In the general population, caffeine is a widely consumed food constituent [1
]. Caffeine consumption is also widespread among athletes, likely due to its performance-enhancing effects on exercise [2
]. In most of the studies that examine the effects of caffeine ingestion on exercise performance, the participants ingest caffeine administered in the form of a capsule and wait 60 min before starting the exercise session [3
]. This waiting period is used with the idea that plasma levels of caffeine reach their peak values ~60 min following the ingestion of a caffeine-containing capsule [5
In recent years, however, several studies have explored the effects of alternate sources of caffeine on exercise performance [3
]. Some of the alternate sources of caffeine include chewing gums, bars, gels, mouth rinses, energy drinks, aerosols, and coffee [3
]. These sources attracted the attention of researchers, given that they may provide rapid absorption of caffeine in the body. For example, following the consumption of a caffeine-containing gum, increases in caffeine levels in plasma occur within 5 min [8
]. This rapid absorption may lead to a faster ergogenic effect, which subsequently may be useful in many situations in sport and in exercise settings.
Wickham and Spriet [3
] highlighted that only two studies thus far have examined the effects of caffeinated gels on exercise performance; one reported an ergogenic effect of caffeine on 2000-m rowing-ergometer performance [9
], while another stated that caffeine ingestion did not enhance intermittent sprint performance [10
]. Due to the scarce and conflicting studies examining the effects of caffeinated gels on exercise performance, it is evident that further research with this source of caffeine is warranted.
Two recent meta-analyses reported that caffeine ingestion acutely enhances muscle strength, as assessed by isokinetic peak torque and jumping performance [11
]. In both meta-analyses, all included studies explored the effects of caffeine administered in the form of a capsule or liquid.
In resistance exercise, caffeine ingestion may acutely increase muscle strength, muscle endurance, and muscle power [13
]. However, the effects of caffeine on muscle power in resistance exercise have been explored the least. Grgic et al. [13
] highlighted only four studies [14
] that have explored the effect of caffeine on power (as assessed by barbell velocity). Grgic et al. [13
] suggest that caffeine may have a considerable performance-enhancing effect on barbell velocity in resistance exercise; however, the authors also noted the need for future research on the topic. Given that all four studies that examined the effects of caffeine on muscle power in resistance exercise used caffeine in the form of a capsule, it remains unclear if comparable effects may be observed with caffeinated gel as a source of caffeine. While studies are exploring the effects of caffeine on resistance exercise administered in alternate forms such as coffee and chewing gums [6
], there is a lack of studies utilizing caffeinated gels.
An additional limitation of the current body of evidence that explored the effects of caffeine on power is that almost all studies used performance tests that involved a specific body region in isolation (e.g., upper-body in the bench press exercise). Currently, there is a need for studies that measure power output during exercise tests that require simultaneous coordinated activity of the upper- and lower-body musculature.
This study aimed to explore the effects of caffeinated gel ingestion on: (1) jump performance; (2) isokinetic strength and power of the knee extensor and knee flexor muscles; (3) upper-body power; and (4) whole-body power, in a sample of resistance-trained men. We hypothesized that ingesting a caffeinated gel would acutely enhance exercise performance in all of the employed performance tests compared to the placebo.
The present study aimed to explore the effects of caffeinated gel ingestion on exercise performance of resistance-trained men in tests characterized by a very short duration and maximal exertion. The results indicate that caffeine ingestion in the form of a caffeinated gel had performance-enhancing effects on: (1) vertical jump performance in the SJ and CMJ tests; (2) lower-body isokinetic strength and power; and (3) power of the upper-body musculature. Whole-body power, as assessed on a rowing ergometer test, did not improve following caffeine ingestion. The blinding of the participants was generally effective, and the side effects were minimal.
For the vertical jump performance, our results confirm the recent meta-analytical results by Grgic et al. [11
] that caffeine ingestion before exercise may acutely enhance jump height. Indeed, even the effect size in the SJ and CMJ tests that we observed (d
of 0.18 for both tests) were very similar to the pooled effect size of 0.17 reported in the meta-analysis. Previous studies that reported ergogenic effects of caffeine on jump performance generally used larger doses of caffeine (e.g., 6 mg·kg−1
), as well as a protocol that included a waiting time of 60 min from ingestion to the initiation of the exercise testing [11
]. Our results highlight that ingesting even a smaller dose of caffeine (300 mg; ~3.6 mg·kg−1
) in the form of a caffeinated gel administered 10 minutes before exercise, may also be ergogenic. These findings mirror those of Bloms et al. [26
] who also used both jump techniques and reported that ingesting 5 mg·kg−1
of caffeine improved performance both in the SJ and CMJ tests.
A recent meta-analysis [12
] reported that caffeine ingestion acutely increases strength, as assessed by an isokinetic dynamometer. Our results provide further support for these findings, given that we observed increases in peak torque following the ingestion of caffeine with d
across angular velocities and muscle groups (i.e., knee extensors and knee flexors) ranging from 0.21 to 0.37, and corresponding percent changes ranging from +3.5% to +6.9%. While the ergogenic effects of caffeine were noted at both angular velocities for the knee extensor muscles, a significant effect of caffeine on the knee flexor muscles was observed only at the velocity of 60°·s−1
. This divergent effect between muscle groups might be due to the lower level of muscle activation during maximal contractions at baseline in the knee extensor muscles [27
]. This naturally occurring lower level of activation may provide a greater “room for improvement” in contraction force following the ingestion of caffeine in this muscle group. Smaller muscle groups may have a higher muscle activation level at baseline and, therefore, are less affected by caffeine ingestion [27
]. Caffeine ingestion also improved average power, with a magnitude of improvement similar to that observed for muscle strength.
The ergogenic effect of caffeine on barbell velocity in the bench press exercise was evident across all three employed loads with the effects ranging from small (d
= 0.33; +3.5%) to moderate (d
= 0.59; +12.0%). These results provide further support to findings of the previous studies that explored the effects of caffeine on barbell velocity. For example, Mora-Rodriguez et al. [15
] reported that caffeine ingestion in a dosage of 3 mg·kg−1
, ingested 60 min before exercise, enhanced barbell velocity in the bench press when using external loads amounting to 75% 1RM.
Pallarés et al. [17
] suggested that the effects of caffeine on power might be external load- and caffeine dose-dependent. In that study, caffeine ingested in low and moderate doses (3 and 6 mg·kg−1
) enhanced barbell velocity in the bench press at loads corresponding to 25% and 50% of 1RM. However, when using loads of 75% of 1RM, only the doses of 6 and 9 mg·kg−1
were effective. At the highest load of 90% of 1RM, only 9 mg·kg−1
was effective. The findings presented herein are not in full agreement with the work by Pallarés et al. [17
] given that, in the present study, an absolute dose of 300 mg (~3.6 mg·kg−1
) was ergogenic for barbell velocity across all three loading schemes (including 90% of 1RM).
In contrast to the work by Pallarés et al. [17
], the magnitude of effect in the present study increased with an increase in the load that the participants lifted (Table 2
). The most pronounced effect across loading schemes, amounting to a +12.0% increase in barbell velocity, was evident for the load corresponding to 90% of 1RM. Based on these results, it seems that the effects of caffeine are more noticeable, at least for this exercise, when requirements for the contraction force are the highest. Given the direct importance of high barbell velocity in the development of power [28
], our results suggest that individuals might consider supplementing with caffeine before exercise to achieve acute increases in barbell velocity and, subsequently, stronger stimuli for the development of muscle power.
We did not observe any significant differences between placebo and caffeine conditions in the whole-body power, as assessed by the peak power output produced during the “all-out” rowing ergometer test. Based on these results, it does not seem that caffeine ingestion is ergogenic for whole-body peak power output; however, this could be due to large inter-individual variation in response to caffeine ingestion [29
], and therefore needs to be explored in future studies with larger sample sizes.
4.1. Mechanisms of Caffeine
Caffeine produces its ergogenic effects by binding to adenosine receptors [30
]. After binding to these receptors, caffeine blunts the fatiguing effects of adenosine and subsequently reduces perceived exertion. Indeed, there is substantial evidence that caffeine’s effect of reducing perceived exertion is one of the primary mechanisms for its ergogenic effect on aerobic endurance [31
]. However, the ergogenic effect of caffeine on high-intensity, short-duration tests (such as those performed in the current study) may be related to the release of calcium from the sarcoplasmic reticulum, and the subsequent inhibition of its reuptake [30
]. These actions may be associated with neuromuscular function changes, as well as increased contractile force in skeletal muscles [32
]. For the readers interested, these mechanisms of caffeine are discussed in greater detail elsewhere [30
The limitations of this study include the following: (1) the sample consisted of trained young men, which limited the generalizability of these results to those who are untrained, of older age, or to women; (2) we did not measure plasma levels of caffeine and, therefore, the amount of caffeine absorbed is not entirely clear; (3) an absolute dose of caffeine was used, whereas a relative dose might have been more appropriate (of note here, an absolute dose was given due to the fixed amount of caffeine per 75-mg gel sachet).
One additional limitation [33
] might be that 12 out of 17 participants correctly identified the placebo condition post-exercise; as determined by the 95% CI of the BBI, this identification was not solely due to chance. It is likely that correct identification of the placebo condition in the post-exercise assessment was due to the lack of perceived improvements in performance (only one participant answered “yes” to the perception of improved performance item following the ingestion of placebo). This may especially be evident given the small number of individuals that correctly identified placebos in the pre-exercise evaluation. From that aspect, it is possible that pre-exercise responses are of greater importance than the answers obtained post-exercise. Additionally, based on the findings by Tallis et al. [34
], an argument can be made that the correct identification of the placebo did not confound the results. In that study, the participants experienced similar improvements in isokinetic peak torque both when they were told that they were given caffeine and received a dose of caffeine, and when they were told that they ingested the placebo even though the capsule contained caffeine. While the placebo was identified beyond random chance in the post-exercise assessment, correct identification of caffeine in the post-exercise assessment can be attributed solely to chance, as there was a 95% CI overlap with the null value. These results further support an actual ergogenic effect of caffeine.
4.3. Practical Applications
Ingesting a caffeine dose of 300 mg in the form of caffeine gel 10 min before exercise may elicit an acute ergogenic effect on vertical jump height, muscle strength, and power in an isokinetic strength assessment, as well as barbell velocity in the bench press exercise. Due to these ergogenic effects, trained individuals may consider supplementing with caffeinated gels before exercise for acute increases in performance.