The purpose of this study was to examine the effects of a caffeine-containing energy drink with a dose of 1 or 3 mg/kg of caffeine on muscle performance during half-squat and bench-press exercises. Findings indicate that the ingestion of the energy drink with 1 mg/kg of caffeine was not enough to raise the power output or to modify the force-velocity association during 10-to-100% 1RM power-load tests. However, the ingestion of an energy drink with 3 mg/kg of caffeine increased maximal power output by 7 ± 4% in the half-squat and by 7 ± 2% in the bench-press, in comparison to the ingestion of a placebo energy drink (P < 0.05). In addition, 3 mg/kg of caffeine moved the relationship found between the force production and velocity upwards in both the half-squat and the bench press. Thus, an energy drink with at least 3 mg/kg of caffeine is necessary to significantly enhance muscle performance.

Apart from Seidl et al. [33] who investigated the effects of an energy drink on cognitive performance, the first authors to investigate the outcomes of caffeine-containing energy drinks on physical performance were Alford and co-workers [23]. Since then, a small number of studies have been geared to examining the effects of caffeine-containing-energy drinks on physical performance or sports tasks, mainly because of the relative novelty of these beverages [19–25, 34]. Most of them have used the most popular energy drink, Red Bull®, which contains 80 mg of caffeine per 250 mL of product (one serving). When the investigations included the comparison between a placebo beverage and one serving of the energy drink, equivalent to ~1 mg of caffeine per kg of body weight, most [19–22], but not all studies [23], failed to find a better physical performance derived from the energy drink ingestion. On the other hand, the ingestion of two or three servings of energy drink (equivalent to ~2-3 mg of caffeine per kg) improved [24, 34] or tended to improve [25] physical performance. These outcomes combined with the results of the present investigation suggest that the physical benefits attributed to caffeine-containing energy drinks are present with at least 3 servings, equivalent to ~3 mg/kg of caffeine.

The effects of caffeine ingestion on muscle strength have been previously investigated during the realization of either isometric maximal voluntary contractions (MVC) or isotonic 1 RM tests [12]. Overall, the ingestion of ~6 mg/kg of caffeine raised maximal force production during both assessments, while lower caffeine doses have not been extensively studied (see review [28]). Regarding muscle power production and caffeine ingestion, most studies have used a 4–30 s maximal cycling test. In these studies, the results are confusing since ~6 mg/kg of caffeine increased [6, 35–37] or did not changed [38–43] maximal cycling power with similar 3-to-7 mg/kg caffeine doses. The experimental design used for the present investigation contains some novelties in comparison to previous studies about caffeine and muscle performance.

First, we have selected a power-load test to assess muscle performance after caffeine ingestion instead of single-resistance trials (i.e., MVC, 1RM, Wingate test, etc). This test includes maximal concentric contractions over a wide range of resistances and thus, it allows a better identification of maximal power and strength production. Similar power-load tests have been successfully used to assess the effect of training [44] and age [45] on muscle performance. Second, we have used two doses of caffeine to assess the dose–response benefits of this substance on muscle performance. These doses (1 and 3 mg/kg) were chosen based on previous publications on endurance performance tests in which the ingestion of 3 to 9 mg/kg of caffeine produced comparable benefits, while 1 mg/kg was found to be non ergogenic [7, 14]. Third, we have measured the effects of caffeine ingestion on upper-body and lower-body exercises. It has been suggested that lower-body muscles are more sensitive to caffeine ingestion due to their lower activation level [28]. With this experimental design, we can conclude that caffeine increases both maximal muscle strength and muscle power even with a dose of 3 mg/kg. In addition, the effects of caffeine on lower-body and upper-body muscles were alike.

Originally, the ergogenic effects of caffeine on physical performance were attributed to an enhancement of muscle fat oxidation and thus to a better glycogen sparing capacity derived from the intake of this substance [46]. However, caffeine has been found to be ergogenic in short-term activities where increased fat oxidation and/or carbohydrate sparing capacities are not the limiting factor for performance [27]. More recently, it has been found in animal models that caffeine may directly affect the muscle via enhanced Ca++ release from the sarcoplasmic reticulum [47] or via enhanced motor unit recruitment by inhibiting adenosine actions on the central nervous system [48]. In a previous study with humans, we found that 6 mg/kg of caffeine improved knee extensor muscle strength and cycling power production due to a higher voluntary contraction (central effects) with no effects on electrically evoked contractions (no effects on muscle contractile properties). Although we did not assess the source of the benefits found with caffeine-containing energy drinks in the present investigation, we did find the tendency for a lower time to maximal power output (Figure 3). A lower time to maximal power suggests a better intra- and inter-muscular coordination during the muscle contraction, likely mediated by improved motor unit recruitment [49].

Figure 3 Time to maximal power output during half-squat and bench-press concentric actions one hour after the ingestion of 1 and 3 mg/kg of caffeine using a caffeinated energy drink or the same drink without caffeine (0 mg/kg). Data are mean ± SD for 12 participants. * 3 mg/kg different from 0 mg/kg (P < 0.05). † 3 mg/kg different from 1 mg/kg (P < 0.05). Full size image

In a recent study with 176 participants, Badillo and Medina [50] found a very good association (R2 = 0.98) between load and propulsive velocity during the concentric phase of the bench press exercise. The mean velocity attained with 100% 1RM was 0.2 m/s and it increased progressively to 1.4 m/s when the load was reduced to 30% 1RM. According to these data, the authors conclude that measurement of propulsive velocity can be used for training or testing as a good predictor of the relative load (% 1RM) using a regression equation [50]. In the present study, we found a similar correlation between load and propulsive velocity in both half-squat and bench-press exercises (Table 2). In addition, with the ingestion of the placebo drink, the velocities attained during the propulsive phase of the bench press at 100% and 30% 1RM were similar to the ones found by Badillo and Medina (0.4 ± 0.1 and 1.5 ± 0.1 m/s, respectively). On the other hand, the ingestion of the energy drink with 3 mg/kg of caffeine raised bench press velocity to 0.6 ± 0.1 m/s at 100% 1RM and to 1.6 ± 0.1 m/s at 30% 1RM (Figure 2), moving the association between load and velocity upwards. Thus, when using the propulsive velocity to predict the relative load that represents a given resistance, the ingestion of caffeine or caffeine-containing energy drinks might represent a source of error.

Previous studies have found that caffeine or coffee ingestion may increase resting energy expenditure by 3-7% [51, 52]. However, in the present investigation with energy drinks, we did not find a thermogenic effect after the ingestion of 1 or 3 mg/kg of caffeine (Table 1). While we measured energy expenditure for 15 minutes, previous investigations have used more prolonged measurement periods (from 150 min to 24 h) to determine the thermogenic effect of caffeine. Thus, it could be necessary to enlarge the measurement period for the determination of resting energy expenditure to clarify if caffeine-containing energy drinks also raise energy expenditure. The acute ingestion of caffeine produces mild psychostimulant effects, which are thought to be the reason for its extensive use in the general population [31]. However, the ingestion of moderate-to-high amounts of this substance could also produce negative effects such as anxiety, headaches, elevated heart rate and blood pressure, increased sweating and urine production or insomnia [32]. The ingestion of an energy drink with 1 mg/kg of caffeine increased mean blood pressure by 5 ± 3 mmHg and heart rate 2 ± 3 beats per minute. However, this caffeine dose did not raise the prevalence of typical side effects in comparison to the placebo energy drink (see Table 3). The ingestion of an energy drink with 3 mg/kg of caffeine increased mean blood pressure by 8 ± 2 mmHg and heart rate by 4 ± 3 beats per minute in addition to a tendency for a higher frequency of abdominal/gut discomfort, incidence of tachycardia and heart palpitations and perceived anxiety (non significant). Therefore, it seems that caffeine-containing energy drinks, like pure caffeine ingestion, produce some minor side-effects in the subsequent hours to the ingestion. However, these side-effects would be only present with a caffeine dose of 3 mg/kg.