exercise for 48 h before the visit. Height and weight were measured, and after a 5-min warm-up on a cycle ergometer (Monark 808e; Vansbro, Sweden), the participant was seated in the isokinetic dynamometer (Biodex System 4; Biodex, Shirley, NY) for a practice trial consisting of one bout of 40 repetitions of knee extension/flexion of the dominant leg at a velocity equal to 180°·s−1. Subjects were instructed to exercise maximally throughout the bout. Settings of the isokinetic device, including seat angle, chair location, and lever arm length, were noted and used for subsequent trials within each subject.
Solutions ingested included anhydrous caffeine (Gallipot, St. Paul, MN; 5 and 2 mg·kg−1 body weight) and placebo, which were housed in identical containers containing one package of a commercially available noncaloric lemon-flavored beverage (Crystal Light, Northfield, IL). Treatment order was assigned to subjects using a single-blind, randomized, counterbalanced, crossover design. Subjects were unaware of the order of their treatments, yet one co-investigator prepared all drinks and provided them to subjects before their trials. For example, subjects were provided their solution on the first day of testing during their familiarization trial, and this process ensued during remaining trials. They were provided specific instructions with each drink to mix it with 8 oz of cold water and drink it 1 h before their exercise trial.
Subjects were instructed to refrain from intense exercise and caffeine intake for 48 h before each trial. During three separate days separated by at least 48 h, they ingested one of three beverages 1 h before each trial, which was confirmed by requiring subjects to return the empty container to the investigators during each visit.
After a 5-min warm-up on the cycle ergometer, subjects completed two maximal bouts of 40 repetitions of knee extension/flexion of the dominant leg at a velocity equal to 180°·s−1. Bouts were separated by 3 min of passive recovery, during which the subject remained in the dynamometer, yet the strap placed on the exercising leg was loosened. Subjects were provided strong verbal encouragement during exercise, although they received no feedback regarding their performance during the protocol. Peak and average torque (ft·lb), power (W), total work (ft·lb), and work fatigue (%) were recorded for both knee extension and flexion across both bouts. Pilot testing revealed a coefficient of variation for peak extension torque, peak flexion torque, and extension total work equal to 5.3%, 6.5%, and 7.8%, respectively.
Assessment of RPE.
The Borg 0-10 category ratio scale (7) was initially explained to subjects during their familiarization trial, and instructions were repeated before subsequent trials. During exercise, subjects reported their RPE 25 repetitions into each exercise bout.
Data are reported as mean ± SD and were analyzed using SPSS Version 16.0 (SPSS, Inc., Chicago, IL). A 3 (treatment) × 2 (sets) ANOVA with repeated measures was used to examine differences in muscular performance and RPE between the caffeine and placebo treatments. The Greenhouse-Geisser correction was used to account for the sphericity assumption of unequal variances across groups. Tukey post hoc test was used to locate differences between means when a significant F-ratio was obtained. Statistical significance was accepted at P < 0.05.
The protocol was well tolerated by all subjects. Of 15 men, 7 (47%) correctly identified the caffeine trials because of symptoms such as cessation of headache, feelings of additional energy, and increased anxiety.
Figures 1A and 1B reveal the effects of caffeine on peak knee extension/flexion torque during isokinetic exercise. Knee extension and flexion torque were significantly different (P < 0.05) between bouts 1 and 2. For peak knee extension torque, there was no main effect or interaction (P > 0.05), yet for peak knee flexion torque, there was a significant (F(2,28) = 4.24, P < 0.05) main effect of treatment, with the 5-mg·kg−1 trial revealing higher torque than placebo in bout 1.
Figures 2A and 2B reveal the effects of caffeine on average knee extension/flexion torque during isokinetic exercise. Knee extension and flexion torque were significantly different (P < 0.05) between bouts 1 and 2. There was a significant interaction (F(2,28) = 5.47, P < 0.05) for average knee extension torque, although no main effect (P > 0.05) was revealed for either flexion or extension.
Figures 3A and 3B demonstrate the effects of caffeine on knee extension/flexion total work during isokinetic exercise. Total work declined (P < 0.05) from bout 1 to bout 2. There was a main treatment effect (P < 0.05) for both knee extension (5-mg·kg−1 dose vs placebo and 2 mg·kg−1 in bout 1) and flexion total work (5-mg·kg−1 dose vs placebo in bout 1).
Data revealed no difference in work fatigue across treatments (P > 0.05). In the 5- and 2-mg·kg−1 and placebo treatments, knee extension work fatigue was equal to 64.1% ± 3.9%, 65.2% ± 4.0%, and 64.4% ± 4.3% in bout 1 and increased (P < 0.05) to 67.8% ± 5.6%, 67.0% ± 4.2%, and 66.3% ± 5.5% in bout 2, respectively. For knee flexion, work fatigue was equal to 57.0% ± 4.2%, 60.1% ± 4.0%, and 60.4% ± 5.0% in bout 1 and 57.6% ± 4.6%, 55.1% ± 9.4%, and 56.9% ± 5.7% in bout 2, respectively, with no difference (P > 0.05) observed across bouts.
Figures 4A and 4B reveal the effects of caffeine on knee extension/flexion power during isokinetic exercise. For knee extension power, there was a significant difference (P < 0.05) in power across bouts 1 and 2, as well as a significant main effect (F(2,28) = 4.61, P < 0.05) and interaction (F(2,28) = 5.87, P < 0.05). Knee extension power in the 5-mg·kg−1 trial was significantly higher than that in placebo. For knee flexion power, there was a significant decrement (P < 0.05) in power between bouts 1 and 2. Data revealed a main effect for treatment (F(2,28) = 3.80, P < 0.05), with power higher (P < 0.05) with 5-mg·kg−1 caffeine versus placebo in bout 1.
RPE was significantly increased from bout 1 (5.47 ± 1.50, 5.47 ± 1.50, and 5.27 ± 1.53 in the 5-mg·kg−1, 2-mg·kg−1, and placebo treatments, respectively) to bout 2 (6.47 ± 1.25, 6.47 ± 1.25, and 6.27 ± 1.58 in the 5-mg·kg−1, 2-mg·kg−1, and placebo treatments, respectively; F(1,14) = 32.59, P < 0.05), yet no effect (P > 0.05) of caffeine on RPE was exhibited.
The primary aim of this study was to examine the effect of two doses of caffeine on peak and average torque, power output, fatigability, and total work during two bouts of high-intensity exercise consisting of maximal knee extension/flexion. Existing data for this exercise mode are equivocal because some studies (6,23) show no benefit of caffeine intake, whereas another (24) revealed enhanced performance with caffeine. Our data demonstrate that 5-mg·kg−1 caffeine improved multiple indices of performance versus placebo in the initial bout of exercise, yet the 2-mg·kg−1 dose was not ergogenic in any case. Doses of caffeine equal to approximately 160 mg, equivalent to the low dose administered in the present study, do not alter fatiguing knee extension/flexion exercise in active men who are caffeine consumers.
Data from the present study support previous results (24) showing improved isokinetic performance with caffeine intake, yet there are a few methodological discrepancies between studies. Subjects in the study of Jacobson et al. (24) were collegiate football players who were low caffeine consumers (72 ± 25 mg·d−1), and they were given a caffeine dose equal to 7 mg·kg−1 body weight (approximately 707 mg). Because caffeine ingestion enhanced performance in both studies, it is likely that caffeine habituation does not mediate the ergogenic potential of caffeine, at least for isokinetic exercise. In the present study, subjects completed two maximal bouts of 40 repetitions at 180°·s−1, whereas subjects in their study completed 3 repetitions at 30°·s−1, 3 repetitions at 150°·s−1, and 15 repetitions at 300°·s−1, identical to a study (6) that showed no effect of caffeine on isokinetic strength or endurance in male sprinters. In the study of Jacobson et al. (24), extension torque at 30°·s−1, a contraction velocity that is commonly used to assess muscle strength, was increased by 8% with caffeine intake, whereas extension torque at 180°·s−1 (Fig. 1A) was unchanged in the present study. To our knowledge, this is the only study that has revealed ergogenic benefits of pure caffeine for dynamic exercise focusing on strength. It may be that their subjects were more accustomed to the low-velocity demands of knee extension exercise because of their familiarity with heavy resistance training, which is typically performed at slow speeds. In addition, Jacobson et al. (24), the present study (Figs. 3 and 4), and others (16,21,31) reveal evidence of increased muscle endurance, in the form of more repetitions to fatigue during dynamic resistance training, with caffeine intake. Nevertheless, other studies revealed no change in muscle endurance with caffeine intake (1,22). These equivocal data suggest that there is variation in subjects' individual performance responses with caffeine ingestion. Consequently, scientists need to examine individual data in response to caffeine ingestion and should be aware that aggregate data revealing no effect of caffeine intake do not necessarily mean that no one benefited from caffeine ingestion. Often, individual data are more important than group means in studies investigating the efficacy of various ergogenic aids, and concluding no treatment effects across groups without considering individual responses is inaccurate and flawed. A simple technique to identify "responders" would be to determine a priori a meaningful difference in performance and use this as a criterion to identify participants who do and do not benefit from caffeine ingestion.
These interindividual differences may be mediated by discrepancies in caffeine metabolism between individuals (19). A single substitution in the gene coding for caffeine degradation causes some persons to be slow caffeine metabolizers, whereas those who are homozygous for the allele metabolize caffeine more rapidly (27). A recent study (9) demonstrated that habitual caffeine consumption is related to these genotypes, which may explain the discrepancy in individual responses to caffeine's physiological effects.
The present study is unable to identify exact mechanisms explaining caffeine's ergogenic effects during high-intensity exercise, which are currently unresolved. Recent articles (2,10) thoroughly described multiple peripheral and central mechanisms that serve as possible mediators of the ergogenic effects of caffeine. They concluded that caffeine's effects on high-intensity exercise are not due to increased reliance on lipid (15) or enhanced motor unit recruitment (18) and are more than likely located outside the muscle fiber (26). Data in rats (11) revealed improved run time to exhaustion due to caffeine's action as an adenosine antagonist, but to our knowledge, this has yet to be demonstrated during high-intensity exercise. Caffeine-induced attenuations in RPE and leg pain have been reported during endurance exercise (13,14,25), yet no change in end-exercise RPE was revealed in men completing fatiguing resistance training after ingestion of 6 mg·kg−1 caffeine (21), similar to the findings in the present study. However, increased performance coincident with no change in RPE suggests that perceived exertion may be blunted in response to caffeine intake. Alternatively, caffeine ingestion may alter mood by reversing the effects of caffeine withdrawal, as suggested by Yeomans et al. (32).
The present study maintained a few limitations. Our results can only be generalized to the ergogenic effects of anhydrous caffeine rather than energy drinks or other caffeine-containing products, of which less is known about their effects on exercise performance. Recreationally active men were recruited as subjects in the present study, so results cannot be applied to women or trained individuals. Two studies (4,8) revealed that caffeine is not ergogenic in untrained men completing exercise dependent on oxygen-independent metabolism, yet it is unknown if this is primarily due to their lack of training or inherent physiological differences between more active individuals, which makes them more susceptible to the ergogenic effects of caffeine. Yet, our conclusions are strengthened by the incorporation of a familiarization trial that reduced the learning effects of repeated exercise trials and subject adherence to all pretest guidelines.
In summary, the present study revealed improved performance during fatiguing knee extension and flexion exercise in active men when 5 mg·kg−1 caffeine, but not 2 mg·kg−1, was ingested 1 h before exercise. Muscular performance was improved only during bout 1, which suggests that caffeine may not alter performance when the muscle is already fatigued. It seems that low doses of caffeine likely ingested as part of a regular diet are not ergogenic during high-intensity, fatiguing contractions of the knee extensors and flexors.
An abstract of this study was presented at the annual meeting of the American College of Sports Medicine in May 2009.
This project was not funded by National Institutes of Health or other entities.
The researchers thank the participants for their effort during the study, as well as Miriam Caraveo for assistance with data collection. This study was funded by a Research and Scholarly Activity grant from CSU-San Marcos.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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