Volleyball is defined as a high-intensity, intermittent sport that requires lower body muscular strength and power, along with jump ability and sprint performance over short distances (1). Plyometric jump training (PJT) is an effective training strategy that can improve muscular strength and power, jumping ability, sprinting speed, and change of direction ability in volleyball players (2, 3). In addition to critical variables such as training intensity, volume-load, frequency, training surface, and other program design considerations for designing an optimal PJT program (4–7), some strength and conditioning coaches in volleyball also aim to incorporate sports supplementation to achieve further improvements in physical performance during the short-term preparation phase (8). Indeed, when training time is limited, sports supplementation may be recommended to facilitate performance improvements (9).

While knowledge regarding the effects of sports supplements on athletic performance is continually expanding, it is well-established that caffeine (CAF) significantly contributes to enhancing sports performance and improving physical capabilities related to sports by promoting neuromuscular (i.e., increase the contractile properties of the muscle fibers) and metabolic (i.e., improvements in Na⁺–K⁺ pump) adaptations (10–12). In fact, research indicates that CAF supplementation is effective in enhancing adaptations related to jumping ability, sprinting, change of direction, and maximal strength in volleyball players who undergo various training programs (13–15).

Prior studies have explored both the immediate and long-term effects of CAF on athletes’ physical performance, employing different doses based on body mass (BM), which has resulted in inconsistent conclusions (10–15). For example, a dosage of 3 mg per kg of BM has been identified as optimal (16), while male athletes showed significant performance improvements with a dosage of 6 mg per kg of BM (10). Research by Nemati et al. (14) indicated that a 6 mg per kg of BM intake led to better performance in volleyball-specific assessments compared to the 3 mg per kg of BM dosage. Furthermore, Wu and Jiang (17) reported that basketball players who consumed 6 mg per kg of BM before PJT exhibited greater physical performance adaptations than those who ingested 3 mg per kg of BM.

Although previous research has generally examined CAF dosages ranging from 3 to 6 mg per kg of BM (10–17), the effects of gradually increasing CAF doses on exercise performance adaptations following a 4-week PJT program in volleyball players remain uncertain. In fact, progressively increasing the dosage from 3 to 6 mg per kg of BM over a 4-week period may serve as an effective strategy, emphasizing the body’s improved tolerance to CAF absorption throughout the training phase (10, 18, 19). This approach could be more beneficial than maintaining a high and constant dose through a duration, which may lead to gastrointestinal issues, feelings of anxiety, or other side effects (10, 12). Indeed, caffeine’s ergogenic effects are thought to be mediated through various mechanisms, including modulation of the Na⁺/K⁺ pump and antagonism of adenosine receptors, leading to increased neuronal excitability and reduced perception of fatigue (10–14). The current study aimed to explore a progressive CAF intake approach, hypothesizing that a gradual increase in dosage may allow for a more controlled adaptation of these systems. Specifically, it is proposed that this method could enhance tolerance by allowing adenosine receptors to gradually downregulate in response to increasing CAF levels, or reduce side effects by minimizing the initial shock to the nervous system associated with a sudden high dose (12, 17). While the performance-enhancing effects of CAF are well-established (13–16), the optimal method of administration remains a subject of investigation. This study focuses on a progressive CAF intake strategy, driven by the hypothesis that a gradual increase in dosage may improve tolerance and reduce side effects compared to a constant dosage. This may be attributed to several mechanisms, including a potential for gradual desensitization of adenosine receptors, which are responsible for caffeine’s stimulatory effects, or a more controlled physiological adaptation to the stimulant, minimizing the likelihood of adverse reactions (15, 17, 18). It is important to note that these assumptions are hypothetical and require investigation. Therefore, this study sought to investigate various strategies for CAF supplementation, specifically comparing a consistent dosage of 3 mg and 6 mg per kg of BM with a progressive increase from 3 mg to 6 mg per kg of BM over the course of 4 weeks during the preparations phase of training using the PJT intervention. We hypothesize that a gradual increase in CAF dosage may serve as an effective approach to enhance exercise performance adaptations in volleyball players during the short-term PJT program.

There were no significant differences among the groups at baseline values (p > 0.05). All participants completed the training sessions, and there were no injury reports related to the PJT intervention or testing. Additionally, none of the athletes reported experiencing side effects from CAF supplementation, including nervousness, increased vigor, or gastrointestinal issues.

There was a significant main effect of time (p = 0.001) which indicates small to large benefits in exercise performance gains for volleyball players following the 4-week PJT intervention (Table 3; Figures 2–5).

A significant group by time interaction (p = 0.001 to 0.044, n²_p = 0.247 to 0.650) were observed in between the training groups, which indicates more adaptations for the CAF- 6 and CAF- 3 to 6 groups versus CAF-3 and PL groups in the performance adaptations. In fact, the CAF-6 and CAF- 3 to 6 groups showed significantly greater improvements compared with the CAF-3 and PL groups in the CMVJ (7.6% [p = 0.001 and 0.001] and 6.9% [p = 0.011 and 0.005] versus, 4.3 and 4.1%) (Figure 2A), SPJ (2.5% [p = 0.001 and 0.001] and 2.3% [p = 0.010 and 0.010] versus, 1.4 and 1.4%) (Figure 2B), BLJ (1.1% [p = 0.013 and 0.006] and 1.0% [p = 0.029 and 0.015] versus, 0.7 and 0.7%) (Figure 2C), 10-m sprint (−7.5% [p = 0.023 and 0.015] and −7.4% [p = 0.024 and 0.015] versus, −5.2% and −5.0%) (Figure 3A), T-CODS (−5.4% [p = 0.019 and 0.007] and −5.2% [p = 0.034 and 0.013] versus, −3.8% and −3.5%) (Figure 3B), 1RM strength (5.2% [p = 0.008 and 0.006] and 5.3% [p = 0.006 and 0.005] versus, 2.5 and 2.4%) (Figure 4), Ppeak (10.1% [p = 0.003 and 0.002] and 9.4% [p = 0.016 and 0.009] versus, 6.2 and 6.0%) (Figure 5A) and Pmean (8.7% [p = 0.001 and 0.001] and 8.2% [p = 0.003 and 0.001] versus, 5.1 and 4.9%) (Figure 5B), respectively, following the training period.

This research aimed to investigate the impact of various CAF supplementation strategies, utilizing different dosages, on the exercise performance adaptations of male volleyball players following a 4-week PJT intervention during the pre-season. The findings revealed that a 4-week PJT intervention could lead to significant improvements in the exercise performance of volleyball players. Furthermore, the greater adaptations were observed when players consumed 6 mg/kg of CAF or when there was a gradual increase in CAF intake from 3 to 6 mg/kg throughout the 4-week intervention. Conversely, the intake of 3 mg/kg of CAF over the 4-week training period did not yield any additional benefits compared to the PL conditions, indicating that this lower dosage does not enhance exercise performance adaptations in volleyball players over a short duration.

Our research corroborates findings from several earlier studies that indicate the efficacy of PJT as a training method for improving athletes’ jumping ability, sprinting speed, and agility, particularly among volleyball players (2, 3, 25, 32). In this investigation, we observed significant small to large ESs in the improvements of CMVJ, SPJ, BLJ, 10-m sprint, and T-CODS following a 4-week PJT intervention for volleyball athletes. These advancements may stem from adaptive changes in neuromuscular characteristics resulting from the PJT program (4–6). Previous studies have suggested that participation in PJT can lead to increased motor unit recruitment, enhanced coordination among various muscle groups, and improved neural activation of the primary muscles involved (5, 33). Furthermore, PJT is linked to the engagement of fast-twitch muscle fibers, increased stride length and frequency, and a swift transition from eccentric to concentric muscle actions, all contributing to small and moderate gains in jumping ability, sprinting speed, and change of direction performance in volleyball players who reported in previous studies (2, 4–6, 25, 33).

Additionally, the ergogenic effects of CAF were only observed in athletes who consumed 6 mg/kg of CAF or those who gradually increased their CAF intake from 3 to 6 mg/kg of over the course of the 4-week intervention. This indicates that a dosage of 3 mg/kg of CAF is insufficient to elicit physiological adaptations associated with caffeine, as it did not yield greater improvements than the PL group. In fact, the adaptations observed in both the PL and CAF-3 groups were similar (i.e., small to moderate ES) throughout the 4-week PJT intervention. Conversely, strategies involving 6 mg/kg of CAF and a gradual increase from 3 to 6 mg/kg of CAF led to superior enhancements (moderate to large ESs) in jumping ability, sprinting, and T-CODS among volleyball players. These results align with previous studies suggesting that for volleyball players or during PJT interventions, a caffeine dosage of 6 mg/kg is more effective than 3 mg/kg in achieving additional gains (13, 14, 17). It appears that ingesting a higher dose of CAF, rather than a lower dose (such as 3 mg/kg of body mass), prior to engaging in PJT has a counterproductive effect on adenosine receptors in both the central and peripheral nervous systems (10). This results in an increased central drive and a reduction in the perception of pain and fatigue during physical activity. Additionally, CAF triggers the release of serotonin in the cerebral cortex, influencing the central nervous system (11). This mechanism enhances the performance of the sympathetic nervous system while inhibiting the activity of certain neurons (12, 14). Consequently, PJT is conducted with increased arousal, leading to enhanced activation of muscle fibers via improvements in Na⁺–K⁺ pump as well as neuromuscular adaptations (14–17) resulting in significant improvements in jump performance, sprinting, and T-CODS.

Our results demonstrated that starting with 3 mg/kg of CAF and increasing to 6 mg/kg yielded similar performance gains as maintaining a constant dose throughout the 4 weeks. These findings may lend support to our assumption regarding the body’s capacity to tolerate and absorb CAF as well as lower CAF ingestions than consistent dose, indicating that athletes may not need to begin their training with high doses of CAF; rather, they could start with a lower dose and progressively increase it each week (18, 19). Nonetheless, it is crucial to recognize that our suggestions are speculative, based solely on a 4-week intervention, and may not be generalized to longer training durations and additional studies are necessary to investigate the effects of different CAF dosage strategies on performance adaptations in other athletes.

The results of the present study showed that a 4-week PJT protocol produced small gains in strength performance, and these gains were the same as those seen in athletes consuming 3 mg/kg of CAF, suggesting that low-dose CAF intake did not provide additional benefits for further strength gains and both the CAF-3 and PL groups showed small ESs following the PJT intervention. In fact, consuming 6 mg/kg of CAF or increasing caffeine from 3 to 6 mg/kg during short-term training was effective in promoting greater gains. In line with previous studies highlighting the positive effects of PJT on strength gains with the same ESs (7, 17), our study suggests that a strategy of using high-dose or gradually increasing CAF doses is optimal for volleyball players in order to achieve greater strength adaptations in short-term interventions (moderate vs. small ES). The greater strength gains after the short-term intervention were mainly attributed to the improvement of the neuromuscular junction (7), as well as the increased recruitment of motor units, the improvement of muscle internal and external coordination, and the enhanced storage and utilization of elastic potential energy (34). In addition, the intake of CAF (i.e., 6 mg/kg or 3 to 6 mg/kg) before PJT may stimulate higher activation of muscle fibers and promote calcium release during exercise (10, 17), thereby enhancing adenosine receptors in the central and peripheral nervous systems, reducing fatigue during training sessions, leading to more muscle fiber recruitment and further improvement in strength performance (12, 17).

More importantly, the intake of CAF, from low to high doses over the course of 4 weeks, produced similar and moderate ES in strength improvements to those seen in athletes who consumed high doses of CAF throughout the training period. This finding underscores the potential benefits of gradually increasing CAF intake for better absorption and body tolerance during the intervention (10, 18). Nevertheless, these strategies appeared to be effective only for short-duration training. Caution is warranted when applying these results to long-term training durations, and additional studies are required to better understand the long-term implications of progressive and consistent CAF intake on strength performance.

The results of the present study indicated that a 4-week PJT intervention resulted in small to moderate ESs in the improvements of Ppeak and Pmean among volleyball players. Notably, the consumption of 6 mg/kg of CAF, or an increase in CAF intake from 3 to 6 mg/kg, led to greater adaptations compared to other interventions (moderate vs. small ES). The primary effects of PJT that contributed to the enhancements in Ppeak and Pmean can be attributed to increased motor unit recruitment and more forceful muscle contractions (5, 25, 30). Specifically, PJT improved the rate of force development and engaged fast-twitch muscle fibers, resulting in greater force production (4, 6) as evidenced by the Wingate anaerobic power test. These findings align with previous research that emphasized the beneficial impact of PJT on athletes’ power output (5, 35). Additionally, the findings of our study revealed that the administration of 6 mg/kg of CAF, or a gradual increase from 3 to 6 mg/kg, resulted in more pronounced adaptations that support the ergogenic properties of CAF in power production. In fact, the intake of higher doses of CAF (such as 6 mg/kg or the range of 3 to 6 mg/kg) stimulates the sarcoplasmic reticulum to release increased amounts of calcium, as demonstrated in an in vitro study (36), which enhances the recruitment of motor units (11). This leads to more forceful muscular contractions and adaptations (13), ultimately resulting in greater improvements in Ppeak and Pmean.

Moreover, CAF enhances the production of neurotransmitters that activate the brain, leading to heightened alertness and an improved mood (12). It also aids in neuromuscular recruitment and decreases the time needed to achieve maximum effort during PJT (17), resulting in beneficial adaptations in the power output of volleyball players. Our research, however, revealed that a progressive CAF dosage strategy is equally effective as a consistent dosage over the 4-week PJT, potentially providing the way for new supplementation approaches. Nonetheless, while our findings suggest potential for short-term adaptation, the long-term effectiveness of such dosing strategies remains to be confirmed, and further research is necessary to fully comprehend the long-term effects of both progressive and consistent CAF consumption on Wingate anaerobic power performance.

Collectively, our results indicate that a lower overall CAF consumption, achieved through a 4-week intervention employing a novel supplementation strategy (gradually increasing from 3 to 6 mg/kg), yields comparable enhancements in the physical performance of volleyball players relative to a consistent high-dose (i.e., 6 mg/kg) CAF regimen. It appears that incrementally increasing CAF dosages may attenuate the potential for receptor desensitization while promoting the upregulation of key physiological pathways, such as heightened the Na⁺/K⁺ and improved calcium-handling efficacy (10, 11). These adaptive responses may contribute to sustained performance advantages by mitigating receptor downregulation and preserving neuromuscular sensitivity to CAF (12). Although calcium release and Na⁺/K⁺ activity are crucial in elucidating the acute effects of caffeine, their integration within the context of dosing strategies remains a comparatively under-investigated area (16). Gradual CAF administration may serve to minimize receptor desensitization and facilitate the upregulation of adaptive mechanisms, including enhanced Na⁺/K⁺ efficiency and calcium handling capacity (10, 36). Therefore, future research should examine the role of tolerance theory, balancing processes of desensitization and upregulation, to optimize the ergogenic potential of CAF in the context of training adaptations.

This research has several methodological constraints that warrant consideration. The first limitation is the small sample size, which presents valuable opportunities for future investigations. However, the sample size was determined through G*power analysis (17) to ensure statistical adequacy. Additionally, it is essential to note that the results are exclusively applicable to male volleyball players, thereby limiting their applicability to female athletes and individuals engaged in different sports disciplines. Moreover, the lack of laboratory measures to assess physiological and neuromuscular adaptations restricts the ability to elucidate the metabolic and muscular changes, including anaerobic capacity and physical performance enhancements, that may have occurred in the athletes. Accordingly, further research is needed to explore the ergogenic effects of CAF ingestion at varying dosages on performance adaptations following PJT. Given these limitations, the findings should be considered preliminary, highlighting the importance of additional studies to validate or refute our results.

Our study suggests that the implementation of a short-term PJT may lead to improvements in the exercise performance of volleyball players during the pre-season. Additionally, administering 6 mg/kg of caffeine or gradually increasing the dosage from 3 to 6 mg/kg of caffeine could enhance these adaptations. Given these results, it is recommended to consider high doses of caffeine as an ergogenic aid to optimize performance adaptations in volleyball players. Furthermore, the strategy of progressively increasing caffeine dosage proves to be as effective as maintaining a consistent dosage throughout the 4-week PJT, offering a viable option for caffeine supplementation with reduced overall consumption while achieving similar training benefits.