This study is the first to investigate the effect of acute variable CAF supplementation compared to PLA on discipline-specific performance and combat activity in judo. Our study results support the hypothesis that the ergogenic effect of CAF is dose-dependent in the range of the studied dosages, and that regular customary consumption of CAF-containing products alters the effect of CAF supplementation. The main finding is that, in general, 6 and 9 mg/kg CAF improved TOTthrows in SJFT compared to 3 mg/kg, PLA or BASE. Importantly, the commonly used 3 mg/kg dosage did not substantially improve performance compared to PLA or BASE. Nine mg/kg CAF exclusively increased combat activity compared to PLA or BASE. Among CAF non-consumers 6 mg/kg was as effective as 9 mg/kg in enhancing SJFT performance. Among judoists who habitually consumed CAF-containing products, only 9 mg/kg CAF was more effective than 3 mg/kg or BASE. Thus, we conclude that with regard to combat sports, higher (6–9 mg/kg) than currently recommended CAF dosages (3–6 mg/kg) [1, 2, 5] are apparently more effective in terms of discipline-specific performance.
CAF easily crosses the blood-brain barrier and acts in the CNS [13, 15]. It also crosses cellular membranes of other tissues , thus many mechanisms explaining its ergogenic action were uncovered [3, 5, 7,8,9,10, 37,38,39]. Possible CAF mechanisms of action include: competition with adenosine receptors [8, 13, 14]; an increase in calcium ion release from the sarcoplasmic reticulum [13, 15]; norepinephrine elevation [4, 20, 21] and increasing HR [4, 8, 40, 41]; stimulates beta-endorphin secretion and decreasing pain perception  or altering skeletal and neuromuscular functions [15, 43, 44] and promoting a thermogenic response ; interference in substrate utilisation during exercise, by decreasing reliance on glycogen utilisation and increasing fat oxidation . CAF supplementation purportedly increases time to exhaustion [10, 46, 47], modulates central fatigue , reduces RPE , improves agility and decision making [4, 11, 12, 19] and increases alertness and cognitive performance [8, 17, 18]. CAF exerts significant ergogenic effects on strength and power; nevertheless, there is a need for future studies that explore the optimal CAF form and dosage to maximise its effect on muscles .
Combat sports require high levels of power, strength, dynamics and agility . CAF effects in high-intensity intermittent combat sport exercises are not highly recognised. It is supposed that in combat sports CAF may indirectly contribute to an increase in blood lactate concentrations after specific tests and simulated combats [25, 44, 49]. This effect is probably due to CNS stimulation [23, 44], higher energy availability for muscles  and/or reduced pain perception [23, 44]. All of the mentioned mechanisms may lead to greater ability to exercise with higher intensity and to a higher volume of performed effort. These could be the direct reasons for increase in blood lactate concentration. It is also possible that CAF enhances muscle contractions, increases technical performance and/or delays fatigue during combats .
A few studies examined the effects of CAF on performance in combat sports. Yet, there are no trials that compared different supplementation strategies (doses) in judo. One previous study in endurance athletes suggested that significant performance increases can only be achieved by low to moderate CAF doses (3–6 mg/kg), while high doses (9 mg/kg) may overstimulate the CNS . In the current study, we showed that CAF doses of 6 and 9 mg/kg resulted in better SJFT performance compared to BASE, PLA or 3 mg/kg. Further, at 9 mg/kg CAF the number of throws between SJFTR3 and SJFTR2 was comparable. Astley et al.  noted that acute ingestion of 4 mg/kg CAF increases the number of throws in SJFT compared to PLA in young judoists, while Lopes-Silva et al.  and Felippe et al.  did not observe any effects of 6 mg/kg CAF alone on the number of throws. Inconsistences between findings in the aforementioned studies may arise from different methodological approaches, e.g., different athlete weight categories, alternative pre-supplementation preparation (e.g., 5-day weight loss weight ) or small sample sizes [25, 51].
Other studies that investigated the effects of 5 mg/kg of CAF supplementation on judo performance [44, 52] utilised measures we did not implement, namely the Wingate Anaerobic Test , countermovement jump test, handgrip strength test or judo grip strength test . Athayde et al.  did not observe any positive significant effect of CAF supplementation in neuromuscular tests. Souissi et al. , however, noted that CAF ingestion improves peak and mean power after morning compared to afternoon supplementation. CAF effectiveness was also investigated in wrestling [53, 54], taekwondo [55, 56], Brazilian jiu-jitsu (BJJ) [49, 57] and boxing . Four  and 5 mg/kg  doses were ineffective; only 10 mg/kg CAF  improves performance in wrestlers. Further, supplementation with 5 mg/kg CAF produced mixed results in taekwondo athletes [55, 56]; 3 mg/kg enhances combat intensity and muscular performance in BJJ [49, 57] and 6 mg/kg induces greater duration of high-intensity actions in boxing .
Judo combat is dependent on the opponent’s strategy of attacks and defense, and it is composed of acyclic tasks, factors which make it difficult to identify the effect of CAF during combats. However, high-intensity efforts lead to central and muscle fatigue (related to metabolite accumulation, energy substrate depletion, neuromuscular junction failure and muscle contractile potential disturbance) . These phenomena apparently explain why during consecutive matches the number of attacks decreases . However, in our study we did not observe substantial differences in the number of attacks in three consecutive judo combats at any CAF dose, PLA or BASE. There is only one previous study  that measured the number of attacks during a judo match. Ingesting 5 mg/kg CAF did not increase the number of attacks . This finding is contrary to our results, where we observed increased TOTattacks in 3 combats at 6 and 9 mg/kg CAF comparing to PLA.
Ingesting CAF can influence both HR at rest [8, 40] and during exercise [4, 41], namely due to CNS activation (e.g., catecholamine release), phosphodiesterase inhibition, adrenal cortex stimulation (corticosteroid release) and influence on the renal system . Low CAF doses (3 mg/kg) do not alter HR at rest [29, 50]. In our study 6 and 9 mg/kg CAF increased HRRA and HR1minAF compared to 3 mg/kg, and 9 mg/kg additionally compared to PLA. However, 3 mg/kg CAF did not affect HR, and this result is in accordance with the current literature [29, 50]. In contrast, two former studies that measured HR in judo did not observe any significant differences in HR after 4  or 6 mg/kg  CAF supplementation.
CAF ingestion may reduce RPE, which could be caused by the ability of CAF to cross the blood-brain barrier and compete with adenosine receptors [8, 13, 14]. This mechanism is also responsible for reduced pain perception and sleepiness . On the one hand, our current study did not show any direct significant effect of CAF supplementation on RPE measured by the Borg scale and are in line with a study by Fellipe et al. , who utilised 6 mg/kg CAF. On the other hand, it should be emphasized, that in the current study 6 and 9 mg/kg CAF doses improved judo-specific performance. An increase in performance is inherent with, among others, greater work performed. In practice, the participants were able to perform more effectively without perceiving greater exertion. Thus, it must be stated that CAF supplementation positively affected RPE. Contrarily, report by Lopes-Silva et al.  showed reduced RPE in judoists who ingested 6 mg/kg CAF, but no increase in performance.
Habitual caffeine consumption
It is important to note that, to our knowledge, there is no data on the influence of customary CAF consumption on the effectiveness of CAF supplementation in combat sports. This aspect was investigated in athletes who represented other sport disciplines [20, 61, 62]. Moreover, none of the studies showed significant differences in performance between CAF users and nonusers [20, 61, 62]. However, in CAF non-consumers, the post-ingestion effect lasted 3 h longer compared to customary consumers . In our study we showed that CAF consumers needed to ingest 9 mg/kg to achieve better SJFT results, while in non-consumers the dose of 6 mg/kg was equally effective as 9 mg/kg. Further, in CAF consumers, in contrast to non-consumers, the implemented supplementation did not affect HR after exercise. Habitual CAF consumption also had no effect of combat activity.
Implications for performance
Knowledge of the impact of different CAF dosages on judo performance could have significant beneficial implications for athletes who compete in international matches. Our data can therefore be translated into a real-world setting and hold implications for improving performance and overall competition efficiency. Information on whether higher CAF doses promote more throws and/or attacks could be important to establish discipline-dependent recommendations for CAF usage. Importantly, given the physiologically similar nature of the effort, our results could also be used also in ergogenic supplementation support of athletes in wrestling, BJJ, and other “grappling” combat sports disciplines.
Limitations and strengths
It should be noted that our research has some limitations. First, we did not analyse blood biochemical marker (e.g., CAF) concentration in athletes during our study, which in our opinion would be worth performing in future investigations. However, our primary outcome was a practical assessment of the final CAF influences on discipline-specific exercise capacity, which we achieved. Second, the result of combat activity may be also related to the opponent’s experience level and the load that the opponent makes during the combat. Still, in our opinion this is in practice an unavoidable problem in combat sports. However, to minimise this effect, judoists fighting with each other were selected in terms of weight category and experience level, as well as in the same order of opponents (in T1-T5). Further, the result of combat activity may be affected by the fatigue that occurred after SJFT. According to RPE expressed by the Borg scale [30, 63], the participants perceived the exertion after SJFT as somewhat hard. Finally, we did not account for carry-over effect. However, it needs to be emphasised that at each laboratory visit we administered a single CAF dose to each participant, and each participant visited the laboratory five times with at least 7 days of wash-out. Since the elimination half-life of CAF ranges between 2.5 and 10 h , we believe that this wash-out period was sufficient to remove CAF from the judoist’s systems.
The unquestionable strength of our study is application of a four-fold crossover design protocol and implementation in all study participants each of the three CAF doses as well as PLA; such an approach has never been deployed in judo before. For the scientific value of our work, double-blind masking and full control of compliance of the delivery of the administered supplement, which was taken at the set time under the control of a member of the research team, are also important. Moreover, there was close cooperation among the research team and the coaches to ensure proper study protocol conduct (e.g., motivated athlete effort and commitment and analysed and assessed the correctness of techniques and actual attacks during combats). It is also worth noting that we used the discipline-specific judo performance test. The SJFT was developed to evaluate both anaerobic and aerobic potential, and it can be considered more appropriate to evaluate judo performance capacity than the Wingate Anaerobic Test [31,32,33]. It is also worth taking into account the high training level of studied judoists and high familiarisation of the studied group to perform all efforts. We also accounted for customary consumption of CAF-containing products. We revealed that recommended doses of CAF in judo should vary according to usual caffeine consumption.