- Journal Details
- Format
- Journal
- eISSN
- 1899-7562
- First Published
- 13 Jan 2009
- Publication timeframe
- 5 times per year
- Languages
- English
Caffeine is one of the most commonly ingested psychoactive substances in the world (Mitchell et al. 2014). A national survey from the U.S. indicated that 85% of the population ingests at least one caffeinated beverage per day (Mitchell et al. 2014). Caffeine is also a well-researched ergogenic aid, with relevant studies dating back to 1907 (Rivers and Webber, 1907). For research purposes, caffeine is generally ingested in doses from 3 to 6 mg/kg, approximately 60 minutes before exercise (Grgic et al. 2020a). Current evidence indicates that caffeine ingestion, compared to placebo, may acutely enhance muscle and aerobic endurance, muscle strength, and power, as well as speed and jumping performance (Grgic et al. 2020a).
Generally, studies that explore the effects of caffeine on exercise performance include two trials. In one trial, the participants consume the caffeine dose, while in the other, they consume a placebo. It is assumed that placebo ingestion does not influence exercise test results, so any increases in performance are attributed to caffeine's physiological mechanisms. However, there are several instances where isolated placebo ingestion provided an ergogenic effect (Beedie and Foad, 2009). For example, Pollo et al. (2008) found that placebo administration (along with the suggestion that it was caffeine), enhanced total work in a set of leg extensions. Duncan et al. (2009) used a design where the researchers informed participants that they ingested caffeine on one occasion and placebo on another, even though they ingested placebo in both cases. Increases in muscle endurance were observed when the participants thought that they ingested caffeine. Given that placebo may enhance performance, researchers have suggested that future studies should include a non-supplement, control trial in their designs (Beedie et al. 2018). A study that includes all three trials (i.e., caffeine, placebo, and control) would inform and quantify the isolated effects of caffeine and placebo on exercise performance (Beedie et al. 2018).
A recent meta-analysis found a significant ergogenic effect of caffeine ingestion on repetition velocity in resistance exercise (Raya-González et al. 2020). However, all 12 studies included in the meta-analysis only compared the effects of caffeine vs. placebo. In other words, no available studies explored the isolated effects of caffeine and placebo on repetition velocity. Accordingly, we aimed to examine the effects of placebo and caffeine compared to a control condition on mean repetition velocity in resistance exercise. Based on previous research (Duncan et al. 2009; Pollo et al. 2008), we hypothesized that caffeine and placebo ingestion would enhance performance compared to a control condition.
This study employed a randomized, crossover, double-blind study design, where the participants were required to visit the laboratory on four occasions. In the first visit, the participants filled out a validated questionnaire for estimating their habitual caffeine intake (Bühler et al. 2014). In line with the validation study, this questionnaire utilized a 24-hour recall for the assessment of habitual caffeine intake (Bühler et al. 2014). Additionally, one-repetition maximum (1RM) in the bench press was estimated using the methods by Balsalobre-Fernández et al. (2018). The participants were also familiarized with the test, which involved an assessment of repetition velocity in the bench press with loads of 50%, 75%, and 90% of 1RM. After the first visit, the participants were randomized to the three experimental conditions: (a) placebo; (b) caffeine; and (c) control. Before every session, the participants were instructed to give maximum effort in the exercise tests. A minimum of 3 and a maximum of 6 days was provided between trials. All testing sessions were conducted in the morning hours (between 07:00 and 09:00 am), and all participants performed the trials in a fasted state. Both caffeine and placebo were provided in gelatin capsules of identical appearance, 60 minutes before starting the testing session. Caffeine was provided in the dose of 6 mg/kg, while the placebo contained 6 mg/kg of dextrose. In the control condition, the participants came to the laboratory and waited for 60 minutes before beginning the testing session; however, they did not ingest any capsule.
Power analysis was performed using G*Power (version 3.1.9.2, University Düsseldorf, Germany) and the following parameters: “ANOVA, repeated measures, within factors” was assumed as the statistical test, expected effect size (
Fifty minutes after capsule ingestion (placebo or caffeine) or after passive rest (control condition), the participants performed ten minutes of self-selected warm-up. In each session, the participants were instructed to keep the warm-up consistent. In the three experimental conditions, mean repetition velocity was assessed using a valid and reliable
Video recording of the lift in slow motion,
Frame-by-frame inspection of the recorded video material,
Manual selection of the beginning and the end of the concentric portion of the movement.
The push-off phase was considered as the beginning of the movement. The end of the movement was considered the moment when the participants fully extended the elbows. The eccentric phase lasted two seconds in each repetition and the participants were instructed to perform the eccentric action in a controlled manner. The application calculated the time (in m/s) between two frames and provided mean repetition velocity data. In each session, the participants performed two, one, and one repetition of the bench press exercise with loads of 50%, 75%, and 90% of their 1RM, respectively. For 50% of 1RM, the better repetition, in the context of higher mean velocity, was used for the analysis. All repetitions were carried out with maximal intended concentric velocity. A three-minute rest interval was provided between sets or loads.
The effectiveness of the blinding was tested pre- and post-exercise in the placebo and caffeine trials (Saunders et al. 2017). For this assessment, we asked participants the following question: “Which supplement do you think you have ingested?” This question had three possible answers: (a) “caffeine”; (b) “placebo”; (c) “do not know”.
The differences between the caffeine, placebo, and control trials in mean velocity at 50%, 75%, and 90% of 1RM were examined using one-way repeated measures ANOVA. In the case of a significant main effect, post hoc analysis was performed using pairwise comparisons between each condition with a paired t-test. We initially set the statistical significance threshold at
The results of the one-way repeated measures ANOVA for mean velocity at 50% of 1RM indicated a significant main effect of condition,
Mean repetition velocity at 50%, 75%, and 90% of one-repetition maximum (1RM) in the three experimental conditions.
| Variable | Caffeine trial | Placebo trial | Control trial |
|---|---|---|---|
| Mean repetition velocity at 50% 1RM (m/s) | 0.83 ± 0.11 | 0.82 ± 0.11 | 0.80 ± 0.09 |
| Mean repetition velocity at 75% 1RM (m/s) | 0.56 ± 0.09 | 0.53 ± 0.08 | 0.53 ± 0.09 |
| Mean repetition velocity at 90% 1RM (m/s) | 0.37 ± 0.08 | 0.34 ± 0.08 | 0.33 ± 0.09 |
| Data are reported as mean ± standard deviation; 1RM: one repetition maximum | |||
All pairwise comparisons and their p-values and the adjusted p-values using the Holm-Bonferroni correction. For each outcome, p-values are ranked from highest to lowest
| Variable | Pairwise comparison | Paired t-test | Rank | Adjusted statistical significance threshold |
|---|---|---|---|---|
| Mean velocity at 50% of 1RM | Caffeine vs. placebo | 0.478 | 3 | 0.05 |
| Placebo vs. control | 0.036 | 2 | 0.025 | |
| Caffeine vs. control | 0.003 | 1 | 0.017 | |
| Mean velocity at 75% of 1RM | Placebo vs. control | 0.666 | 3 | 0.05 |
| Caffeine vs. placebo | 0.001 | 2 | 0.025 | |
| Caffeine vs. control | 0.0003 | 1 | 0.017 | |
| Mean velocity at 90% of 1RM | Placebo vs. control | 0.376 | 3 | 0.05 |
| Caffeine vs. placebo | 0.0002 | 2 | 0.025 | |
| Caffeine vs. control | 0.0001 | 1 | 0.017 | |
| 1RM: one repetition maximum | ||||
The results of the one-way repeated measures ANOVA for mean velocity at 75% of 1RM indicated a significant main effect of condition,
The results of the one-way repeated measures ANOVA for mean velocity at 90% of 1RM indicated a significant main effect of condition,
When evaluated pre-exercise, 20% and 44% of all participants identified caffeine and placebo trials beyond random chance, respectively. When evaluated post-exercise, 28% and 52% of all participants identified caffeine and placebo trials beyond random chance, respectively.
The main finding of this study is that caffeine ingestion, compared to placebo, was ergogenic for mean velocity in the bench press exercise at 75% and 90% of 1RM. When compared to the control condition, caffeine ingestion was ergogenic for mean velocity across all analyzed loads. However, there were no significant differences between placebo and control conditions, suggesting that isolated placebo ingestion may not be ergogenic for mean repetition velocity in resistance exercise.
The results presented herein are not in agreement with previous research that explored the effects of placebo ingestion on resistance exercise-related outcomes. Specifically, Pollo et al. (2008) and Duncan et al. (2009) reported that placebo ingestion was ergogenic for total work and muscle endurance in the leg extension exercise, respectively. Additionally, Costa et al. (2018) also reported that placebo ingestion, coupled with a suggestion that the capsule contains caffeine, was ergogenic for movement velocity at 50%, but not at 60%, 70%, or 80% of 1RM. These results are in contrast to those presented herein likely because of the methodological differences in the studies. Specifically, previous research used a design whereby a placebo was provided to the participants together with the suggestion that the capsules contained caffeine (even though caffeine was not ingested in any of the trials). However, in the present study, we used a double-blind study design, where neither the investigators nor the participants knew the content of the capsules, and there were no suggestions provided to the participants that may influence their exercise performance.
Caffeine ingestion may result in side-effects (both positive and negative), such as increased heart rate, headache, and increased focus (Juliano and Griffiths, 2004). Due to these side-effects, some participants may be able to correctly identify the content of the capsules. This is important to consider as correct supplement identification may influence the outcomes of an exercise task and lead to bias in the results, despite the “double-blind” study design (Saunders et al. 2016). Therefore, for studies that examine the effects of caffeine on exercise performance, it is essential to explore the effectiveness of the blinding of the participants to the caffeine and placebo trials. Many studies do not even perform this procedure (Grgic, 2018; Grgic et al. 2018). Some studies that also used 6 mg/kg of caffeine reported that 75% to 92% of all participants were able to correctly identify the caffeine trial (Carr et al. 2011; Tarnopolsky and Cupido, 2000). In this study, the blinding effectiveness was high, given that only 20% to 28% of the participants were able to correctly identify the caffeine condition beyond random chance. Given the effective blinding and the lack of the ergogenic effects of placebo ingestion, it seems that the improvements following caffeine ingestion are mainly due to caffeine's physiological effects, such as increased motor unit recruitment (Bazzucchi et al. 2011). The study by Tallis et al. (2016) further confirms this idea. Here, the participants were tested on four occasions: (a) "told caffeine – given caffeine"; (b) "told caffeine – given placebo"; (c) "told placebo – given placebo"; and (d) "told placebo – given caffeine". Improvement in isokinetic torque was observed only on the two occasions when the participants truly ingested caffeine (i.e., "told caffeine, given caffeine" and "told placebo, given caffeine" conditions).
When assessing the effects of caffeine on repetition velocity, it is important to use different loads, as some authors have hypothesized that caffeine's effects on repetition velocity might be external-load-dependent (Grgic et al. 2019). Interestingly, the ES of caffeine (when compared to both control and placebo trials) increased with the increases in external load lifted. At 50%, 75%, and 90% of 1RM, ES ranged from 0.19–0.29, 0.32– 0.34, and 0.36–0.46, respectively. These results suggest that the ES of caffeine on repetition velocity is the greatest when the force production requirements are highest. We observed a similar pattern in our recent study where ES ranged from 0.20–0.29 for loads up 50% of 1RM, 0.36–0.50 at 75% 1RM, and 0.57–0.61 at 90% 1RM (Grgic et al. 2020b). Nevertheless, it warrants mention that the 95% CIs in the present study somewhat overlapped between the analyses for different loads and the loads used in each testing session were increased from lowest to highest (i.e., the order of loads was not randomized). These limitations make drawing firm conclusions on the relationship between caffeine ES and load lifted difficult.
There are several limitations to the present study that need to be considered when drawing practical inferences. Even though the blinding can generally be classified as effective, we should also consider that the number of participants that correctly identified caffeine or placebo was higher when the blinding was evaluated post-exercise vs. pre-exercise. In this context, an argument can be made that the pre-exercise responses are more important, because the improvements during the testing session (or lack thereof) may influence the post-exercise responses. We instructed the participants to provide maximum effort in each session. However, there might have been a lack of motivation in some participants in the control condition as there was no substance ingestion associated with this trial. The participants performed all testing sessions in a fasted state, which is not a very common practice for most individuals performing resistance exercise and is not in accord with the current sports nutrition guidelines (Aird et al. 2018). This also has relevance because caffeine's effects may be smaller when caffeine is ingested in a fed vs. fasted state (McLellan et al. 2016). Additionally, given that the study was conducted in a sample of young resistance-trained men, the findings cannot necessarily be generalized to older populations, women, and those who are untrained. Finally, the included participants were generally “low” habitual caffeine users with a median intake of 0.6 mg/kg/day (Filip et al. 2020). Therefore, future research is needed to explore this topic among participants with higher habitual caffeine intake.
In this study, we found that caffeine ingestion, compared to placebo, provided an ergogenic effect on mean velocity in the bench press exercise at loads of 75% and 90% of 1RM. When compared to the control condition, caffeine ingestion was ergogenic for mean velocity across all analyzed loads. No significant differences were found between placebo and control conditions. Given that the blinding of the participants was generally effective, and that there were no significant ergogenic effects of placebo ingestion, it seems that the improvements in performance following caffeine ingestion can be mainly attributed to caffeine’s physiological mechanisms of action.
Mean repetition velocity at 50%, 75%, and 90% of one-repetition maximum (1RM) in the three experimental conditions.
| Variable | Caffeine trial | Placebo trial | Control trial |
|---|---|---|---|
| Mean repetition velocity at 50% 1RM (m/s) | 0.83 ± 0.11 | 0.82 ± 0.11 | 0.80 ± 0.09 |
| Mean repetition velocity at 75% 1RM (m/s) | 0.56 ± 0.09 | 0.53 ± 0.08 | 0.53 ± 0.09 |
| Mean repetition velocity at 90% 1RM (m/s) | 0.37 ± 0.08 | 0.34 ± 0.08 | 0.33 ± 0.09 |
| Data are reported as mean ± standard deviation; 1RM: one repetition maximum | |||
All pairwise comparisons and their p-values and the adjusted p-values using the Holm-Bonferroni correction. For each outcome, p-values are ranked from highest to lowest
| Variable | Pairwise comparison | Paired t-test | Rank | Adjusted statistical significance threshold |
|---|---|---|---|---|
| Mean velocity at 50% of 1RM | Caffeine vs. placebo | 0.478 | 3 | 0.05 |
| Placebo vs. control | 0.036 | 2 | 0.025 | |
| Caffeine vs. control | 0.003 | 1 | 0.017 | |
| Mean velocity at 75% of 1RM | Placebo vs. control | 0.666 | 3 | 0.05 |
| Caffeine vs. placebo | 0.001 | 2 | 0.025 | |
| Caffeine vs. control | 0.0003 | 1 | 0.017 | |
| Mean velocity at 90% of 1RM | Placebo vs. control | 0.376 | 3 | 0.05 |
| Caffeine vs. placebo | 0.0002 | 2 | 0.025 | |
| Caffeine vs. control | 0.0001 | 1 | 0.017 | |
| 1RM: one repetition maximum | ||||
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