The acute intake of caffeine (CAF) has been found effective in enhancing exercise performance in a wide range of resistance-based exercises based on strength-endurance [1–3], and strength-power exercise modalities [4, 5]. The ergogenic effect of CAF has been found when consumed at doses ranging from 3 to 13 mg/kg body mass (b.m.) and ingested in the form of capsules 30 to 90 minutes before exercise [6, 7], although the use of high doses of CAF normally increases the prevalence of caffeine-associated side-effects . Interestingly, the manner of CAF administration seems to be less relevant than the dosage and timing, as ergogenic effects of caffeine on resistance-based exercise has been found after consuming caffeinated energy drinks  and gels , and coffee .
Mechanisms responsible for ergogenic effects of CAF are linked to the impact of this substance on various tissues, organs and systems of the human body [12–15]. Specifically, the hydrophobic nature of CAF permits a high capacity of distribution, while its lipophilic nature enables CAF to enter all tissues, entering intracellular tissue water and penetrating the blood-brain barrier . The effect of CAF on multiple body tissues makes it difficult to accurately determine the key mechanism of action during exercise. Nevertheless, several mechanisms, such as reduced muscle pain and perceived exertion , enhanced fat oxidation , increased muscle oxygen saturation  and local changes within the exercising muscle , have been proposed to explain caffeine’s ergogenic effects, although most of them explain the effect of CAF on submaximal intensity exercise. To date, the capacity of CAF to block the fatiguing effects of adenosine seems the most plausible explanation for the wide ergogenic effects of this supplement on maximal exercise performance [21, 22]. Briefly, evidence in animal  and human models  supports the capacity of CAF to act as an adenosine A1 and A2A receptor antagonist, inhibiting the brake that endogenous adenosine imposes on the ascending dopamine and arousal systems .
Given that sex has been recognized as an important factor of athletic and sports performance through the impact of endocrine differences , specific recommendations for both females and males are necessary to achieve the best possible sport results with the use of CAF. However, CAF supplementation studies have primarily focused on males or a mixed gender population and little is known about the effects of CAF on muscular performance in women [13, 14]. Specifically, a systematic review  has suggested that the effects of CAF during resistance exercise may be reduced in women when compared to men ingesting the same CAF dosage. This may be produced by the fluctuation of female sex hormones across the menstrual cycle and their interaction with CAF metabolism . In addition, some of the caffeine-induced stimulant effects are of smaller magnitude in women than in men . However, recent evidence suggests that the ergogenic effects of CAF on muscle performance is present in the early follicular, late follicular, and mid luteal phases , suggesting a stable effectiveness of CAF to increase muscle performance across the menstrual cycle. Furthermore, two recent investigations have found that the ergogenic effect of CAF may be of similar magnitude in men and women, although these investigations were carried out under aerobic conditions lasting from approximately 6 to 60 min [16, 17]. Hence, to date, it is unknown if the ergogenic effects of CAF related to resistance exercise performance observed in male subjects [1–3, 5] apply to female athletes, and it is unfeasible to determine if the magnitude of caffeine’s ergogenic effect on resistance exercise is similar in athletes of both sexes.
Only two previous studies analyzed the ergogenic effects of CAF on maximal strength and local strength-endurance in females [4, 18]. Goldstein et al.  showed that the acute intake of CAF (6 mg/kg/b.m.) significantly increased bench press performance (1-repetition maximum − 1RM) with no significant enhancement in the number of repetitions performed at 60% 1RM. Likewise Sabblah et al.  showed significant improvements in the results of the 1RM test in the bench press exercise after ingestion of 5 mg/kg/b.m. of CAF in both male and female subjects. However, the ingestion of CAF did not produce any effect during the 1RM squat exercise and during a strength-endurance test at 40% 1RM in a bench press exercise protocol in women, while a tendency for increased performance in the strength-endurance test was found in male subjects. Thus, the scarcity of data impedes to determine whether acute CAF intake increases resistance exercise performance, and it is difficult to ascertain if this potential ergogenic effect is of a similar magnitude of the effects found in men.
There is also a lack of information on how habituation to CAF may impact the ergogenic effect of CAF in women because previous studies did not select samples of women habituated to CAF [4, 18]. Habitual CAF intake modifies physiological responses to acute ingestion of this stimulant by the up-regulation of adenosine receptors [19, 20]. In animal models, the acute ingestion of CAF (10 mg/kg/b.m./day for two weeks) increased the number of binding sites for adenosine in the brain cortex . Then, the chronic intake of CAF results in newly-created adenosine receptors, reducing in part the competitive blockage of CAF on adenosine receptors, ultimately reducing its ergogenic effects in a progressive manner . Under this background, habituation to CAF due to chronic intake would produce a progressive reduction of CAF ergogenicity in those athletes consuming CAF on a regular basis, because the newly created adenosine receptors may bind to adenosine and induce fatigue. A progressive habituation to the performance benefits of CAF has also been proposed in investigations with humans by comparing the ergogenic effect of CAF in naïve/low CAF consumers vs. individuals with habitual CAF intake. However, the differences in the research protocols and thresholds to consider one participant as a habitual CAF consumer make it difficult to obtain concrete conclusions. Hence, the current evidence indicates that CAF habituation can decrease its ergogenic effects, but neither the time course of tolerance nor the CAF dose necessary to create habituation are known at this time.
In men, habituation to CAF reduced the ergogenic effects of acute intake of 3-to-9 mg/kg/b.m. of CAF during the bench press exercise  and doses up to 11 mg/kg/b.m. may be necessary to obtain minor effects of acute CAF uptake on maximal muscle strength [1, 2]. Pickering et al.  suggested that the reduction in the ergogenic effects of CAF in habitual users can be modified using doses greater than the daily habitual intake, however a study by Wilk et al. [1, 2] showed no benefits (except in maximal strength) from acute ingestion of CAF when the doses of CAF were above their habitual intake. In another study, Wilk et al.  found a positive effect of CAF (3 and 6 mg/kg/b.m.) on mean power output and mean bar velocity during the bench press throw in athletes habituated to CAF, and performance enhancements were obtained even when the dose of CAF did not exceed the value of habitual consumption. It should be noted that results presented by Wilk et al. [1, 2, 25] apply only to males habituated to CAF and there are no such analysis in women habituated to CAF.
Since there is no available data regarding the influence of acute CAF intake on maximal strength and strength endurance in women habitually consuming CAF, we decided to assess the acute effects of different doses of CAF (3 and 6 mg/kg/b.m.) on maximal strength (1RM) and local strength-endurance during the bench press exercise in women habituated to CAF. We hypothesized that both doses, 3 and 6 mg/kg/b.m., would enhance muscular strength but none of the investigated doses would enhance local strength-endurance.