The use of nutritional ergogenic aids has become more popular for professional and recreational athletes to enhance their performance and to accelerate their recovery process (14). In sports sciences, a nutritional ergogenic aid can be defined as substances or procedures used for the purpose of enhancing performance. Although the term nutritional ergogenic aids is the most common name in scientific literature referring to anything that enhances performance, these products are also commonly known as nutritional supplements, dietary supplements, or sports supplements (89,90). Nutritional ergogenic aids marketed in the form of dietary supplements accounted for approximately $660 million in US sales in 2013 (Internacional E. Vitamins and Dietary Supplements in the US. 2014. http://www.euromonitor.com/vitamins-and-dietary-supplements-in-the-us/report). In addition, 80% of German athletes (17), 89% of American university athletes (43), 98.6% of Canadian university athletes (80), and 88.57% of Irish athletes confirmed taking at least 1 supplement (105).
Tennis is an intermittent sport with match duration from 1 hour to more than 5 hours characterized by short bouts of high intensity intermittent exercise (4–10 seconds), a short break between points (10–20 seconds), and moderate rest between games and sets (90–120 seconds) (38,75). Because of intermittent activity during tennis play, tennis players could enhance their performance on court with use of several ergogenic aids; caffeine (CAFF) may delay fatigue in long matches, creatine (Cr) may enhance the resynthesis of phosphocreatine, β-alanine (BA) and sodium bicarbonate (SB) may buffer lactic acid, and nitric oxide precursors may promote cardiovascular responses.
Another aspect is thermoregulation in tennis players. A few tournaments during the year are played in extreme weather conditions (>40°C) (i.e., Australian Open). Recent published studies in tennis (46,102) simulating these conditions in the extreme conditions mentioned above (the trials were realized at 36.8 and 39.3°C) found that physical performance deteriorated after 2 hours. Tournaments in heat conditions could lead to heat-related illnesses such as heat exhaustion and heat stroke. Also, it was reported that tennis players need at least 24 hours after matches to recover. See the review by Kovacs (77) for more information. Some recovery strategies could speed recovery such as cold treatments, compressive clothing, and fluid replacement. Regarding hydration techniques, ranges of sweat losses could vary between 1 and 2.5 L/h, and tennis players should avoid 2% dehydration during tennis matches. Some researchers have reported hydration strategies such as to drink >200–400 mL of a fluid replacement beverage per changeover (76).
The aim of this review is to clarify which ergogenic aids can improve performance in a sport as complex as tennis, which requires a mixture of short-distance speed between 0 and 20 m (47) agility (111) and power (107), combined with medium to high aerobic and anaerobic demands (38). This review looks specifically at 3 ingredients with an abundance of scientific support (i.e., CAFF, Cr, and SB) and 2 others that show promise in the scientific literature (i.e., nitric oxide modulators (NO) and BA).
CAFF (1,3,7 trimethylxanthine) is metabolized by the liver and, through enzymatic actions, and results in 3 metabolites: paraxanthine, theophylline, and theobromine (51,53,57).
The most common administration method for CAFF is oral consumption. It is interesting to note that 74% of Spanish athletes in national and international events consume CAFF at different doses (31) and 27% of American and Canadian youths (11–19 years) also take CAFF before competitions (127). This supplement has global effects on the central nervous system (affecting cognitive performance and mood states), including hormonal (catecholamine excretions), metabolic (glycogen sparing), muscular (enhancing endurance and strength and power values), cardiovascular (increasing heart rate), pulmonary (higher values of ventilation), and renal functions (more blood flow) during rest and exercise (117). Since 2004, when the World Anti-Doping Agency eliminated CAFF from the list of banned substances, its consumption by athletes has increased (31,35).
The recommended CAFF dose to obtain significant improvement in performance is 3–6 mg/kg bw (body weight) (14,26,30). Therefore, the use of lower doses (i.e., less than 2 mg/kg bw) has shown inconsistent results from a performance perspective (10,68). CAFF is rapidly absorbed by the body and appears in the blood within 5–15 minutes (52), reaching a peak between 45 and 60 minutes after ingestion, without showing statistical differences when it is administered as capsules or in beverages (80). However, CAFF chewing gum has demonstrated faster absorption when compared with CAFF capsules (71). The half-life of CAFF is between 2.5 and 10 hours (84).
In endurance sports, CAFF intake has demonstrated a strong performance enhancing effect with low to medium doses (i.e., 3–6 mg/kg bw) (51–53,83,92); however, similar results are not obtained with doses higher than 9 mg/kg bw (50,100). The same results have been observed in activities that require short efforts such as multiple sprints (i.e., tennis rallies) (48,115). Until recently, there has been much debate about the usefulness of CAFF in sports that are highly dependent on strength levels. Recent research has demonstrated that small to moderate CAFF doses (3–6 mg/kg bw) increase strength and power output (32,49,96,131). However, maximum strength is only enhanced with high doses (9 mg/kg bw), and the secondary effects associated with CAFF ingestion should be considered (98). CAFF ingestion could also repair the detrimental effects on neuromuscular performance associated with the circadian rhythms when training early in the morning (96), the effects being more evident in the lower-body musculature (97).
Research on the effects of CAFF on tennis performance has been less extensive than in endurance sports. There is little evidence showing CAFF's effect on tennis performance (Table). As mentioned before, tennis is an intermittent sport because performance is the product of the interaction of different qualities (speed, power, endurance, etc.) (38,75). CAFF has demonstrated performance enhancement in prolonged exercise, such as multiple sprints, strength, and muscle power, all of which are qualities required for success in tennis. To our knowledge, all research that has assessed tennis performance related to CAFF ingestion used small to moderate CAFF doses (between 3 and 6 mg/kg bw). In one study, Klein et al. (74) found that CAFF administration of 6 mg/kg bw had positive impacts on enhancing tennis performance during a tennis skill test when compared with placebo (PLAC). However, the effect of CYP1A2 (a liver enzyme that contributes to CAFF metabolism) increasing has no apparent influence on tennis performance (74). In another study, 4.5 and 4 mg/kg bw of CAFF was administered to men and women tennis players respectively. The study evaluated different parameters during a 4-h tennis match (i.e, sprint performance, hitting accuracy and games won) (41). Only women reported improvements in the number of games won with respect to PLAC conditions, with no changes observed in men between the different protocols. The researchers claim that the differences found could be due to the normally lower CAFF consumption among females versus males (70 versus 110 mg/d) although relative CAFF dose across genders might be similar. However, another study performed with the same group and with the same CAFF doses for men and women (4.48 mg/kg bw) did not report any benefits in any of the physical parameters that were measured (124).
Hornery et al. (65) compared the consumption of CAFF (3 mg/kg bw), a 6% carbohydrate (CHO) solution, cooling use, and PLAC during 4 simulated tennis matches; only the CAFF protocol was able to reduce the effects of fatigue during tennis matches and increase serve velocity in the final set of the matches. Strecker et al. performed 2 studies to determine the influence of CAFF ingestion on tennis skills performance. In the first study (122), the subjects received a CAFF dose of 3 mg/kg bw combined with a CHO solution or PLAC before 90-minute trials of simulated tennis against a ball machine. For every 30 minutes of the match, the subjects performed a tennis skill test consisting of 15 groundstrokes (forehand/backhand) in all 4 directions: cross-court and down-the-line to a specific target on the court. Although forehand performance was enhanced in CAFF protocols, backhand performance did not reach statistical differences between CAFF versus PLAC. The second study (123) used the same CAFF doses as in the previous study (3 mg/kg bw) but in a liquid form, and the subjects of the study played 90 minutes of simulated matches. The CAFF protocol showed an increase in tennis performance at latter stages of the matches. The CAFF dose administered did not have a negative effect on hydration status before or during matches when compared with PLAC conditions.
Reyner and Horne (110) studied whether CAFF ingested in small amounts (80 mg) could counteract the detrimental effects associated with a 33% reduction in sleep (5 hours) compared with normal sleep (8 hours) in a serving accuracy test. The researchers concluded that CAFF ingestion is no substitute for lost sleep. However, the weakness of the study, in our opinion, was that the dose of CAFF administered did not reach the ergogenic threshold of 3–6 mg/kg bw. Future studies should consider enhancing the CAFF doses administered to know whether CAFF ingestion could really counteract the effects associated with sleep loss. In a classic study, Vergauwen et al. (128) compared the consumption of CHO (0.7 mg/kg bw), CHO + CAFF (5 mg/kg bw), and PLAC and examined the effects on 2 different performance protocols; the Leuven Tennis Performance Test (LTPT) and shuttle run (for protocols details see (129)). On each occasion, they performed each test before and after 2 hours of strenuous training sessions. These protocols showed that CHO + CAFF ingestion did not produce any benefits compared with CHO conditions.
Finally, a recent study by Gallo-Salazar et al. (44) showed that 3 mg/kg bw CAFF in liquid form increased handgrip force in both hands, running pace at high intensity, and the number of sprints compared with the PLAC protocol, whereas other aspects such as ball velocity during the service test remained unchanged during simulated tennis matches.
Side effects associated with CAFF are mixed. Historically, it has been reported that ingestion of CAFF affected fluid balance causing an increase in the urination rate and, consequently, increased dehydration. However, a recent study by Killer et al. (73) has shown that ingestion of moderate doses of CAFF did not affect the rate of fluid reduction and, hence, the rate of dehydration. If CAFF ingestion is high (>9 mg/kg bw) (100), these negative hydration effects do occur. Therefore, CAFF is a useful and safe substance that has been shown to benefit performance in low and moderate doses (3–6 mg/kg bw). Only the use of high doses (>9 mg/kg bw) seems to cause undesirable effects such as increased urine flow, gastrointestinal problems, heart palpitations, etc. CAFF ingestion before matches/training sessions may be a useful ergogenic aid to increase tennis performance, although future studies should determine the optimum dose.
Cr or α-methylguanidinoacetic acid is a nitrogenous compound that naturally exists in the skeletal muscle in equilibrium with phosphocreatine (70). The first studies of Cr supplementation began in the 1900s; and in the last century, the studies on this ergogenic aid have increased substantially. Cr is produced endogenously, mainly in the liver, at a rate of 1–2 g/d and an additional 1–2 g/d of Cr is obtained from dietary intake (27,95). This substance has been proven to be an important stimulant aid for neuromuscular (130) and cardiovascular diseases (91), and in the near future, it appears that this substance may have even more therapeutic effects (i.e., cancer, type 2 diabetes, etc.) (54). Cr is currently considered to be an effective ergogenic supplement by different nutritional and sports medicine organizations (18,126).
The most common use of Cr administration starts with a loading phase, consisting of 4 repeated doses of 5 g separated by 5–7 hours during 3–5 days and a maintenance dose of 3–5 g/d, which show a 17–20% increase in intramuscular Cr levels (95). Other protocols have proven to have the same success or even better results, such as doses of 0.25 g Cr/kg fat-free mass/d (19), 3 g of Cr per day during 30 days (69), or 20 doses of 1 g of Cr during the day (114). Furthermore, Cr bioavailability is better when it is consumed in conjunction with carbohydrates (CHO). Ideally, the CHO loading should be ingested 30 minutes after Cr ingestion to produce peak Cr and insulin concentrations (95). Oral administration of low-medium doses of Cr in humans (1–5g) reaches its maximum plasma Cr concentrations in less than 2 hours, whereas doses above 10 gr reach maximal plasma concentrations of Cr over 3 hours (114). Therefore, the clearance rate of Cr from the blood is highly variable and dependent on intramuscular Cr levels, hormone levels, muscle mass, and kidney function (103).
In general, the studies with Cr supplementation have been based on sports highly dependent on strength and hypertrophy levels (9,118). However, the increase in muscle mass and strength values associated with Cr ingestion have drawn the attention of intermittent sports (e.g., soccer, handball) because of the fact that different physical capacities such as repeated sprints, agility performance, jumping ability, and maximum lower-body strength are necessary for success in intermittent sports. However, other studies did not find improvement in performance in repeated sprints and other variables associated with performance in intermittent sports (28). Regarding tennis performance, Cr ingestion has been less reported; only 2 studies have used Cr ingestion to observe the effects on performance. Eijnde et al. (37) used a Cr dose of 20 g/d during 5 days (divided into 4 doses per day) with 8 well-trained tennis players and evaluated performance on the LTPT and the 70-m shuttle run on 2 different occasions (Cr protocol versus PLAC protocol). No significant differences were reported between treatments in any of the variables, and they concluded that short-term high dose Cr ingestion does not benefit tennis performance.
Pluim et al. (104) observed the effects of both Cr supplementation over short (6 days) and medium terms (4 weeks) compared with PLAC condition period in tennis players. A Cr intervention with a loading phase of 0.3 mg/kg bw during 6 days and a maintenance phase of 0.03 mg/kg bw during 28 days was used. No gains in body weight were reported in the short-term intervention, but gains were reported in the medium term between Cr versus PLAC (+1.4 versus −0.2 kg). Some aspects related to tennis performance were evaluated (i.e., sprint velocity over 5, 10, and 20 m, upper and lower-body strength values, and groundstrokes performance drills). No differences were found for the short or medium term in any variable. As a result, it was concluded that Cr should not be recommended to tennis players.
The controversial secondary effects of Cr ingestion lack supportive scientific evidence. According to the literature, it seems that Cr ingestion may be related to an increase of 1–2% in body weight (79) possibly associated with water retention. Other secondary effects linked to Cr consumption, such as gastrointestinal, renal, and liver damage, have only been anecdotally reported. Future investigations should clarify the issue (18). Currently, Cr is a safe ergogenic aid regarding athlete's health (116) possibly with the ability to positively impact tennis recovery.
SB (NaHCO3) is an extracellular buffer with an important role in maintaining a stable electrolyte gradient between intracellular and extracellular environments (20). SB has been extensively studied in recent years mainly for its properties as a buffering agent. In normal human conditions, arterial blood pH is 7.4 and human muscle pH is normally 7.0. After exhaustive exercise, arterial pH tends to fall to 7.1 and muscle pH to 6.8 resulting in fatigue (94).
The acid-base balance has been studied since the 1930s. In those years, some scientists postulated that the ingestion of alkaloid agents might reduce the decline of muscle pH (33). Studies on SB and athletic performance have been published since the 1980s. The best time for NaHCO3 ingestion is 60–120 minutes before the event, and it must be diluted preferably in about 400 mL of water (101,108); peak blood alkalosis can be expected ∼120–150 minutes after ingestion (24).
The optimum dose of SB ingestion has been a cause of debate. Costill et al. reported several studies demonstrating the efficacy of SB in enhancing performance in several sports (i.e., swimming and cycling) (29,45). In a well-designed experiment (93) with different doses (0.1, 0.2, 0.3, 0.4, and 0.5 g/kg bw), McNaughton et al. were the first to establish that 0.3 g/kg bw was the minimum dose with which changes were noticeable in the variables measured in the study: total work performed and peak power output. Men and women responded the same way to the ingestion of SB, showing the same, or nearly the same, improvements with this ergogenic aid (24). A different published meta-analysis that described SB as a useful ergogenic aid to improve athletic performance reported a small to moderate effect size (0.44 versus 0.36, respectively) (87,101). Training status seems to impact the effect seen from SB use (i.e., untrained people benefit more from SB intake when compared with high-performance athletes, particularly in repeated bouts protocols, time exhaustion test, and short [<2 minutes], medium [2–10 minutes], and long protocols [>10 minutes]) (103). However, although the effects are less pronounced in highly trained athletes, there seems to be evidence that in events characterized by high-intensity protocols and those that recruit large muscle groups, athletes can benefit from SB intake (109). Effects on neuromuscular performance are not clear; although some studies reported positive results (25,36), others have not (133).
Regarding tennis, Wu et al. (134) developed the only study with SB ingestion. Nine male college tennis players in a randomized crossover, PLAC-controlled, and double-blind study investigated the intake of SB (0.3 g/kg bw) or PLAC (0.209 g/kg bw NaCl); the researchers investigated the effect on a skilled tennis performance test (Loughborough Tennis Skill Test) before and after a simulated game of tennis (a duration of approximately 50 minutes). This study suggested that SB supplementation could prevent the decline in skilled tennis performance after a simulated match. Others suggest that SB could be useful for tennis performance (13) through improvement in RSA performance, a quality that has been demonstrated to be important in intermittent sports (e.g., tennis, soccer) (39,106).
SB is associated with a wide spectrum of secondary effects: gastrointestinal upset, diarrhea, and cramps (23). Several strategies have been suggested to minimize the secondary effects, such as familiarization trials and intravenous administration (109). The consumption of food alongside SB reduces gastrointestinal side effects relative to the same dose taken on an empty stomach, and serum increases of bicarbonate seem to be highest when ingested with food (24).
Because of the lack of research regarding the intake of SB on tennis performance, more studies need to be developed regarding this topic. Likewise, because of the minor effect reported in highly trained athletes, interventions would need to examine the effectiveness, or otherwise, of SB in highly trained tennis players. Additionally, studies that combine extracellular buffers (such as SB) and intracellular buffers (BA) need to be conducted to determine whether this substance should be considered an ergogenic aid to tennis performance.
BA is found in muscles in combination with L-histidine forming the dipeptide carnosine. This is found in high concentration in the mammalian skeletal muscle. It is synthesized by the enzyme carnosine synthase from the amino acids L-histidine and BA (34). Although it was discovered more than 100 years ago, the use of this substance to enhance athletic performance is still a new topic (56). It shows a good muscle buffering capacity (MBC) of H+ at a higher rate during intense exercise and is perhaps the most important intracellular buffer (1). The majority of the body's carnosine, over 99%, is present in muscles, whereas other places in the body have small quantities (e.g., brain) (34), with more pronounced quantities in fast twitch fibers at the end compared with those in slow twitch fibers. Furthermore, studies have demonstrated that men have approximately 20–25% more carnosine content than do women (86). However, BA is a nonessential amino acid synthesized by the liver (88), which can be ingested through a diet containing animal sources (meat) or through dietary supplements (6). The study of BA has attracted interest because of its direct relation to the synthesis of carnosine. The body is unable to absorb carnosine directly from the bloodstream (88), and concentrations of BA in the muscle are relatively small compaired with histadine and carnosine synthetase (61). Endogenous synthesis of BA is limited to a small amount produced in the liver (88). The synthesis of carnosine in skeletal muscle may be limited by the availability of BA in the diet (113).
The most commonly used dosing regimen to enhance performance provides a total dose of 4–6.4 g/d over several weeks. This total dose is typically achieved by ingesting multiple doses per day (i.e., 4–6 doses) (72,121) in individual dosing amounts of 4–10 g that have shown to cause a 40–80% increase in intramuscular carnosine (57,72). The washout period may take >9 weeks to return to baseline levels (7,119) with a decline rate of 2–4% per week on average, which is a longer (7) and slower process if compared with other substances such as CAFF. A recent meta-analysis showed that the median effect of BA supplementation is 2.85%, being especially effective in events of between 60–240 seconds and >240 seconds but not reaching statistical differences in events whose durations are <60 seconds (58).
Highly trained anaerobic athletes have greater buffering capacity and intramuscular carnosine than untrained people or endurance athletes (99), but BA supplementation improves the carnosine deposits in all of the cases mentioned before. Regarding neuromuscular performance, the ingestion of BA does not seem to improve maximum strength (60,72). These finding are not surprising considering that the improvement in buffering capacity and maximum neuromuscular performance is not limited by acidosis (6). In the tennis field, no studies have been conducted yet, but in complex intermittent sports such as tennis in which the bouts of exercise require that players have a good buffering capacity, BA could be an interesting ergogenic aid. In fact, other alkalinizing agents such as SB have shown good results against the decline in performance during simulated tennis matches (134). Although the main buffering of H+ is generated by bicarbonate, the pH of carnosine (6.83) is closer to the physiological system than the pH of bicarbonate (6.37), which means that it may be used primarily in high-intensity exercise (66). Therefore BA supplementation could contribute to the ability of muscle carnosine to buffer between 7 and 25% of the acid produced (6,56,86). The contribution of carnosine for these purposes may differ depending on the fiber type involvement with greater contributions coming from type II fibers (86).
Secondary effects reported with BA ingestion are symptoms of paresthesia (an unpleasant sensation characterized by the irritation of the skin and prickly sensation) and are reduced or eliminated when the quantity is less than 800 mg per day. To avoid these symptoms, BA should be administered in several doses during the day (58) because of the fact that symptoms of paresthesia are associated with peak blood values of BA serum (120). However, these symptoms of paresthesia were not observed when BA was ingested in conjunction with carbohydrates. (61) This suggests that administering BA with food reduces the maximum concentration in serum by up to 50% because of delayed gastric emptying (56).
In tennis, no studies have been conducted regarding the use of BA, but in complex intermittent sports such as tennis, in which the bouts of exercise require that players have a good buffer capacity, BA could be an interesting ergogenic aid that should be studied. Furthermore, coingestion with other buffering agents such as SB could be another area to explore in future publications.
Nitric oxide (NO) is a labile lipid-soluble gas synthesized at several locations in the body with antioxidant and vasodilator properties that also regulate the use of glucose and oxygen (3). The production of nitric oxide occurs in 2 different ways: NO synthase (NOS) dependent and NOS independent (12). Importantly, it is the first gaseous chemical that has been shown to be produced by living cells to send intracellular signals. The different properties (i.e., vasodilator mechanism) have caught the attention of the exercise physiology field because of the potential beneficial effects of this substance as an ergogenic aid.
NOS-dependent pathway L-arginine (L-Arg) is a semiessential amino acid and also a precursor to nitric oxide (NO), which can be synthesized by the kidneys where L-Arg is formed from L-citrulline. The dietary intake of L-Arg is close to 4–5 g/d (125). Scientific findings have reported that L-Arg supplementation varied between 1.5 and 20 g/d in different studies, with durations of between 1 and 180 days (22,42). The half-life L-Arg after oral ingestion of 6 g is between 50 and 120 minutes (15), and its excretion varies according to the food consumption and renal function of individuals.
The ingestion of nitrate and nitrite can also be reduced to nitric oxide (NO). Nitric oxide deposits can be obtained exogenously through diet, knowing that some kinds of vegetables contain large amounts of nitrates (i.e., beets, spinach, or lettuce). The most common supplementation reported is between 300 and 600 mg of nitrate per day for 1–15 days, eliciting favorable physiological effects (65,71). After bolus nitrate ingestion, plasma nitrate ingestion (nitrate) peaks after 1–2 hours and plasma nitrite peaks after 2–3 hours (71). Finally, baseline values return to normal 24 hours after ingestion.
Over the last several years, consumption of L-Arg has increased considerably among athletes because it increases the blood's acute vasodilatation and has been associated with a neuromuscular and cardiovascular performance enhancement (4). A study on NCAA athletes showed that 8% of males and 5% of females regularly used L-Arg as an ergogenic aid (85). However, the scientific evidence is not so clear. The acute effects of L-Arg administration showed that the majority of the studies have been developed using aerobic protocols, with different results. Although some studies showed an improvement in some parameters with L-Arg ingestion, such as reduced oxygen consumption (V[Combining Dot Above]O2), cost of moderate-intensity cycle exercise, and time to exhaustion (8,135), other studies did not find the same results (81). The scientific literature regarding chronic ingestion of L-Arg is more extensive compared with the literature regarding acute ingestion of this ergogenic aid. However, the results are inconclusive as well. Although some studies showed L-Arg use to have positive effects on cardiovascular and neuromuscular performance (21,22), other studies did not find any differences (2,132). Bescos et al.(11) have developed the only study pertaining to tennis, which included 9 highly trained male tennis players. They followed 3 different diets during 3 days (with 5.5, 9, and 20 g/d of L-Arg) with washout periods of 4 days between trials. Participants performed a submaximal treadmill test until 85–90% V[Combining Dot Above]O2max in which oxygen uptake, heart rate, and blood lactate were measured. No differences were noted between the various protocols with various doses of L-Arg.
The most common method of NOS-independent pathway intake reported is through beetroot juice. A recent meta-analysis by Hoon et al. (63) showed that untrained or recreational athletes showed better improvements with nitrate intake, reported modest improvement with protocols until exhaustion, and showed small improvements in time trial protocols that, though not statistically significant, might be useful for elite athletes. However, the controversy over the usefulness of nitrate ingestion in elite athletes continues because although some studies reported benefits in highly trained rowing athletes (64), other studies did not find improvement in time trial performances (62) or at 1,500 m (16). Regarding tennis, the only study to have been developed is by Aksit et al. (3). The objective of this study was to establish a relationship between tennis performance test results and NOx levels (the sum of nitrate + nitrite). Twenty well-trained tennis players performed three 4-minute bouts and 2 minutes of continuous groundstrokes with balls shot from a tennis ball machine at speeds of 50, 55, 62, and 70 km/h. After this exercise, the participants had 20 minutes of passive rest. After each period and during the recovery phase, NOx levels, glucose, lactate levels, and lactate elimination speed were measured. The study suggested that no significant correlation was found between NOx levels and tennis performance. However, it was suggested that the addition of loads in the third period of tennis training may be beneficial and that the relationship between performance on court and NOx levels and glucose should be studied in real game situations (i.e., official tennis matches).
Secondary effects associated with L-Arg and NO are not well reported in the literature. Basically, the most common side effect reported with L-Arg is diarrhea. An excellent review that has been published recently by Álvares et al. (5) showed that low oral doses of L-Arg (≤20 g) could obtain the same results as those of higher doses (21–30 g) without secondary effects such as nausea, diarrhea, etc., which are associated with doses above 20 g. Regarding nitrate ingestion, it has been reported that supplementation through vegetable sources (mainly beetroot juice) is unlikely to be harmful or have side effects for the organism, even at higher doses. However, nitrite in higher doses may cause hypotension, especially if combined with other vasodilatory drugs (82).
Because of the limited evidence available from studies using L-Arg or NOx in the scientific literature on tennis performance, we should be cautious and wait for further studies to clarify whether this ergogenic aid could be useful in sports as complex as tennis.
Despite the limited evidence we have about ergogenic aids on the tennis court (except for CAFF), a series of recommendations are presented to coaches, strength and conditioning coaches, and tennis-related medical personnel.
CAFF, in small doses (3 mg/kg bw) may improve tennis performance (i.e., more points won with the serve), although further studies should be performed with different doses to determine whether there is an optimum dose.
Although SB and BA, because of their buffer capacities, could have a place in a sport such as tennis in which the ability to recover between efforts is critical, additional studies should be performed to determine their usefulness in the world of tennis. As for L-Arg and NOx, studies in real game situations could be developed to consider them for use as ergogenic aids.
Finally, because of the weight gain in tennis players associated with Cr ingestion and the lack of scientific evidence (because little has been published about Cr on tennis performance), more research is needed during competitive matches and during training blocks to determine whether it may be appropriate at certain times of competition/training.
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