The effect of repeated coffee mouth rinsing and caffeinated gum consumption on aerobic capacity and explosive power of table tennis players: a randomized, double-blind, placebo-controlled, crossover study

ABSTRACT

Background

Athletes require proper nutrition to enhance training and performance. Studies indicate that alternative sources of caffeine, such as caffeinated chewing gum, mouth rinses, energy gels, and coffee can improve performance. Therefore, this study investigated the impact of consuming caffeinated gum (CG) and repeated coffee mouth rinsing (CMR) on professional male table tennis players’ aerobic capacity and explosive power.

Method

A randomized, cross-over, placebo-controlled, and double-blinded study was conducted with eighteen male table tennis players (Age: 21.86 ± 2.40 yr, Height: 173.80 ± 6.88 cm, Weight: 61.81 ± 10.32 kg). In each test session, the participants were randomly placed in one of the three conditions including i) Chewing caffeinated gum (CG, n = 6), ii) Coffee mouth rinsing (CMR, n = 6), iii) Starch capsule as a placebo (PLA, n = 6). All participants consumed caffeine with an average dose of ∼3 to 4.5 mg·kg⁻¹. Also, a one-week interval was considered a washout period for each condition. First, the participants were given the required supplement and performed functional tests such as throwing medicine balls and Sargent’s jump tests. Then, the maximum oxygen consumption (VO_2max), time to exhaustion (TTE), oxygen consumption equivalent at primary ventilatory threshold (VO₂ at VT₁), and oxygen consumption equivalent at respiratory compensation point (VO₂ at RCP) were measured during the Bruce test. All data were analyzed using SPSS Windows software, repeated measure analysis ANOVA, and Bonferroni post hoc tests at p < 0.05.

Results

The current study’s findings illustrated that TTE significantly increased in CG (p = 0.000) and CMR (p = 0.012) conditions compared to PLA, but no significant difference was observed between CMR and CG (p = 1.00). VO₂ at VT₁ was significantly higher in CG (p = 0.004) and CMR (p = 0.000) compared to PLA; however, no significant difference was observed between CMR and CG (p = 0.335). VO₂ at RCP increased significantly in CG (p = 0.000) and CMR (p = 0.000) compared to the PLA condition, and despite this, no significant difference was observed between CG and CMR (p = 1.000). Nevertheless, there were no significant differences between the three conditions in VO_2max, throwing a medicine ball, and Sarjent’s jump height.

Conclusion

The study found that CMR and CG had a relatively positive impact on male table tennis players’ aerobic capacity; however, they did not significantly improve their explosive power.

KEYWORDS:

• Caffeine
• caffeinated gum
• coffee mouth rinsing
• aerobic capacity
• time to exhaustion
• table tennis

1. Introduction

Table tennis is a racquet sport that originated in England in the 1880s. This sport has been part of the Olympic Games since 1988 and is recognized worldwide as the fastest ball game [1]. Table tennis has an alternating movement profile, consisting of short rallies of approximately three seconds with short rests and, similar to other non-endurance sports activities, has an endurance or aerobic component [2]. Also, table tennis players must hit the ball at high speed (over 50 km/h) more than 30 times per minute during short rallies with a rest time of less than 15 seconds. As a result, many table tennis experts have pointed out that this sport’s movement skills and physical fitness are essential features [3]. Most scientists worldwide agree that table tennis is an aerobic metabolic sport that requires much endurance and is often accompanied by intense anaerobic metabolism in short intermittent periods [1]. Hence, this exercise requires significant energy from both anaerobic and aerobic systems [1]. Of course, the oxidative pathway is the most critical energy pathway for table tennis players [4]. In addition, physical fitness is an essential element for all athletes and forms the basis for developing techniques, tactics, strategies and mental development [5].

On the other hand, sports nutrition has been considered one of the most critical factors affecting the performance of athletes [6]. From the past until today, athletes have been looking for ways to improve their performance during sports activities or competitions. One of the strategies that can help athletes in this way is the reasonable use of dietary supplements that can improve the athlete’s speed and physical performance [6]. Although training plays a vital role in developing athletes’ movement and skill capabilities, nowadays, much attention is paid to nutritional compounds and especially energy supplements [6]. Caffeine is one of the energizing supplements that has attracted the attention of many athletes and trainers [7]. Researchers have concluded that approximately 74% of elite athletes use caffeine as an energy aid before or during an event [8,9]. Consuming caffeine in moderation increases body energy; reduces physical and mental fatigue; improves physical, cognitive, and motor performance; increases wakefulness, alertness, and sense of empowerment; increases reaction speed, accuracy and concentration; strengthens short-term memory; increases problem-solving ability and ability to make correct decisions and neuro-muscular coordination [10]. Caffeine is quickly metabolized in the liver, easily passes through the blood-brain barrier, and can soon affect the central nervous system (CNS) [11]. There are several hypothesized mechanisms for improving performance by caffeine: 1- release of calcium from the sarcoplasmic reticulum, 2- preservation of muscle glycogen stores by inhibiting phosphodiesterase, 3- antagonistic effects of caffeine adenosine receptors in the central nervous system [12,13]. Caffeine binds to A₁ and A₂ adenosine receptors, reducing the influence of the parasympathetic system [14]. The synthesis of neurotransmitters such as dopamine and catecholamines at the peripheral level improves the activity of the sodium-potassium pump (Na⁺-K⁺) and increases the bioavailability of calcium (Ca²⁺) in myoplasm [14,15]. Therefore, it is possible that one or a combination of these factors, through increasing actin binding to myosin, causes more muscular contraction and, as a result, more power, also, by increasing lipolysis and helping to maintain glycogen reserves, which is effective in aerobic performance and improves sports performance after consuming caffeine [15]. Caffeine is consumed in different forms by athletes, such as caffeinated beverages, coffee, tea, capsules/pills, chewing gum, mouth rinses, energy gels, and pre-workout supplements [16]. Like any other supplements, caffeine consumption may come with side effects such as tachycardia, heart palpitations, anxiety [17], headache, insomnia, and sleep quality disturbances [18], or potentially associated with adverse effects or changes in glucose metabolism, appetite, food intake, protein synthesis, attention, learning, and memory; that all of these factors can reduce sports performance [18]. Furthermore, to minimize the side effects of directly consuming caffeine supplements, the intake of alternative sources, such as caffeinated gum and coffee mouth rinsing, are investigated [17].

Caffeinated gum has been around for over 20 years as an alternative form of caffeine delivery. Chewing caffeinated gum can rapidly increase caffeine in the bloodstream compared to the capsule form [19]. The caffeine in the gum may enter the oral cavity through two routes: the buccal mucosa or through the gut due to swallowing saliva containing caffeine and to be absorbed and accelerate the speed of its transfer to the blood through these routes [17]. In a study, it was found that the use of caffeinated gum in doses between 2 and 6 mg/kg in increasing the performance of several types of sports, such as cycling [20,21], team sports tests [22], endurance running [23,24] and jumping performance [25] have been effective. On the contrary, intake of 2.7 and 5.4 mg of caffeine per kg of body weight via caffeinated gum did not increase the number of throws performed during the Special Judo Fitness Test (SJFT) compared to decaffeinated gum [26]. Vanir et al. in a study investigated the effect of 300 mg of caffeine in caffeinated gum and found that consuming it 10 minutes before exercise leads to energizing effects on jumping performance, maximum isokinetic torque, speed, upper body movement and total body power output during a rowing test [25]. Additionally, Pirmohammadi et al. showed that 300 mg of caffeinated gum can improve the explosive power of lower body muscles and the performance of table tennis players [27].

On the other hand, coffee is the most common form of caffeine consumption. It is one of the most essential natural energizing compounds that is used as a drink to improve performance [28]. Coffee is a complex matrix of hundreds of compounds [29]. These are consumed with broad variability based on serving size, bean type (e.g. common Arabica vs. Robusta), and brew method (water temperature, roasting process, grind size, time, and equipment) [30]. Coffee’s constituents, including but not limited to caffeine, have neuromuscular, antioxidant, endocrine, cognitive, and metabolic (e.g. glucose disposal and vasodilation) effects that impact exercise performance and recovery [30]. Coffee’s physiologic effects are influenced by dose, timing, habituation to a small degree (to coffee or caffeine), nutrigenetics, and potentially by gut microbiota differences, sex, and training status [30]. Coffee and its components improve performance across a temporal range of activities, from reaction time, through brief power exercises and into the aerobic time frame in most but not all studies [30]. Researchers have recently shown that sensing nutrients in the mouth can quickly affect the brain’s neural function and motor outputs [31]. These findings are precious because one can benefit from a method such as reeling in nutritional compounds without worrying about digestive discomfort caused by food supplements [12]. The non-swallowing “mouth rinsing” developed two decades ago suggests that carbohydrate (CHO) or caffeine mouth rinsing may enhance aerobic endurance activities and cognitive performance through central mechanisms [15]. Drinking caffeinated solutions increases motor output and brain activity without increasing plasma caffeine levels; increasing the activity of oral receptors improves central conduction arousal and physical performance [12,31]. Many studies have examined the effects of coffee mouth rinsing (CMR) on the athlete’s performance. For example, Taheri et al. in 2023 showed that CMR significantly increased Sarjent’s jump height in futsal players [32]. Also, Pirmohammadi etal. showed a significant increase in Sarjent’s jump test results in CMR and CG conditions compared to PLA in table tennis players [27]. In Dolan’s study on collegiate lacrosse athletes, the positive effect of caffeinated mouth rinsing on physical performance in the Yo-Yo Intermittent Recovery Test (Yo-Yo IRT) was not shown [33]. On the other hand, to determine the effect of caffeine on ventilatory equivalent indices, Ruiz-Moreno et al. conducted a study on 11 aerobically trained people to investigate the ergogenic effect of caffeine on ventilatory threshold and the results showed that the acute consumption of caffeine (i.e. the first day of consumption) with a dose of 3 mg per kilogram of body weight before exercise, increased the workload obtained in VT₂, the heart rate in VT₂ and VO₂ at the same time [34]. The ergogenic response of caffeine gradually decreased with daily consumption of this substance for twenty days. Therefore, from a practical point of view, caffeine consumption at a dose of 3 mg per kilogram of body weight may be used acutely to increase exercise intensity at the second ventilatory threshold and may represent a potential benefit for the athlete’s training time and competition [34].

Based on the information provided, sufficient research is needed on the impact of various sources and methods of consuming caffeine, such as CG and CMR, on athletes’ performance, particularly table tennis players. Additionally, the existing studies in this area must be more consistent in their findings. Therefore, the current study aimed to investigate the consumption of coffee mouth rinsing and caffeinated chewing gum on the aerobic capacity and explosive power of male table tennis players.

2. Methodology

2.1. Participants

Eighteen male table tennis players with approximately three years of Iran national league experience and three sessions a week of training voluntarily participated in the current study. The demographic information of the participants is listed in Table 1. Participants had no known diseases or medical issues, no history of an allergy to caffeine, and were not consuming any supplements or medications during data collection period. Furthermore, the participants did not smoke or drink alcohol or caffeinated beverages during data collection. According to the above conditions, all these eighteen participants were selected from the table tennis players who volunteered to participate in this study. Participants were informed of study procedures and provided consent before intervention implementation. The current study was reviewed and approved by the Human Research Ethics Committee of Shiraz University (ethics approval code: IR.US.PSYEDU.REC.1402.020, 2023) and carried out in accordance with the Declaration of Helsinki. Additionally, all participants were members of the same training camp, and their training regime was the same under the supervision of trainers.

Table 1. The anthropometric data of participations.

Characteristic	Mean ± SD(n = 18)
Age (years)	21.86 ± 2.40
Height (cm)	173.80 ± 6.88
Weight (kg)	61.81 ± 10.32
BMI (kg/m^2)	20.39 ± 2.63

2.2. Study design

This study was conducted in a randomized, cross-over, placebo-controlled, and double-blinded manner (Figure 1). Before the investigation, participants attended a familiarization session where they were introduced to all testing protocols and procedures. They also completed the consent form, personal characteristics, and a daily caffeine consumption and intolerance questionnaire. It should be noted that according to the data of the habitual caffeine intake questionnaire, the subjects’ average consumption of drinks and foods containing caffeine before entering this study was ∼2.1 mg·kg⁻¹ per day. During each test session, participants were randomly assigned to one of three conditions: i) Chewing caffeinated gum (CG, n = 6), ii) Coffee mouth rinsing (CMR, n = 6), or iii) Starch capsule as a placebo (PLA, n = 6). Participants first received their assigned supplement, then medicine ball throwing and Sargent’s jump tests were administered. Afterward, participants performed the Bruce test using a respiratory gas analyzer device (Gas Analyzer: Cortex Biophysics, Germany & COSMOS treadmill). The aerobic capacity indicators, including maximum oxygen consumption (VO_2max), time to exhaustion (TTE), oxygen consumption equivalent at primary ventilatory threshold (VO₂ at VT₁) and oxygen consumption equivalent at respiratory compensation point (VO₂ at RCP), were measured during the Bruce test. A one-week interval was provided as a washout period between conditions (PLA, CMR, and CG), and each test session measured medicine ball throwing Sargent’s jump and Bruce tests. Participants were instructed to avoid consuming any dietary sources of caffeine 24 hours before each exercise test session.

Figure 1. Cross-over and double-blinded study design in three conditions.

2.3. Kinetic oxygen conception parameters

Ventilatory Threshold (VT): The VT was visually determined using the modified V-slope method as described by Sue et al. [35], which is a modification of the method described by Beaver et al. [36]. The ventilatory equivalent method (the point at which VE/VO₂ begins to rise without an increase in VE/VCO₂) and end-tidal methods (partial pressure of end-tidal oxygen tension (PetO2) starts to grow without a decrease in partial pressure of end-tidal carbon dioxide (PetCO2)) was used as a complement [37]. In the current study, the oxygen consumption equivalent at VT₁ (VO₂ at VT₁) was measured and reported.

Respiratory Compensation Point (RCP): The respiratory compensation point was comprehensively determined from the point where PetCO₂ decreased, VE/VCO₂ began to increase, and the inflection point of the VE/VCO₂ slope [38]. In the current study, the oxygen consumption equivalent at RCP (VO₂ at RCP) was measured and reported.

2.4. Supplementation

Chewing Caffeinated Gum (CG): The amount of caffeine given to each table tennis player during the study was determined based on their body weight before the study. They chewed Military Energy Gum (MEG, USA) containing 100 mg of caffeine ten minutes before the tests began. If a player’s weight was less than 65 kg, they received 200 mg of caffeine (n = 13), and if their weight was more than 65 kg, they were given 300 mg of caffeine (n = 5) [24]. After chewing CG for ten minutes, the subjects immediately performed the medicine ball and Sargent’s jump test, and after five minutes of active rest (walking), the Bruce test was taken from them (Figure 2). In addition, this regime ensured all table tennis players consumed a moderate dose of ∼3 to 4.5 mg·kg⁻¹.

Figure 2. The protocol for taking supplements and performing tests.

Coffee Mouth Rinsing (CMR): Each participant performed mouth rinses with espresso coffee five times. In each mouth rinsing, the participants used 25 ml of the solution prepared from 7 grams of caffeinated coffee. Fifteen minutes before the start of the tests, the participants gurgled 25 ml of the prepared solution for 5 seconds and then poured it out completely. This action was repeated 15 minutes, 10 minutes, 5 minutes, and immediately before the test. Then, they performed the medicine ball and Sargent’s jump tests. After 5 minutes of active rest (walking), 25 ml of espresso coffee solution was mouth rinsed on another occasion, and then the Bruce test was performed [27,32] (Figure 2). In addition, this regime ensured all table tennis players consumed a moderate dose of ∼3 to 4.5 mg·kg⁻¹. To facilitate the task, coffee was prepared in advance for each subject and poured into containers of the same shape. The temperature of the coffee was maintained to the extent that it did not cause mouthburns; that is, it was lukewarm. The amount of water used, temperature, and pressure applied to prepare the coffee were the same for all subjects. After rinsing their mouth with coffee, each subject emptied the solution into a container at the test site. The amount of liquid discharged was then measured and compared with the amount consumed to ensure that the subject only rinsed their mouth with coffee and did not drink it [32].

In PLA condition, participants consumed one capsule containing 5 grams of starch 60 minutes before the start of the tests. All trials were conducted between 9 and 11 am, and all participants were given the same breakfast containing 350 to 400 kcal, consisting of 64% carbohydrates, 20% protein, and 16% fat, two hours before the exercise test session [27]. Participants were advised to maintain their regular diet throughout the testing period, abstain from food an hour before testing, and avoid strenuous exercise 24 hours before each trial. Participants had access to water and were free to consume it as required during the tests.

2.5. Dose and preparation of coffee

According to estimates, each single shot of espresso coffee has 60 milligrams of caffeine [32]. However, in the present study, to more accurately estimate the amount of caffeine in each single shot, the coffee solution prepared by High-Performance Liquid Chromatography (HPLC) was analyzed, and the amount of caffeine was reported. It should be noted that the espresso coffee was obtained from the combination of 7 grams of ground coffee (%100 Arabica Lavazza Espresso Italiano – single shot). The amount and temperature (180–190 F) of the water and the pressure (9–10 bars:900–1000 Kpa) applied to prepare coffee were the same. This coffee’s approximate amount of caffeine was 60 mg, and the water pressure used to make coffee was 20 bar. Also, the espresso machine (Zigma20 bar RL-333N) was the same for preparing all coffees.

2.6. Functional tests

Throwing medicine ball: First, the subjects stood behind the starting line while the shoulders were along the throwing field. The 3 kg medicine ball was held under the chin, and from a standing position, without moving the legs, they threw the ball toward the field of the test. The distance thrown was measured in meters [39].

Sarjent’s jump height: Sargent’s jump test was used, and all evaluations in Sargent’s jump test were performed three times in a row (1 minute of passive rest between them), and the best score was recorded as the final result [40].

Bruce’s test: Bruce’s test consists of seven stages, each lasting 3 minutes, and in each step, in addition to increasing the speed of the treadmill, also added %2 to its slope. How to increase the speed and incline of the treadmill in the Bruce test is as follows: first stage: speed of 2.7 km/h and %10 slopes; second stage: speed of 4 km/h and %12 slopes; third stage: speed of 5.5 km/h and %14 slopes, fourth stage: speed of 6.8 km/h and %16 slopes, fifth stage: speed of 8 km/h and %18 slopes, sixth stage: speed of 8.8 km/h and %20 slopes, and seventh stage: speed of 9.6 km/h and %22 slopes [41]. Additionally, before beginning the test, the first 3 minutes were for respiratory gas checking without movement. After the seventh stage, the last 3 minutes were for recovery with a 2.7 km/h speed without slope.

2.7. Data analysis

The collected data were analyzed using both descriptive and inferential statistical methods. The normality of the data distribution was determined by using the Kolmogorov-Smirnov test. The repeated measure analysis ANOVA test was employed to identify the main effect with characteristics over time between different time points intragroup and between groups on the indicators (medicine ball throwing, Sargent’s jump height, VO_2max, TTE, VO₂ at VT₁ and VO₂ at RCP). The Bonferroni post hoc test was used to determine pairwise differences [42]. The data were analyzed using SPSS software (version 26, IBM-SPSS Inc., Chicago, IL, USA), and the level of statistical significance was set at p ≤ 0.05. Also, figure production was performed using GraphPad Prism (version 8.4.3).

3. Results

Descriptive characteristics (including mean and standard deviation) are reported in Table 2.

Table 2. Means and Standard Deviation (SD) of the measured variables in the three conditions (n = 18).

Variables	PLA	CMR	CG
VO2max (ml/min/kg)	50.500 ± 5.982	50.277 ± 5.613	50.111 ± 5.143
TTE (min)	11.580 ± 1.220	12.114 ± 1.402	12.263 ± 1.304
VO2 at VT1 (L/min)	24.264 ± 1.578	26.480 ± 2.1341	25.894 ± 2.275
VO2 at RCP (L/min)	27.938 ± 2.645	31.346 ± 3.742	31.355 ± 2.929
Sarjent’s jump height (cm)	42.000 ± 5.79	42.722 ± 4.86	43.000 ± 5.32
Throwing medicine ball (m)	6.577 ± 0.97	6.816 ± 0.84	6.870 ± 0.92

PLA: Placebo, CMR: Coffee Mouth Rinsing, CG: Caffeinated Gum, VO_2max: maximum rate of oxygen consumption, VO₂ at VT₁: oxygen consumption equivalent at primary ventilatory threshold, VO₂ at RCP: oxygen consumption equivalent at respiratory compensation point, TTE: Time to Exhaustion, ml: milliliter, min: minute, kg: kilogram, L: Liter, m: meter, cm: centimeter.

The results of the analysis of variance with repeated measures showed that the main effect in TTE was significant [p = 0.000, F = 10.854, pEta2 = 0.390], and the TTE was significantly higher in CG (MD = 0.683, p = 0.000, 95% CI [0.374–0.993]) and CMR (MD = 0.534, p = 0.012, 95% CI [0.110–0.959]) compared to PLA, but no significant difference was observed between CMR and CG (MD = −0.149, p = 1, 95% CI [−0.626–0.328]) Figure 3(b). As well as, the main effect on VO₂ at VT₁ was significant [p = 0.000, F = 16.056, pEta2 = 0.486], and the VO₂ at VT₁ was significantly higher in CG (MD = 1.630, p = 0.004, 95% CI [0.498–2.762]) and CMR (MD = 2.216, p = 0.000, 95% CI [1.061–3.370]) compared to PLA, but no significant difference was observed between CMR and CG (MD = 0.586, p = 0.335, 95% CI [−0.341–1.512]) Figure 3(c). In addition, a significant difference was observed between the studied conditions in VO₂ at RCP [F = 25.674, p = 0.000, pEta2 = 0.602]. Also, VO₂ at RCP increased significantly in the CG compared to PLA (MD = 3.417, p = 0.000, 95% CI [−4.792– −2.042]) and CMR compared to PLA (MD = 3.408, p = 0.000, 95% CI [1.974–4.841]) and despite this, no significant difference was observed between CG and CMR (MD = −0.009, p = 1, 95% CI [−1.574–1.556]) Figure 3(d). However, the results demonstrated that there were no significant differences in VO_2max [p = 0.877, F = 0.131, pEta2 = 0.008] Figure 3(a), throwing medicine ball [p = 0.928, F = 0.075, pEta2 = 0.004] and Sarjent’s jump height [p = 0.596, F = 0.525, pEta2 = 0.030], between the three conditions (Figure 4) (Table 3).

Figure 3. Means and standard deviation (SD) of VO_2max, TTE, VO₂ at VT₁ and VO₂ at RCP in three conditions. PLA: Placebo, CMR: Coffee Mouth Rinsing, CG: Caffeinated Gum.

*: Significant difference compared to PLA.

Figure 4. Means and standard deviation (SD) of Sarjent’s jump height and Throwing medicine ball in three conditions. PLA: Placebo, CMR: Coffee Mouth Rinsing, CG: Caffeinated Gu.

*: Significant difference compared to PLA.

Table 3. Comparison of the variables data between three conditions.

		CMR (n = 18)		CG (n = 18)
		PLA	CG	PLA	CMR
VO2max (ml/min/kg)	MD	−0.222	0.167	−0.389	−0.167
	Sig	1.000	1.000	1.000	1.000
	95%CI	−2.320–1.876	−2.168–2.501	−1.939–1.161	−2.501–2.168
TTE (min)	MD	0.534	−0.149	0.683	0.149
	Sig	0.012	1.000	0.000	1.000
	95%CI	0.110–0.959	−0.626–0.382	0.374–0.993	−0.328–0.626
VO2 at VT1 (L/min)	MD	2.216	0.586	1.630	−0.586
	Sig	0.000	0.335	0.004	0.335
	95%CI	1.061–3.370	−0.341–1.512	0.498–2.762	−1.512–0.341
VO2 at RCP (L/min)	MD	3.408	−0.009	3.417	0.009
	Sig	0.000	1.000	0.000	1.000
	95%CI	1.974–4.841	−1.574–1.556	2.042–4.794	−1.556–1.574
Sarjent’s jump height (cm)	MD	0.722	−0.278	1	0.278
	Sig	1.000	1.000	0.622	1.000
	95%CI	−1.537–2.982	−2.159–1.603	−1.025–3.025	−1.603–2.159
Throwing medicine ball (m)	MD	0.239	−0.058	0.297	0.058
	Sig	0.423	1.000	0.369	1.000
	95%CI	−0.172–0.650	−0.382–0.256	−0.189–0.784	−0.256–0.382

PLA:Placebo, CMR: Coffee Mouth Rinsing, CG: Caffeinated Gum, MD: Mean Difference, CI: Confidence Interval, VO2max: maximum rate of oxygen consumption, VO2 at VT1: oxygen consumption equivalent at primary ventilatory threshold, VO2 at RCP: oxygen consumption equivalent at respiratory compensation point, TTE: Time to Exhaustion, ml: milliliter, min: minute, kg: kilogram, L: Liter, m: meter, cm: centimeter.

4. Discussion

This study investigated the effect of caffeinated gum and repeated mouth rinsing with espresso coffee on professional male table tennis players’ aerobic performance and explosive power. Seven indicators were measured and analyzed to investigate the aerobic performance of the table tennis players. The results showed that TTE significantly increased in CG and CMR conditions compared to PLA, VT₁ was considerably higher in CG and CMR compared to PLA, and RCP increased significantly in CG and CMR compared to PLA conditions. However, there were no significant increases in VO_2max. Additionally, two indicators were measured to investigate the tennis players’ explosive power. The results showed no significant increases in throwing the medicine ball and Sarjent’s jump height between all three conditions.

About these findings, there are also some studies consistent with present research results. For example, in a study, Hogervorst et al. have shown that 300 mg caffeine consumption during 2.5 h of exhaustive exercise improved TTE by about 27% in well-trained cyclists [43]. On the other hand, to determine the effect of caffeine on ventilatory equivalent indices, Ruiz-Moreno et al. conducted a study on 11 aerobically trained people to investigate the ergogenic effect of caffeine on ventilatory threshold and the results showed that the acute consumption of caffeine (i.e. the first day of consumption) with a dose of 3 mg per kilogram of body weight before exercise increased the workload obtained in VT₂, the heart rate in VT₂ and VO₂ at the same time [34]. In addition, Britzke et al. also showed that consumption of caffeine (6 mg/kg of body weight) or placebo compared to the control condition (no supplement or placebo) does not affect the maximum oxygen consumption of the participants, But it increases TTE [44]. Moreover, Taheri et al. in 2023 showed that CMR significantly increased Sarjent’s jump height in futsal players [32]. However, some other studies are contrary to the results of the present study. For example, in the study of Dolan in 2017 on collegiate lacrosse athletes, the positive effect of caffeinated mouth rinsing on physical performance in the Yo-Yo Intermittent Recovery Test (Yo-Yo IRT) was not shown [33]. Additionally, Grgic et al. investigated the effect of 300 mg of caffeine in caffeinated gum. They found that consuming it 10 minutes before exercise leads to energizing effects on jumping performance, maximum isokinetic torque, speed, upper body movement, and total body power output during a rowing test [25]. Methodological differences, demographic characteristics of the participants, and their numbers, the type of supplements consumed, and their dosage can all be the reasons for the contradictory findings compared to the present study.

According to the results of previous research and the present study, it is essential to mention that several main mechanisms have been stated for the effect of caffeine on aerobic performance. One of the proposed processes is maintaining muscle glycogen reserves by inhibiting phosphodiesterase and the antagonistic effects of caffeine on adenosine A₁ and A₂ receptors in the central nervous system caused by caffeine consumption [12,45]. Stimulation of the sympathetic nervous system by caffeine affects several metabolic pathways to improve endurance performance, Such as increasing intramuscular fat and triglyceride lipolysis, preserving carbohydrate stores (glycogen-sparing effect of caffeine consumption) for use during endurance exercise, delaying fatigue by affecting the CNS [46]. Consuming caffeine before activity increases the mobilization and oxidation of fatty acids. This leads to the preservation of muscle glycogen reserves and prolongation of the TTE [46]. Some studies have shown that aerobic exercise and high doses of caffeine (6–9 mg/kg body weight) can change the pathway of substrate oxidation [13]. The following proposed process is that caffeine causes the release of calcium from the sarcoplasmic reticulum, thereby increasing muscle contractility and endurance in laboratory conditions [13,47]. Another proposed mechanism is that caffeine acts as an A₁ and A₂ adenosine receptor antagonist due to its similarity to the molecular structure of adenosine. After consumption, caffeine binds to adenosine receptors, increasing the concentration of neurotransmitters such as dopamine and norepinephrine, which in turn have been proposed as critical regulatory mechanisms to explain the energy-enhancing effects of caffeine (such as TTE) [13,47]. According to the proposed possible mechanisms, it seems that the increase in TTE, VT₁ and RCP in the present study can be due to the above mechanisms.

Also, rinsing the mouth with coffee may have the potential to increase exercise performance due to the activation of the sensory-motor cortex of the brain [17]. In particular, the mouth contains bitter taste receptors sensitive to caffeine [17]. It seems; that stimulation of these bitter taste receptors may activate neural pathways associated with information processing and reward in the brain. Physiologically, rinsing the mouth with a caffeinated solution, such as coffee, can reduce the potential for gastrointestinal discomfort when consuming caffeinated sources [17]. Caffeinated solutions increase motor output and brain activity without increasing plasma caffeine levels; increasing the activity of oral receptors, improves central conduction and arousal, and in this way, it may improve physical performance [12,31].

About the explosive power of the participants, the findings of the medicine ball test showed that the explosive power of the upper body muscles in CMR (%CV = 3.65%) and CG (%CV = 4.56%) increased compared to PLA, but it was not significant. Also, the explosive power of the lower body muscles as a result of Sargent’s jump test of male table tennis players in CMR (%CV = 1.71%) and CG (%CV = 2.38%) improved, but it was not significant. In contrast to the results of the present study, a study showed that Consumption of caffeinated gum with two doses of caffeine (2.7 and 5.4 mg/kg body weight) could not increase the number of throws performed by elite judo athletes during judo performance during its two specific tests (SJFT) [26]. These researchers considered the inadequacy of the prescribed dose, the habit of consuming caffeine in the participants, and the need to repeat the administration of caffeine gum as the reasons for the non-significance of the results. Also, by the opinion of Pickering and Grgic in 2018, stated that the energizing effects of caffeine can be related to the performance level of athletes [48]. Considering that the subjects were elite judokas, this issue was another reason for the ineffectiveness of the supplement used in that study. In the present study, we used expert table tennis players and the same argument can be considered. In the study of Pfeiffer et al. in 2017 on volleyball players, no significant effect of the caffeine supplement with carbohydrates (CHO+CAF) on performance tests of vertical jump, agility and ability were reported [49]. Additionally, in the present study, it seems that these improvements could be significant if the number of participants was greater. In line with the findings of this research, Pirmohammadi et al. in 2023, showed a substantial increase in Sarjent’s jump test results in CMR and CG conditions compared to PLA in table-tennis players [27]. In addition, Chia et al., in 2017 in a review study, stated that the consumption of caffeine at a dose between 3 and 6 mg/kg of body weight before exercise for athletes in ball games improves the performance of speed and vertical jump [50]. Despite the positive results, they considered the effectiveness of caffeine to be different depending on various factors, including the nature of the game, physical condition, and the existence of a caffeine consumption habit. Also, many studies have investigated the effect of caffeine on jumping performance and showed that caffeine supplementation increases vertical jump height during single and repeated jumps with a small effect size from 0.17 to 0.22 [47,51]. Moreover, Nemati et al. in 2023 showed that consuming 6 mg/kg of body weight of caffeine significantly increased Sarjent’s jump height in volleyball players [52].

Caffeine appears to have direct effects on muscle contraction. The proposed pathway is through the mobility of calcium ions (Ca²⁺), which facilitates the production of force by each motor unit [17]. Caffeine binds to A₁ and A₂ adenosine receptors and reduces the effect on the parasympathetic system. The synthesis of neurotransmitters such as dopamine and catecholamines [14] at the peripheral level improves the activity of the sodium-potassium pump (Na⁺ - K⁺) and increases the bioavailability of calcium (Ca²⁺) in the myoplasm [15]. Therefore, it is possible that one or a combination of the mentioned factors can cause more and more muscular contraction and, thus more power by increasing actin binding to myosin. Despite the strong evidence in the field of the positive effect of the CMR on improving the performance of athletes, there are many differences between the findings and its impact on physical performance [12]. One of the possible reasons for the non-significant results of using caffeine solution could be the lack of increase in plasma caffeine concentration after rinsing the mouth with it, and probably an increase in plasma concentration is required to create the energy-enhancing effect of caffeine [53]. In the current research, this issue can be considered as one of the factors of the non-significance of the findings.

The present study had some limitations that need to be considered. Firstly, due to financial constraints, the authors could not measure the changes in plasma levels of caffeine after CMR and CG. It is also worth noting that the sample size in this research was small due to the size of a table tennis team, which typically has 4 to 6 players. Therefore, future studies should include the evaluation of these variables to expand on our findings.

5. Conclusion

In conclusion, according to the results of the present study, it can be said that CG and CMR supplementing can be approximately effective on the aerobic capacity of table tennis players, especially TTE. Also, it seems that consuming early absorption sources of caffeine (CG and CMR) could relatively improve the explosive power of male table tennis players; however, there were no significant changes. Furthermore, no substantial difference was observed between CG and CMR effects on aerobic capacity and explosive power. Additionally, based on the findings of this study, table tennis players may benefit from incorporating CMR and CG techniques before and during matches to improve their performance.

6. Institutional review board statement

The current study was reviewed and approved by The Human Research Ethics Committee of Shiraz University (ethics approval code: IR.US.PSYEDU.REC.1402.020, 2023) and carried out in accordance with the Declaration of Helsinki.

Informed consent statement

Informed consent was obtained from all subjects involved in the study.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Additional information

Funding

The author(s) reported there is no funding associated with the work featured in this article.