Coffee contains caffeine, and is the second most consumed beverage behind water, with more than 2 billion (estimated) cups consumed globally per day. It is commonly consumed by athletes to improve physical and cognitive performance [1
]. It has been well established that 3–6 mg per kilogram of body mass (mg/kg/bm) of caffeine usage, 60 min prior to exercise, improves strength and aerobic-muscular endurance performance via the most probable mechanisms, with adenosine A1
receptor antagonism, and NA+
ATPase pump activation [2
]. In addition, the hypoalgesic effect of caffeine may decrease muscle pain perception during high-intensity resistance exercise that induces pain through blocking central and peripheral adenosine receptors that influence pain signaling [4
]. Considering the improvements in reaction time, cognition, and mood after caffeine consumption, Astorino ve Roberson [3
] suggested that the ergogenic mechanisms of caffeine are likely to be multifactorial and have the potential to enhance performance. However, doses above 500 mg may cause the opposite reaction, with performance decrements, anxiety, and tension [2
Despite the initial controversy on whether biologically active compounds (ferulic acid, chlorogenic acid, caffeic acid, etc.) found in coffee blunts the ergogenic effects of caffeine [5
], the use of coffee as a form of caffeine administration may have comparable effects to anhydrous caffeine on exercise performance [8
]. Trexler et al. [11
], in their between-group design study, reported anhydrous caffeine is not superior to caffeinated coffee for sprint and muscular endurance performance and pointed out the interindividual variation in responses to caffeine intake in the anhydrous caffeine group may be related to large standard deviations in self-reported habitual daily caffeine consumption. The standardization of individual habituation may make the ergogenic or ergolytic effect of caffeine more detectable [12
]. Similarly, Richardson and Clarke reported that number of squat repetitions performed under decaffeinated + anhydrous caffeine conditions were significantly greater than those performed under decaffeinated coffee, anhydrous caffeine, and placebo conditions. Further, bench press performance after caffeinated coffee ingestion was not enhanced with similar magnitude to anhydrous caffeine [10
]. Although the multiform effects of caffeine on strength and endurance of lower and upper body has been reported by previous studies [13
], further investigations as to whether improvement in lower body endurance with caffeine intake persists over multiple sets in a dose dependent manner was suggested [10
Excess anhydrous caffeine intake can lead to gastrointestinal problems, tachycardia, sinus, tingling sensations, and negative health effects with long-term use [2
], and dose–response studies have increased recently [18
]. However, most of them suggested between 3–6 mg/kg/bm classically; the optimal dose required to have ergogenic effects may differ based on sex [21
], muscle group size [18
], and habitual caffeine consumption [22
]. Evident differences between sexes in terms of body size, lean body mass, and hormonal functioning may affect the results for the same dosage of caffeine [23
]. Given that female participants accounted for only 13% of the total number of participants included in the studies investigating the ergogenic effect of caffeine [23
], it would be difficult to generalize the recommendations. This was confirmed by Sabblah et al., showing 5 mg/kg/bm of anhydrous caffeine increased 1 repetition maximum (RM) strength performance in both males and females, but for muscular endurance, tendency towards improvement appeared in males only [21
]. In another study, by Arazi et al., 5 mg/kg/bm, but not 2 mg/kg/bm of anhydrous caffeine, decreased pain perception during muscular endurance tests in female karate athletes [20
]. Inconsistent results between males and females can also be related to oral contraceptive use that may decrease the caffeine clearance, which was considered by only a few studies [23
]. Further, both 3 and 6 mg/kg/bm of anhydrous caffeine doses significantly improved lower body muscle strength, with no effect on upper body reported by Tallis and Yavuz [24
]. Moreover, only 6 mg/kg/bm of caffeine increased peak force during repeated contractions, inferring optimal doses to improve muscle performance may be related to specific muscle mass. However, none of the above studies were conducted with coffee forms and on female athletes. In addition, 6 mg/kg/bm of caffeine intake using coffee consumption equates to 2 cups of strong-tasting coffee, which may irritate the gastrointestinal tract. Coffee consumption, especially in the morning hours, may not be appealing to athletes because of time commitment to ingest strong and hot beverages [10
]. All of these studies in the literature point out that the effects of different doses of caffeinated coffee on muscular endurance of the lower and upper body, over multiple sets, should be examined in females.
Diurnal variation in muscles and cognitive performance oscillate higher at midday, and lower during the early morning hours [25
]. Additionally, in congested match fixture, athletes can have training very early in the morning, unsurprisingly, with declining muscle performance that affect long-term training adaptations. To overcome this, higher beneficial effects of caffeine in the morning may be considered. A related study by Mora-Rodriguez et al. reported that 6 mg/kg/bm of anhydrous caffeine counteracted the decline in muscle performance observed in the morning, but had little effect on neuromuscular performance, and increased the rate of negative side-effects reported in the evening [26
]. However, effects of lower doses and coffee form on muscle and cognitive performance in the early morning were not investigated. Further, beneficial effects of caffeine on cognitive function including increased arousal, reaction time, and vigilance in sports settings has been well established [27
]. There is a limited number of studies that investigate caffeine intake on cognitive performance in female athletes. Only Ali et al. examined female athletes’ responses to 6 mg/kg/bm of anhydrous caffeine, showing non-significant but positive trends (p
= 0.072) in cognitive function [29
]. Although the positive effect of 3 mg/kg/bm of anhydrous caffeine on the velocity of half-squat exercise [30
] and peak aerobic cycling power [31
] were reported to be similar in all three phases of the menstrual cycle, Kumar et al. demonstrated cognitive performance declines in the high progesterone, luteal phase of the menstrual cycle [32
]. Muscle performance is defined by the characteristics of a complex network of mental and physical elements, investigating caffeine’s effect on cognitive performance will help us better understand its ergogenic properties. Furthermore, despite the documented ergogenic effect of caffeine on exercise performance, knowledge of its effect on cardiac recovery is somewhat limited [33
]. The current study is the first to investigate the effects of various doses of caffeinated coffee, which may minimize overdose risk with its relatively low caffeine/volume ratio. Therefore, the aim of this study was to investigate the effects of different doses of caffeinated coffee ingestion on muscular endurance, cognitive performance, and heart rate variability (HRV) in female athletes.
To the best of our knowledge, the current study is the first to analyze the effect of various doses of caffeinated coffee on lower–upper body muscular endurance and cognitive performance and HRV in caffeine naive female athletes in the early morning. The main finding was that the ingestion of both 3COF and 6COF increased lower body muscular endurance and cognitive performance by increasing arousal, with no harmful effect on cardiac autonomic function. The difference between 3COF and 6COF was only found in muscle pain perception while 6COF had a significant effect during lower body resistance exercise, but 3COF had none. Although 6COF improves (3.1%) upper body muscular endurance performance in the first set, there was no difference between 3COF and PLA. The current results may have implications for athletes seeking to improve muscular endurance and cognitive performance with no adverse autonomic effect by pre-exercise caffeinated coffee ingestion.
In the current study, it was determined that both 3 and 6 mg/kg/bm caffeinated coffee doses increased lower body muscular endurance performance and the lack of effect on upper body performance is parallel to previous studies [10
], and is in contrast to the studies that have reported that meaningful effects of caffeine is not related to muscle group location and enhances upper body muscular performance [4
]. It is also contrary to a study by Duncan et al. [4
], demonstrating that 5 mg/kg/bm anhydrous caffeine diluted in water increases repetition to failure performance (with the load of 60% of 1 RM), irrespective of muscle group location, beginning the test protocol with the upper body muscle group. Grgic et al. [51
], in their review regarding caffeine ingestion and resistance exercise performance, reported caffeine’s effectiveness on muscular endurance was lessened as fatigue developed, thus, reducing motor unit recruitment and force production. Supportively, it was stressed by the same research group that bench press repetition to failure performance (with the load of 60% of 1 RM) did not increase with caffeine ingestion when the bench press test was the very last in the experimental protocol, therefore, the accumulated fatigue may affect participant performance, and asserted results might have been different if the bench press had been assessed at the beginning of the test protocol [15
]. Although the results of the current study, and previous [10
] results seem to ratify this assertion, no significant effect of 6 mg/kg/bm of caffeine intake on both leg and bench press repetition to failure performance (with the load of 60% of 1 RM) was reported, even when bench press preceded leg press exercise [52
]. However, only one of these studies employed a design with counterbalanced order of bench press and leg press exercises when examining the effect of caffeine on upper and lower body repetition to failure performance, reported significant improvement in second and third sets of leg press, but not in bench press [53
]. Although, a meta-analysis by Warren et al. [17
] reported that magnitude of caffeine erogenicity may be 4–6 times greater in lower and large (knee extensors specifically) muscle groups compared to upper and small muscle groups, such as arm muscles, rationalized with various neural activation levels during maximal voluntary contractions (85–95% for knee extensors and 90–95% (minimal/no room for improvements) for other muscle groups), and promotion of greater muscle recruitment centrally, subgroup analyses pointed that muscle group locations were significant factors for strength, but not muscular endurance performance. Additionally, various responses across the upper and lower body may partially be explained with a non-equivocal number of adenosine receptors on muscle groups depending on its size [16
]. Contrasting results can largely be attributed to methodological variables of studies including caffeine dose–form, resistance exercise load intensity, heterogeneous daily caffeine intake, training status, genotype, and habituation of the participants.
In the current research, attempts were made to standardize daily caffeine consumption by recruiting very low habitual caffeine users, and training status of the participants selected to detect caffeine’s subtle ergogenic effect more clearly as suggested previously [13
]. Despite the equivocal findings on this topic, efficiency of caffeine can be related to training status of participants, which trained athletes can produce maximal efforts with greater motivation and muscle mass (more adenosine receptor concentration and act directly at the muscle via increased NA+
pump activation and Ca2+
release from sarcoplasmic reticulum) [17
], and have higher pain tolerance [24
] compared to untrained. Furthermore, resistance-trained participants may be more sensitive to muscular endurance responses (average 4–5 more repetitions) after caffeine ingestion, especially by overcoming psychological and muscular stress through the end of low intensity (40% of 1 RM) open-end muscular endurance tests by not allowing great variations in performance, keeping it stable over consecutive test sessions to detect subtle differences, and increase statistical power compared to untrained [10
]. It may also be useful to investigate the responses of participants to caffeine with different training status—equivocal findings on this topic need to be further questioned. Heart rate, lactate, and muscle pain perception values (around 170 bpm, 8 mmol/dL, and 8 points, respectively) in the current study showed that participants performing maximally and repeatability-consistency in each condition were high (ICC ranged between 0.94 and 0.97 during muscular endurance tests). Furthermore, improvement in repetition numbers (95% CI for caffeinated coffee conditions were around 0.3–5.0) is parallel to a meta-analysis concerning acute caffeine intake and isotonic muscular endurance [54
A wide range of load intensity (range between 40 and 80% of 1 RM) was used while testing muscular endurance performance with caffeine intake in the literature [4
]. One may speculate that, as the load intensity increases, benefits of caffeine intake seemed to lessen in a dose–dependent manner, and vice versa. Pallares et al. reported the ergogenic dose of caffeine required to increase neuromuscular performance during all-out contraction, depending on the magnitude of load with 3 mg/kg of caffeine, was enough to increase muscular performance against low loads, whereas a higher dose (9 mg/kg/bm) was necessary against higher loads [18
]. It was previously reported that slow-twitch muscle fibers may be more sensitive to caffeine ingestion than fast-twitch fibers [55
]. It can be speculated that caffeine may facilitate the release of calcium; in turn, exercise performance, selectively by depending on a contraction type (slow–fast twitch) of muscle, may explain different results between the current and previous studies measuring muscular endurance performance with various load intensity. However, while a lighter load (30% maximal voluntary contraction (MVC)) time to task failure performance in isokinetic dynamometer was better in caffeine (+45 s) compared to placebo in females, it was not statistically significant with lighter and heavier (70% MVC) loads [56
]. Some potential limiting factors, including use of very low dose (1.5 mg/kg/bm) of caffeine, recruiting physically active but not resistance-trained individuals and nonhomogeneous daily caffeine intake (100–300 mg/day), might have influenced the results of that study. Thus, further investigations is still required to examine the caffeine responses to muscular endurance test, representing real life settings with light (40–50% of 1 RM) and heavy (70–80% of 1 RM) load intensities. A clear majority of studies did not control movement tempo of resistance exercise during muscular endurance tests that may affect results, particularly in the later stages, by destabilizing timing between eccentric and concentric phases, and may be another consideration for future research. Thinking in reverse, reporting of both 3 and 6 mg/kg/bm of caffeine to increase mean movement velocity, and likely repetition number, during 30% of 1 RM bench press [19
], may explain alleged benefits of caffeine on upper body performance in studies that did not control movement tempo, and may generate a situation in favor of caffeine. Different movement velocities have unique physiological variables, and this may be affected by caffeine ingestion; it is required to investigate caffeine’s effect on resistance exercise performance with various repetition durations (2 s vs. 4 s) as well.
Moreover, 3 mg/kg/bm caffeinated coffee provided the same magnitude of performance enhancing effect as 6 mg/kg/bm in this study. This result was similar to the results of Grgic et al. [57
], reporting that 6, 4, and as little as 2 mg/kg/bm caffeine may enhance lower body muscular endurance performance (back squat 60% of 1 RM) while there was no significant benefit of caffeine intake on upper body muscular performance. These results should be carefully interpreted as 23 participants with heterogenous daily caffeine intake performed tests in the morning, and 5 in the evening, whilst no individual responses were reported in the abovementioned study. Caffeine demonstrated to exert its ergogenic effect depending on the time of day [26
]. It is plausible that the participants’ chronotype (expression of circadian rhythmicity), which likely influences psychophysiological and cognitive responses to physical exercise with “morning larks” might have augmented caffeine erogenicity compared to “night owls” or vice versa [25
]. To date, no study has examined the relationship between the ergogenic effect of caffeine and chronotype, and it would be a great topic for future studies to investigate interaction between caffeine and participants’ chronotype on exercise performance, particularly in a dose-dependent manner, to prescribe individual caffeine consumption strategies, depending on time of the day.
Due to regular caffeine ingestion prior to matches and training, athletes may develop tolerance to caffeine. Beneficial responses to low doses of caffeine in the current study, due to the caffeine naive participants, cannot be generalized to the athletes who routinely ingest caffeine. Supportively, Wilk et al. [22
] reported that caffeine doses between 9 and 11 mg/kg/bm did not improve resistance exercise performance in athletes habituated to caffeine, even though acute caffeine intake prior to test sessions exceeded the athletes’ usual daily consumption. Likewise, no effect on performance of squat and bench presses were reported by Karayigit et al. [58
], but they reported a trend (p
= 0.057), and an 8.8% increase in squat endurance performance. The authors also reported that 10 out of 14 participants had higher values in both squat and bench press compared to the placebo, in participants who had 347 ± 56 mg/day habitual caffeine consumption [58
]. Conversely, 3 mg/kg/bm of anhydrous caffeine was found to improve resistance exercise performance in a study [23
] conducted with light habitual caffeine users. Habitual caffeine consumption has been suggested to increase adenosine receptor numbers and may modify cytochrome P450 enzyme function, hereby reducing the erogenicity [59
]. However, Gonçalves et al., refuting and describing tolerance to caffeine as a myth, reported that performance effects of caffeine during a 30-min cycling time trial performance were not influenced by the level of habitual caffeine consumption [12
]. To date, a few studies [60
] directly investigated the moderating effect of various caffeine consumption levels (low, moderate, high) on ballistic and resistance exercise performance that reported no influence of habitual caffeine intake; however, this requires further exploration. Additionally, participants are generally asked to abstain from caffeine consumption, as the current study, for 12–24 h before test sessions in the caffeine literature. Speculation was made by Astorino et al. [13
] that heavy caffeine users’ muscular endurance performance was reduced in placebo conditions compared to caffeine conditions due to the observed withdrawal symptoms, including lethargy and headaches, in which these symptoms would be ameliorated. This prior assertion was not backed by the current study, showing even low doses of caffeinated coffee increased muscular performance in caffeine naive participants, by whom possible withdrawal symptoms is not expected.
One of the novel results of the current study is the measurement, for the first time, of the effects of caffeinated coffee on muscle pain perception during resistance exercise that showed 6COF reduced pain perception during the squat exercise, but not for the bench press. Caffeine seems to increase the motivation of the participants, to complete greater repetitions to failure, by reduced pain perception, parallel to studies reporting the same effect with only higher doses [4
]. Pain perception may be reduced with caffeine ingestion over multiple set research designs. In the current study, pain perception following three sets of squat and bench press exercises were significantly lower for the 6COF trial. Conversely, Grgic et al., suggested that 2–6 mg/kg/bm of caffeine did not reduce pain perception during a single set of squat and bench press 60% of 1 RM repetition to failure protocol [57
]. Although the pain perception was reported to occur with greatest sensitivity in the luteal phase [31
], effects of low doses of anhydrous or coffee form of caffeine on pain perception may be more apparent in the other phases of the menstrual cycle.
Improved pre- and post-exercise cognitive performance measured with the flanker task, as in the current study, was shown previously [28
]. Similarly, Hogervorst et al., reported that 100 mg of anhydrous caffeine given three times at regular time intervals during time trial test improved cognitive performance [27
]. Caffeine may therefore have an important role in individual and team sports in which concentration and reaction times have an influence on match/training performance. Previously, caffeine was shown to stimulate catecholamine secretion so increases heart rate and blood pressure during exercise, in turn, attenuating post-exercise autonomic recovery [33
]. Parallelly, 400 mg of anhydrous caffeine was reported to disrupt autonomic function (LF/HF ratio) during 5- and 15-min post-exercise because of increased sympathetic nerve activity [34
]. However, Sarshin et al. [35
] reported both 3 and 6 mg/kg/bm anhydrous caffeine increases resting cardiac autonomic modulation and accelerates post-exercise autonomic recovery after a bout of anaerobic exercise in recreationally active young men, which was also observed in young men after a submaximal exercise test [62
]. Divergent results can be related to the various factors that study designs have, such as body position, sex, age, and cardiovascular fitness [33
]. In this study, 3 and 6 mg/kg/bm coffee form of caffeine administration had no effect on HRV. Speculation can be made that polyphenols have antioxidant potential, and exist in coffee, and may blunt the adverse effect of caffeine on HRV. Future research should investigate the effects of various forms of caffeine intake on HRV pre- and post-exercise in this regard.
On a practical level, it can be suggested, based on this study’s results, that caffeine naïve female athletes may benefit from 3 and 6 mg/kg/bm of caffeine provided from coffee before training or a match in the early morning to increase physical and cognitive performance. Moreover, 6 mg/kg/bm of caffeine, provided from coffee, equates to 3–4 cups, and can be preferred to decrease muscle pain perception during squat exercises rather than 3 mg/kg/bm of caffeinated coffee. Additionally, because both doses improved arousal, one may prefer to ingest 3 mg/kg/bm of caffeinated coffee in 200 or 300 mL of hot water in a more practical manner. Overall, especially those who describe themselves as “night owls”, they may ingest caffeinated coffee in the early morning, while increasing muscular and cognitive performance; they do not delay cardiac autonomic recovery, and may, in reverse, adversely affect the next training or match performance. It should be noted that brewing method, and type/brand of coffee, may alter caffeine content, and habitual daily caffeine intake of athletes may cause variable responses to the same doses of caffeinated coffee.
A few limitations inherent to our study are as follows. The effectiveness of blinding was not tested by asking participants to identify which coffee had been ingested. It is unknown whether caffeine expectancy might have affected the results of the current study. Although participants were instructed to replicate their 24-h diet prior to each test session, macronutrient intake was not analyzed. Additionally, the absence of ventilator control and blood pressure measurements before and/or during HRV may affect the results. Further, we did not measure neurotransmitter concentrations which would have provided more insights to elucidate exact mechanisms as to how both 3 and 6 mg/kg/bm of caffeinated coffee increased muscular endurance and cognitive performance, while pain perception was reduced only in 6 mg/kg/bm of caffeinated coffee conditions. Finally, the temperature of coffee was not fully standardized, as Richardson and Clarke [10
] stated that the temperature at which coffee consumed can determine caffeine’s bioavailability.