Introduction

Dietary supplement use is greater in athletes than in the general population1. Some supplements, when ingested properly, can improve the athlete’s health and performance, but others are taken even though they have no proved influence. One of the supplements whose effectiveness is indicated in International Olympic Committee (IOC) as well as International Society of Sports Nutrition statements, especially for high-performance athletes, is sodium bicarbonate (NaHCO3)2,3. NaHCO3 supplementation increases extracellular bicarbonate concentration, which causes blood alkalosis4. Because of the greater pH gradient between the muscle cells and extracellular fluids, H+ produced during exercise are transported more easily leads to greater efflux of H+ and La− from the exercising muscle3,4,5. This is particularly important because intramuscular acidosis can cause muscular fatigue based on different mechanisms: (1) impaired glycolysis because of reduced activity of key enzymes such as glycogen phosphorylase and phosphofructokinase; (2) hindered muscle’s contraction capacity due to the competition of H+ with calcium ions; (3) inhibition of oxidative phosphorylation; (4) compromised resynthesis of phosphocreatine at low pH6. The increase in buffer capacity by NaHCO3 supplementation can therefore allow to sustain muscle contractility during intense exercise and delay muscle fatigue5,6.

Improvements in performance contributed to increases in buffering capacity are likely confined to short and high-intensity tasks which can be limited by acid-base disturbances and combat sports are one of them. Wrestling is a high-intensity competitive sport discipline, in which glycolysis is a substantial energy system. For instance, blood lactate concentrations rise up to ~12.5 mmol·L−1 after a simulated wrestling combat7. The anaerobic power is crucial to perform wrestling attacks, to lift and/or throw an opponent during offensive actions in wrestling, as well as to resist the opponent’s attacks8. Thus, wrestling athletes would benefit from NaHCO3 notably. However, studies on NaHCO3 supplementation in wrestling are scarce. In judo, NaHCO3 caused a significant improvement in judo-specific performance, confirming its efficacy in intermittent supramaximal specific to discipline bouts of exercises, in which fatigue is evident9. The Special Judo Fitness Test (SJFT) result was improved in 0.3 g·kg−1 NaHCO3 than in placebo (PLA) treatment9. Furthermore, in striking disciplines such as boxing or taekwondo, the same dose of NaHCO3 increased exercise capacity related to the total punch efficacy10 or the attack time of a simulated taekwondo combat11, respectively. In contrary, in a study by Felippe et al.12 only combined treatment with NaHCO3 and caffeine resulted in significant increases in judo performance compared with PLA. In a previous study by our lab, a dose of up to 0.1 g·kg−1 NaHCO3 had no effect on wrestling performance in specific dummy throw test (DT) compared to PLA13. However, even though the doses used in the study seemed to be too small to improve the power in two Wingate tests (WT) and the number of wrestling suplex throws, the time-to-peak power decreased significantly with NaHCO3, but only in the second WT13.

It is therefore possible that NaHCO3 supplementation effect is more pronounce later in the multiple-bout workout when the capacity of dealing with H+ is significantly exhausted. For instance, in a study by Artioli et al.9 the improvement in peak and average power was observed only in the two final bouts of four 30-s upper body WTs9. Similarly, Tobias et al.14 observed that NaHCO3 improved mean power only in forth bout of 30-s upper WT by ~9% and ~14%, respectively. Olivera et al.15 also noted an increase in total mechanical work done in the last two bouts (bouts 3 + 4: + 5.93%, p = 0.02) of four 30-s sprints.

Since it seems that sodium bicarbonate may influence the latter bouts of high intensity exercise, we aimed at assessing the gender-related influence of progressive-dose NaHCO3 loading regimen on the difference in power between two bouts of Wingate tests separated by wrestling-specific exercise performance test, simulating combat during an intense competition round. We analyzed both genders separately since it seems possible that females respond to sodium bicarbonate to a lesser extent than males. That can result from the differences in muscle anatomy and physiology. On one hand men are usually stronger and more powerful, and on the other hand women are less fatigable16. Moreover, females have smaller type II fibers than men17,18, while males have greater glycolytic capacity19,20. Finally, the exercise-induced pH drop is also greater in males20. Therefore, we hypothesized that NaHCO3 supplementation in men will have a greater beneficial effect on muscle power and throwing performance.

Results

Wingate anaerobic power indices

The interaction with gender was significant for average power (AP) (p < 0.0001), power drop (PD) (p < 0.0001), and peak power (PP) (p < 0.0001). However, no significant differences in AP, PD and PP were found after NaHCO3 and PLA interventions neither in females nor in males (Table 1). There were no significant gender interactions for the differences between WT2 and WT1 (Δ WT2-WT1) in AP, PD and PP. Δ WT2-WT1 in AP, PP and PD were not significantly affected by NaHCO3 and PLA treatments in both genders (Table 2).

Table 1 Power characteristics before and after supplementation in female and male wrestlers.
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Table 2 The difference in power between WT2 and WT1 (Δ WT2-WT1) before and after supplementation.
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The interactions between treatment x period as regards power were significant in seconds 12 (p = 0.0106) and 16 (p = 0.0398) in all wrestlers. Gender interaction was significant in each second of WT. Moreover, gender x treatment interaction was significant in seconds: 10 (p = 0.0343), 11 (p = 0.0438), 12 (p = 0.0153), 15 (p = 0.0461), 16 (p = 0.0365), 17 (p = 0.0280), 21 (p = 0.0248), 23 (p = 0.0377), 26 (p = 0.0474), 28 (p = 0.0304), 29 (p = 0.0181) and 30 (p = 0.0359). In females significant changes were observed in seconds: 1 (p = 0.0204), 12 (p = 0.0180), 21 (p = 0.0070), 25 (p = 0.0343), 28 (p = 0.0083), 29 (p = 0.0294) and 30 (p = 0.0463). In males in seconds: 12 (p = 0.0269), 16 (p = 0.0409) and 17 (p = 0.0082).

In all participants the difference in power indices between WT2 and WT1 (Δ power WT2-WT1) improved significantly NaHCO3post vs NaHCO3pre in seconds: 12 (p = 0.0413), 16 (p = 0.0199) and 21 (p = 0.0430) (Fig. 1a). Furthermore, Δ power WT2-WT1 NaHCO3post was significantly lower than PLApost in seconds: 12 (p = 0.0144), 16 (p = 0.0370), 17 (p = 0.0125) and 21 (p = 0.0166) (Fig. 2a). In second 12 Δ power WT2-WT1 decreased significantly on PLA (PLApost vs PLApre) (p = 0.0368). The gender interactions were recorded in seconds: 13 (p = 0.0382), 17 (p = 0.0174), 18 (p = 0.0187), 22 (p = 0.0428), 24 (p = 0.0082), 25 (p = 0.0149), 26 (p = 0.0144), 27 (p = 0.0123), 28 (p = 0.0336) and 29 (p = 0.0349), respectively. In females Δ power WT2-WT1 increased significantly NaHCO3post vs NaHCO3pre only in second 21 (p = 0.0475) and was higher NaHCO3post than PLApost (p = 0.0130) (Figs. 1b and 2b). In second 12 (p = 0.0388), 21 (p = 0.0406), 28 (p = 0.0294) and 29 (p = 0.0279) Δ power WT2-WT1 decreased significantly on PLA (PLApost vs PLApre). In males Δ power WT2-WT1 increased significantly NaHCO3post vs NaHCO3pre only in second 12 (p = 0.0488) and in second 17 was higher NaHCO3post than PLApost (p = 0.0063) (Figs. 1c and 2c). Furthermore, in second 16 (p = 0.0316) and 17 (p = 0.0045) Δ power WT2-WT1 decreased significantly on PLA (PLApost vs PLApre).

Figure 1
figure 1

Difference in power indices between WT2 and WT1 before and after NaHCO3 supplementation. (a) In all participants, (b) in females, (c) in males. Data are presented at mean ± SD. *NaHCO3post significantly different from NaHCO3pre.

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Figure 2
figure 2

Difference in power indices between WT2 and WT1 (Δ WT2-WT1) in NaHCO3post vs PLApost (a) in all participants, (b) in females, (c) in males. Data are presented at mean ± SD. *NaHCO3post significantly different from PLApost.

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Dummy throw test

The interactions between treatment x period was significant (p < 0.0297). In females the number of completed throws was unchanged NaHCO3pre vs NaHCO3post (from 18.2 ± 2.8 to 19.6 ± 2.2 throws, p = 0.3766) (Fig. 3a). However in males, it increased significantly by ~12% form 19.3 ± 2.6 to 21.7 ± 2.9 throws (p < 0.0001, Fig. 3c). No significant changes were also observed PLApre vs PLApost (females: p = 0.9185; males: p = 0.7174) and NaHCO3post vs PLApost (females: p = 1.0000; males: p = 1.0000) (Fig. 3b,d).

Figure 3
figure 3

Total number of throws in dummy throw test. (a) In females before and after NaHCO3, (b) in females before and after PLA, (c) in males before and after NaHCO3, (d) in males before and after PLA. Data are presented at mean ± SD, and individual raw data. *NaHCO3post significantly different from NaHCO3pre.

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Blood sample analysis

Before and after supplementation no significant differences in glucose, lactate and pyruvate concentrations were found neither for female nor for male wrestlers (Table 3).

Table 3 Glucose, lactate and pyruvate concentrations before and after exercise tests in female and male wrestlers.
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Discussion

In this study we showed that progressive supplementation of up to 100 mg·kg−1 sodium bicarbonate did not significantly influence AP, PD and PP characteristics in two Wingate tests. However, it improved power maintenance in the midsection of the 30-s Wingate test and performance in wrestling-specific dummy throw test. We observed that gender was a significant factor potentially influencing the effectiveness of such a treatment. Gender interaction was significant for AP, PD and PP, but possibly the dose was too small to elicit any significant improvement in those parameters in both males and females. Gender was also significant factor influencing the effect of NaHCO3 on power in each second of the Wingate test and on performance in DT test. What is interesting, males significantly increased the number of throws in DT test, while females did not. That may suggest that the response to NaHCO3 treatment is gender specific.

As previously observed, supplementation with NaHCO3 may improve performance in combat sports9,10,11. NaHCO3 resulted in improvement of boxing (punch efficacy: +5.4%)10, taekwondo (the total attack time in combat: +13%)11 and judo (summed number of throws in three bouts of SJFT: +4 throws)9 specific performance, respectively. In contrast, in our study performance in wrestling-specific DT improved significantly on NaHCO3 with significant gender interaction. Then, when analysing genders separately we found that males increased the number of throws by ~11% (~2 throws), while no significant changes were observed in females. This slight, yet important change could contribute to winning in real wrestling competition. Previous studies on NaHCO3 in combat sports did not include female athletes9,10,11,12.

Gender differences in response to NaHCO3 supplementation are especially worth discussing. Papers with female subjects are scarce. Only one of six studies on women showed the improvement after NaHCO3 intake21,22,23,24,25,26. Kozak-Collins et al.21 supplemented seven competitive female cyclists with either 300 mg∙kg−1 NaHCO3 or PLA (NaCl). 2 h after ingestion participants performed interval cycling protocol consisting of repeating intervals of 1 min 95%VO2max cycling and 1 min recovery at 60 W until exhaustion. They did not find any improvement in the number of completed intervals. In comparison, Price et al.27 recruited only male subjects. Investigators also gave them NaHCO3 or PLA (NaCl) before testing. Participants did two intermittent cycling trials comprised of repeated 3-min blocks (90 s at 40%VO2max, 60 s at 60%VO2max, 14-s maximal sprint, 16-s rest). Authors found that compared to PLA, power output was greater throughout exercise during the NaHCO3 trial. Tiryaki and Atterbom22 assessed the effect of NaHCO3 on 600 m running time of trained females and found no differences (121.5 s on NaHCO3 and 120.4 s on PLA). On the other hand, males improved running time in 400 m distance by 1.52 s on NaHCO3 and in 800 m by 2.9s28,29. Even though there are no studies assessing the effect of NaHCO3 on 600 m run in males, it can be expected that it would be also improved. Then, there are four studies on female team sports players23,24,25,26. Macutkiewicz and Sunderland23 observed no influence of NaHCO3 on Field Hockey Skill Tests and the Loughborough Intermittent Shuttle Test in elite female field hockey players. In comparison, Krustrup et al.30 found 14% improvement in Yo-Yo intermittent recovery test level 2 performance (735 ± 61 m on NaHCO3 vs 646 ± 46 m on PLA) in trained males. Moreover, Ducker et al.31 and Miller et al.32 observed improved repeated sprint capacity in males on NaHCO3. In a study by Ducker et al.31 subjects did three sets of 6 × 20 m sprints with 4 min of recovery between sets. NaHCO3 resulted in the best repeated-sprint performance. In a study by Miller et al.32 male athletes were given NaHCO3 or PLA and then performed repeated sprint cycling protocol comprising 10 × 6 s sprints with 60 s recovery. Total work completed during the repeated sprint protocol was higher in the NaHCO3 condition (69.8 ± 11.7 kJ) compared with both - the control (59.6 ± 12.2 kJ) and PLA (63.0 ± 8.3 kJ) conditions. In a study on female team sports athletes NaHCO3 failed to improve total work in prolonged intermittent sprint performance (IST)24. IST consisted of two 36-min “halves” of repeated ~2-min blocks: all-out 4-s sprint, 100 s of active recovery at 35%VO2max, and 20 s of rest. There was a trend toward improved total work in the second half, but it did not reach statistical significance (p = 0.08). Similarly, no improvement was observed in female water-polo players25. After the ingestion of NaHCO3 or PLA the subjects performed a 59-min match-simulation test (MST) that included 56 ×10 m maximal-sprint swims. NaHCO3 increased blood pH, but failed to improve mean sprint times. The only study to show improvement on NaHCO3 in female athletes is a study by Delextrat et al.26. Participants in that study were university basketball players. The supplementation protocol differed from all other studies. Athletes were supplemented with higher dose of NaHCO3 (0.4 g∙kg−1 compared to 0.3 g∙kg−1) and it was a multiday (3 days) loading. NaHCO3 improved mean values of sprint times, circuit times and jump height compared with PLA.

In summary, out of six trials on female athletes only in one NaHCO3 was proven to be effective21,22,23,24,25,26. On the other hand, males seem to benefit more from the supplementation27,28,29,30,31,32. The reason for that might be in physiological differences. Females have smaller type II fibers than men17,18. Type II fibers rely predominantly on glycolytic energy system. It was shown that males have greater glycolytic capacity19,20. In addition, in females pH drops to a lesser extent that in males during the same type of exercise20. All of that would explain the gender differences in the response to NaHCO3 supplementation observed in our and all previous studies.

Furthermore, it is important to observe that several bouts of intense exercise cause muscular fatigue, which may hamper performance during competition or training. In our study, power characteristics in WT1 tended to be higher than in WT2 (Table 1). One of the factors contributing to fatigue is a decrease in intramuscular pH, which causes reduction in enzyme activation, competitive binding of H+ to the active site of troponin, inhibition of oxidative phosphorylation and compromised resynthesis of phosphocreatine6. Sodium bicarbonate supplementation results in better buffering capacity of blood, which may increase the efflux of H+ and La− out of muscle cells and decrease acidosis4.

It was previously established that the effect of NaHCO3 supplementation may be pronounced predominantly in latter stages of exercise9,13,14,15. Artioli et al.9 supplemented their athletes with 300 mg·kg−1 NaHCO3 2 h before exercise. The performance test included four bouts of 30-s upper body WT tests. The significant changes in AP and PP were observed only in the two final bouts. This was attributed to improved resynthesis of phosphocreatine due to alkalosis caused by NaHCO3 supplementation, since low intramuscular pH may hamper this process9.

Tobias et al.14 assessed the effect of one week NaHCO3 ingestion on four-bout upper-body WT performance. Single bout was 30 s long with the load of 5% body mass. Seven-day supplementation resulted in 8% increase in total work done (in all four bouts summed). However, when the bouts were analysed separately a significant increase in AP and PP was present only in the last bout (+9.4%, p = 0.038, and +13.7%, p = 0.018, respectively)14.

A subsequent study by Oliveira et al.15 confirmed those results. They adopted a similar protocol of performance testing (four 30-s WT bouts for upper body interspersed by 3-min recovery) and also observed a significant increase in the total work done (+2.86%, p = 0.02) after 5-day NaHCO3 supplementation compared to PLA. And again the difference was more pronounced in the last two bouts (sum of bout 3 and 4: +5.93%, p = 0.02).

Since aforementioned studies9,14,15 showed that the effect of NaHCO3 is apparent the most in latter stages of intense exercise, we aimed at assessing the gender-related effect of NaHCO3 on the difference between the first and the second WT, which were additionally interspersed by DT. Dummy throw test is a highly strenuous test, specific to wrestling. It is comprised of two alternating modes – slow and fast13. The slow mode lasts 30 s, during which an athlete does four compulsory dummy throws. Whereas, in the quick mode an athlete performs as many throws as possible in 15 s. The test lasts 3 min and comprises four slow and four quick modes, so that it is highly exhausting. Thus, the participants of our study were already fatigued on the onset of the second WT. Even though the difference in PP between WT2 and WT1 tended to be improved by NaHCO3 (by 35.1 W and 35.0 W in females and males, respectively), they were not statistically significant (Table 2).

Furthermore, innovative analysing (in the field of NaHCO3 supplementation) of each second of WTs separately significant improvement (NaHCO3post vs NaHCO3pre) in the difference in power between WT2 and WT1 were observed in seconds 12th, 16th and 21st when all participants were taken together. In females the significant difference was apparent only in 21sts (NaHCO3post vs NaHCO3pre), whereas in males in 12ths (NaHCO3post vs NaHCO3pre). Compared to PLA, on NaHCO3 the difference in power between WT2 and WT1 improved in seconds 12th, 16th, 17th and 21st in all participants. In females, significant improvement was observed in 21sts and in males in 17ths (NaHCO3post vs PLApost). It therefore seems reasonable to emphasize that most of the substantial effects were observed in the case of this supplementation protocol in the middle (12–21s) of the WTs.

In spite of the few significant differences observed in our study, we hypothesise that the dosage of NaHCO3 might have been too small for female and male wrestlers to elicit more apparent improvements. We used up to 100 mg·kg−1 NaHCO3 in days 8–10 of supplementation (Fig. 4). The dosage was well tolerated and did not cause any gastrointestinal (GI) problems, but the effectiveness was slight and moderate. Simultaneously, in previous studies higher doses were usually implemented9,14,15,33. IOC recommends the intake of 200–400 mg·kg−1 NaHCO3 60–150 min prior to exercise3. However, in many athletes these doses result in GI distress34. This may prevent the practical use of supplementation with this compound in the conditions of natural high-intensity effort that is carried out, e.g. in combat sports. On the other hand, smaller doses might be ineffective. For instance, in nine healthy males the dose of 100 mg·kg−1 failed to induce alkalosis, increase base excess and had no influence on work output35. Furthermore, in six males McKenzie et al.36 showed that even though induced alkalosis was greater with 300 than 150 mg·kg−1 NaHCO3, there were no differences in work produced (133.5 and 133.1 kJ, respectively) and time to fatigue in the last bout (106 and 110s) between those two doses. However, comparing all those results to ours is limited because all of them used acute supplementation protocol, while participants in our study ingested NaHCO3 for ten days. In a previous study done by our lab, progressive-dose protocol of NaHCO3 up to 150 mg·kg−1 was enough to improve CrossFit-like performance and ventilatory threshold37. However, NaHCO3 supplementation protocol similar to the one used in the current study (10 days, up to 100 mg·kg−1) improved only time to PP in the second WT test with no further influence on anaerobic capacity and performance13. Nevertheless, we would like to highlight that in our research only highly-trained female and male wrestlers participated. Therefore, the observed changes related to males wrestling-specific performance and more effective maintenance of anaerobic power during high-intensity efforts, that can be considered beneficial at elite sport level, especially considering the short time duration of supplementation and a low dose of NaHCO3. It is worth bearing in mind, however, that a certain limitation to our study is the lack of verification of the bicarbonates concentration in the blood, which should be included in the subsequent studies, preferably in connection with the attempt to evaluate the effectiveness of supplementation of various doses of NaHCO3. Another limitation is the uneven distribution of participants in study groups. It is possible that if the number of participants was equal in each group the gender differences would be more pronounced.

Figure 4
figure 4

Flowchart of the study design.

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Conclusions

Progressive low-dose NaHCO3 supplementation allows in combat sports the advantageous suppression of fatigue-induced power decline in the midsection of the 30-s Wingate test and improvement in wrestling specific dummy throw test. The response to NaHCO3 supplementation seems to be gender dependent. It appears that males can benefit more from the sodium bicarbonate supplementation, possibly because of physiological differences.

Methods

We would like to clarify that in this work we used the data previously collected in sodium bicarbonate studies involving wrestlers, which we conducted in our lab. We have already partially published the selected results obtained from most of the evaluated participants13. However, the data presented here was analyzed in a completely different fashion. The results for female and male wrestlers were analyzed separately to assess whether the response to the supplementation protocol may be determined by the gender of the athletes. We also focused on previously untouched aspects of the detailed change of power indices during each seconds of the Wingate test. Additional athletes were also included. Thus, we can unequivocally state that there is absolutely no risk of duplicate results, but we want to inform potential readers about the details of the data processing. Lastly, from a practical and scientific perspective, the research results which we have presented here are extremely valuable due to the innovative approach we have taken with NaHCO3 supplementation, the detailed analysis of performance indices herein, and the accompanying assessment of gender-related responses to NaHCO3 treatment.

Participants

Forty-six male and thirty-one female wrestlers were initially enrolled in this study. However, thirty-three male and eighteen female athletes participated in the study and were included in the analyses (Fig. 4). Anthropometric characteristics are presented in Table 4. The athletes were members of the Polish Wrestling National Team and/or competed in the highest level of Polish competitions. The inclusion criteria were good health, a valid medical clearance to participate in sports, a minimum of four years of combat sports experience, and doing at least four workout sessions (combat sport) a week. The exclusion criteria were current injury, any health condition preventing from participation, self-declared unwellness, and no interest in proper participation in study protocol.

Table 4 Anthropometric characteristics of female and male wrestlers.
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All athletes reassured that they had not changed their life-styles, training regimen, diet or supplementation, and that they had not been using any medications and supplements with potential ergogenic effects, other than those supplied by the authors of this study. In accordance with the 1975 Declaration of Helsinki, before enrolment all participants had given their written consents to participate in the study protocol. Informed consents were also obtained from the parents of athletes under the age of 18 years, prior to participation in the study. The approval of the Bioethics Committee at Poznan University of Medical Sciences was obtained for this study. This trial was registered at Clinical Trials Gov (website: https://clinicaltrials.gov/ct2/show/NCT03406065; Clinical Trial Identification Number: NCT03406065). The study was registered retrospectively as registration was not required when the study enrolment started. The authors confirm that all ongoing and related trials associated with this intervention are registered. The study complies with the CONSORT statement for randomized trials, as shown in Fig. 4.

Study design and protocol

The study was designed as randomized double-blind placebo-controlled parallel-group trail. The supplementation period lasted ten days. The participants were familiarized with the exercise testing protocol and the equipment on a preliminary meeting with the research team. Anthropometric measurements were also taken on the same day. When enrolled athletes were randomly divided into the treatment groups (the NaHCO3 group or the PLA group). The random allocation sequence and matching were performed using stratified randomization via impartial biostatistics.

The experiment consisted of two separate visits (T1–T2) for each participant. All testing was performed in natural conditions at the Central Olimpic Sports Centers (Spała, Zakopane) and Wrestling Training Centers (Poznań) in Poland. Throughout the study the participants were supplemented with either NaHCO3 or PLA. Exercise tests were conducted before and after each trial at the same time of day. Testing sessions started between 7.30 and 10.00 a.m. each time. To maintain constant conditions the participants were asked to refrain from any strenuous exercise for 24h before the testing.

Supplementation

The participants were supplemented with NaHCO3 for ten consecutive days. Initial dose was much smaller than the dose recommended previously2,3 and was then increased gradually until 0.1 g∙kg−1 was reached. This loading protocol was shown to eliminate any GI side effects13,37. Supplementation protocol is depicted in Fig. 4. Sodium bicarbonate was administered in the form of unmarked disc-shaped tablets (Alkala T, manufacturer–Sanum Kehlbeck GmbH & Co. KG, Germany). The tablets were ingested with at least 250 mL of water and could either be swallowed or dissolved in the mouth. Maltodextrin with NaCl served as PLA. It was administered in a similar tablet prepared by the same producer as of the NaHCO3 tablets.

Daily doses of both NaHCO3 and PLA were split into three even portions. On training days, the tablets were ingested in the morning, in the evening, and 1.5h before training session. On rest days, the supplements were administered in the morning, in the afternoon, and in the evening. To increase adherence the participants were also given personal supplementation plans.

Anthropometric measurements

Anthropometric measurements were taken in the fasted state in the preliminary visit in the morning. Body fat and free-fat mass were assessed based on air displacement plethysmography using the Bod Pod® (Cosmed, Italy)38. Total body water and hydration level were assessed by means of bioelectric impedance, with Bodystat 1500 (Bodystat Inc., UK)39, and via urine specific gravity measurement, with URYXXON® Relax (Macherey-Nagel, Germany).

Exercise tests

Every testing session consisted of two Wingate anaerobic tests interspersed with a dummy throw test. Wrestling-specific performance capacity was measured using a specific dummy throw test described previously13. Wingate tests were performed on a cycloergometer (Monark 894E, Sweden). All recommendations for such tests as proposed by Bar-Or were strictly followed40. External loading was set at 7.5% body weight. The first WT (WT1) was performed 5 min before DT and the second (WT2) 10 min after DT (Fig. 4). Prior to testing all athletes completed 5-min warm-up on cycloergometer at approximately 50 W power. During the test, the athletes were verbally encouraged to exert maximum effort. The recorded results were analysed using the Monark Anaerobic Test Software (ver. 3.0.1, 2009, Monark, Sweden).

Blood samples analysis

Fingertip blood samples were taken twice, one sample before the WT1 and the other 3 min after the WT2. During blood draws the participants seated in an upright position. Blood samples were immediately transferred to microtubes containing 500 µL of 0.6 M perchloric acid. Glucose concentration was measured using a colorimetric enzymatic method with glucose oxidase (Liquick Cor-GLUCOSE, Cormay, Łomianki, Poland). Lactate and pyruvate measurements were performed according to the method described previously13. All biochemical analyses were conducted using a Synergy 2 SIAFRT microplate multi-detection reader (BioTek, USA).

Statistical analysis

The study was designed as a randomized parallel trial. Thus, in statistical analysis a mixed model of repeated measures with known error covariance matrix was used41,42. The random factor was participants nested in groups. Group stand for treatment (NaHCO3 or PLA). Fixed factors were: period (NaHCO3pre-WT1, NaHCO3pre-WT2, NaHCO3post-WT1, NaHCO3post-WT2, PLApre-WT1, PLApre-WT2, PLApost-WT1, PLApost-WT2), gender, times (period) (1–30 seconds of WT). Two-way interactions (gender × treatment, treatment × period, gender × period, and treatment × times (period)) and three-way interactions (gender × treatment × period, and gender × treatment × times (period)) were considered. Tested error covariance matrix structures included: Compound symmetry, Autocorrelation, Toeplitz and Unstructured. The choice of model with adequate covariance matrix structure was done according to Akaike information criterion43. Because gender and gender interactions with other factors were usually significant, those analyses were performed also for both genders separately. Statistical significance was set at p < 0.05. The assumptions of normality and homoscedasticity was tested using the Shapiro-Wilk test for normality. If data did not meet the assumptions then the Box-Cox transformation was used. Data were analyzed by own calculations and using the SAS 9.3 software program. Effect size was calculated as Cohen’s ƒ2, as follows: f 2 = h 2 /(1 – h 2).

Ethical approval

All procedures performed were in accordance with the ethical standards of the institutional and national research committee and with the 1975 Helsinki declaration and its later amendments or comparable ethical standards.

Practical Applications

Our study suggests that 10-day progressive low-dose (from 0.025 g·kg−1 (days: 1–2) up to 0.1 g·kg−1 (days: 8–10)) NaHCO3 supplementation allows the advantageous strengthening of wrestling-specific performance in males and suppression of fatigue-induced average power decline in combat sports, which is a result of specific physical efforts. It can also lead to maintenance of high anaerobic power mainly in the midsection of the 30-second Wingate test. Moreover, the higher dose could be more effective in this respect, which indicates that despite the lack of effect on GI functioning, doses lower than 0.1 g·kg−1 BM do not seem to be effective in combat sports. It seems, however, that the response to NaHCO3 supplementation may be gender dependent, and males could be more prone to sodium bicarbonate supplementation.