BRIEF REVIEW
International Journal of Sports Physiology and Performance, 2008, 3, 413-423
© 2008 Human Kinetics, Inc.
A Review of Fluid and Hydration
in Competitive Tennis
Mark S. Kovacs
Hypohydration is known to impair performance and increases the risk of heat injury.
Therefore, the consumption of appropriate fluid volumes before, during, and after
tennis play is important to maintain physiological homeostasis and performance.
Tennis is a sport that typically has points lasting fewer than ten seconds, with shortto-moderate rest periods between each work bout. This sequence is repeated over
hours. Most fluid and hydration research has focused on continuous aerobic exercise,
which provides vastly different physiological strain compared with tennis practice
and competition. Consequently, practical recommendations on maintaining hydration
status for aerobic continuous exercise may not be appropriate for tennis athletes.
Tennis players can sweat more than 2.5 L·h−1 and replace fluids at a slower rate during
competition than in practice. In warm and hot environments, electrolyte-enhanced
fluid should be consumed at greater than >200 mL per changeover and ideally closer
to 400 mL per changeover. Tennis scientists, coaches, and players need to individualize hydration protocols to arrive at the optimal hydration strategy.
Keywords: hypohydration, heat stress, dehydration, electrolytes, euhydration
Understanding fluid and hydration status is one aspect in optimizing tennis
performance. Tennis is a sport that typically has points lasting fewer than ten
seconds, with short-to-moderate rest periods dispersed between each work bout.1
Tennis is often played in warm-hot environments for multiple hours at a time. This
combination of repeated moderate-high intensity activity over a period of hours
challenges the tennis athlete to maintain adequate hydration status both for
performance enhancement as well as the health and safety of the tennis athlete.
Exercise-related hypohydration (less than optimal hydration) can reduce
performance and lead to health risks.2–4 Substantial research has been performed
on fluid and hydration in aerobic-focused activities;2,5–8 however, tennis players
compete in an intermittent, highly anaerobic state, indicated by high maximal
oxygen consumption (VO2max) and exercising heart rate (HR) that could classify
tennis athletes as highly anaerobically trained athletes.9,10 This may conceivably
require different fluid and hydration requirements than traditional aerobic activities
(eg, marathon running). In this review, physiological mechanisms leading to
performance improvement/decrement will be examined in conjunction with
Kovacs is with Player Development, United States Tennis Association, Boca Raton, FL.
413
414 Kovacs
practical implications for the player and coach. For a general review of physiological
mechanisms related to fluid ingestion and electrolytes, see the work of Sawka and
Coyle.11
A MEDLINE database search found 14,423 entries using the search word
hydration; however, only 8 entries came up when hydration was followed by the
word tennis. When the same search was performed in the SPORTDiscus database,
1280 entries were found for hydration, and when tennis was added, 30 responses
were found. It is clear that the knowledge and resources are available for in-depth
hydration studies in tennis, but little research has been performed. However, the
limited research that has been performed provides a clear picture of the fluid and
hydration challenges of competitive tennis play. The literature on hydration in
tennis was reviewed by using MEDLINE and SPORTDiscus database searches
and cross-referencing these articles using the search terms hydration, dehydration, fluid consumption, hypohydration, and tennis. The purpose of this review
was to develop practical recommendations for tennis athletes based upon the evidence-based scientific literature from both controlled laboratory studies as well as
field testing in both practice and competitive tennis situations.
Indicators of Fluid Loss
The initial drive to drink for most athletes is instigated by the thirst mechanism;
however, thirst is not a good indicator of body water status.12 Ad libitum (voluntary) drinking typically leads to involuntary hypohydration, as 1.5 L of body water
could be lost before thirst is perceived.13 A tennis player’s environment and sweat
rate both are vital factors in contributing to hypohydration; however, a player’s
on-court fluid intake routine is equally important.
The production of sweat will cause an increase in plasma osmolality that
increases the drive to drink fluid, owing to the partial plasma retention of Na+.14
With the consequent increase in osmotic gradient (fluid moves to equalize solute
concentrations) supported by an exercise-induced increase in intravascular albumin and loss of free water,15 fluid is mobilized from the intracellular compartment
to maintain the extracelluar fluid volume.14 This relocation of fluid from the cells
into the vascular system reduces hypovolemia (an important stimulus for thirst
and drinking). Notably, no substantial correlations between postmatch perceived
thirst and sweat rate or body weight percentage change were found in tennis players.16 This observation supports the notion that thirst is not a sensitive indicator of
body water status or a sufficient stimulus to prevent a substantial net body water
loss during exercise in a hot environment.13
A player’s thermoregulatory capacity can be diminished if adequate fluid
intake is not maintained.8,14,17 A fully hydrated, average-sized male tennis player
(mass ~80 kg) contains approximately 48 L of water.12 Tennis practice and matches
can last up to five hours, and players’ sweat rates are often above 1.5 L·h−1. It is
not uncommon for athletes to have sweat rates greater than 2.5 L·h−1,12,18 which
may be more than twice that of the gastric emptying rate (1.2 L·hr−1) for
beverages.19–21 Attempting to keep pace with a sweat rate of greater than 1.5 L·hr−1
is a practical and physiological challenge. During a study of collegiate tennis
players, the athlete’s voluntary water consumption was approximate 1.0 L·hr−1,16
Hydration in Competitive Tennis 415
which is close to the typical gastric emptying rate of 1.2 L·hr−1. The coupling of
voluntary consumption and gastric emptying may be the subconscious desire of
the athletes to avoid gastrointestinal discomfort. Another study examining ad libitum fluid intake during tennis competition found that water intake represented
only 27% of the total fluid loss.22 In a more recent study it was found that a sample
of tennis players consumed on average 1.6 L·h−1 of fluid.18 Hopefully these observations are evidence that contemporary hydration education is having some effect
on tennis players’ fluid consumption habits. These consumption rates, even though
improved, still could lead to a body fluid deficit of approximately 1 L∙h−1 or greater
in warm-to-hot environments (assuming a sweat rate > 2.5 L·h−1). This level of
consumption equates to a 1–2% reduction in bodyweight per hour. In a three hour
match this would lead to hypohydration of 3 to 6%. This magnitude of hydration
deficit may result in a reduce strength and endurance performance by greater than
30% and severely inhibit temperature regulation, inducing headache, nausea, and
other hypohydration-related symptoms.3
Competitive factors, such as a close match or highly energized crowds, can
inhibit the drive or motivation to consume fluids to a greater extent than during
practice.24 The volume of fluid consumed was less than required for replacement
needs, which would, if prolonged, lead to hypohydration. This finding has obvious implications for players preparing for and competing in major tournaments.
Hypohydration and Tennis Performance
Exercise performance can be impaired when an individual is hypohydrated by as
little as 2% of body mass, and a loss of 5% can decrease work capacity by about
30%.13,25 Given sweat rates of approximately 2.5 L·h−1,12 some tennis athletes lose
greater than 3 L·h−1.18 Clearly, hypohydration is a constant threat to tennis performance as well as health and safety. Apart from lowering body fluid levels, a hypohydrated athlete has a reduced capacity to dissipate heat as a result of reduced
blood flow to the skin, which usually results in an increased core body
temperature21,26 (see Figure 1). The combination of hypohydration and increased
core temperature can reduce cardiac stroke volume by ~20% during exercise.27
This reduction results in an increased heart rate and requires the athlete to work at
a greater intensity, which continues the cycle of rising core temperatures. There is
evidence that for every 1% of body mass loss due to hypohydration, heart rate
increases 5 to 8 beats·min−1 and core temperature by 0.2 to 0.3°C11,21,26 (see Figure
1). However, when trying to offset hypohydration, for every 1 L of fluid ingested,
core temperature reduces by ~0.3°C and heart rate by 8 beats·min−1 28 (see Figure
2). When hypohydrated, it is clear the cardiovascular demands are increased to
maintain performance when fully hydrated.
Even a small reduction in body weight (<3%) due to hypohydration from
anaerobic exercise can have a negative effect on tennis players’ 5- and 10-m sprint
times.4 Rehydration (2 mL·kg−1 in an initial bolus within 15 minutes of the completion of exercise followed by 9 mL·kg−1 in equal volumes every 15 minutes for
a total of 90 minutes) during exercise improved sprint times to preexercise levels.4
Given that it takes between 40 and 60 minutes for ingested fluid to improve physiological symptoms of reduced performance such as heart rate, core temperature,
Figure 1 — Effect of body fluid loss on heart rate and core temperature (information
adapted from [9] [12] [18][19]).
Figure 2 — Effect of rehydrated fluids on heart rate and core temperature of hypohydrated
athlete (information adapted from [12]).
416
Hydration in Competitive Tennis 417
blood volume, and plasma osmolality,29 it is more important to prevent hypohydration than trying to rehydrate. This requirement highlights the importance for
tennis athletes to have a planned fluid ingestion schedule before and throughout
the tennis match to prevent, rather than react to, fluid imbalances.
Apart from the detrimental physiological effects that hypohydration has on
tennis athletes, their cognitive function, decision making, and proper execution of
complex skills can also be impaired.30 Tennis is a sport that requires fast reactions
and consistent concentration over an extended period of time. Top performance
also requires players to choose and execute high percentage tennis strokes over an
extended period. Maintaining high cognitive function may be more of a concern
in tennis than in some of the sports that have received the majority of hydration
research (eg, cycling and running).
During tennis play, as the magnitude of hypohydration increases, there is an
accompanying increase in core temperature of between 0.10 and 0.40°C for each
percent decrease in body weight14 (Figure 1). The aim of training programs, ergogenic aids, and recovery time is to optimize training time and performance while
limiting the deleterious effects of increased body temperature and hypohydration.
A homeostatic explanation for hypohydration is that thermoregulation is sacrificed for cardiovascular stability on the assumption that the hypohydrated athlete
will stop playing and move to a cooler environment.14 This strategy may be effective for typical individuals; however, it would not be effective in competitive situations, wherein the inherent desire of athletes to deal with adversity and push
through physiological barriers is at the core of their success.
Electrolytes and Tennis
Because tennis induces large sweat rates, maintaining electrolyte levels is paramount in order to limit the negative effects of hypohydration. Under typical physiologic conditions for acclimated athletes, potassium (K+) and magnesium (Mg+)
concentrations will not be high in sweat.12 Contrary to the belief of many coaches
and athletes that K+ depletion is a major cause of heat-related muscle cramps,
some research supports the relationship between heat-related muscle cramps and
extracellular Na+ depletion—not K+ depletion.12,31 Potassium loss in sweat during
exercise is rather small, relative to whole-body K+ stores and, consequently, has
minimal physiological or performance consequence.32 Magnesium is another
electrolyte that many health professionals recommend, mainly as a deterrent to
heat-related muscle cramps. Magnesium sweat loss is also minimal, but reduced
levels of Mg+ have been seen in people who suffer from muscle cramping; therefore, a noncausation link has been made.33 However, a lower plasma Mg+ level
during exercise is more likely due to compartmental fluid redistribution rather
than to sweat loss.33 So supplementing with Mg+ or K+ may not reduce the likelihood of exercise-induced muscle cramping and is not recommended as a beneficial supplement for hydration purposes,33 but in appropriate doses should be
harmless.
Although exercise-induced muscle cramping has multiple causes, the repeated
high Na+ losses (due to sweating), which reduce extracellular Na+ content, especially if daily Na+ ingestion (through diet) is low, may partly explain why some
418 Kovacs
players may cramp in the later rounds of tournaments or toward the end of a
strenuous training or match day. Hypohydration and electrolyte loss are not the
sole reasons for muscle cramping.31 Although exercise-induced muscle cramping
is still not fully understood, it is plausible that psychological stress may contribute, as anecdotal reports indicate a large number of muscle cramping incidents in
tennis players take place during stressful situations in matches.
The combined effect of large sweat Na+ losses with ingestion of a large quantity of hypotonic fluid (eg, unsalted water), could significantly dilute plasma Na+
(hyponatremia).34 Hyponatremia can have serious consequences, including
fatigue, rises in core temperature, muscle cramping, and reduced performance.35
Hyponatremia is not as common in tennis play as in other sports such as marathon
running. More prominent problems are hypohydration-related fatigue, rises in
core temperature, muscle cramping, and reduced performance.35
Gender has a large affect on fluid loss during tennis play. A study evaluating
a broad range of fluid-electrolyte responses in males and females to competitive
match play in a hot environment found that sweat rates of males were consistently
higher than those of females, even when the per-hour sweat rates were expressed
relative to estimated body surface area.16 Males, as would be expected, had larger
fluid losses than the female subjects, but the fluid intake was similar. Males have
been shown to have greater sweat rates in other, nontennis, exercise research.36,37
As a result, fluid consumption guidelines should be different between the genders.
Males should attempt to consume greater relative fluid levels during training and
competition. At the current time, no gender-specific guidelines for tennis are
available for tennis and research in both the laboratory and field settings is
needed.
Reduced hydration status, susceptibility to low electrolyte levels, and resultant performance decrements are more commonly experienced by athletes in the
later stages of tennis tournaments. During a four-day tournament, more than half
the tennis athletes had less-than-acceptable hydration status as measured by urine
specific gravity (USG) readings of >1.025.16 Tennis athletes should have a USG
of <1.010 before practice or match play to indicate an appropriate hydration level.16 Moreover, compared with Na+ (and chloride), the other electrolytes (Mg+ and
K+ specifically) have low concentrations in sweat;5 therefore, it is recommended
that if the regular diet is providing an adequate amount of electrolytes, Na+ should
be the main electrolyte supplemented in on-court fluid ingestion (20–40 mmol·L−1
of Na+) to help offset the Na+ lost in an athlete’s sweat. This practice is even more
important when training or match sessions are long lasting.
Fluid Intake and Requirements
Before a Practice or Match
A competitive tennis player can practice two or three times throughout the same
day. During tournament time, it is not unusual to play three or even four matches
in a single day. Therefore, fluid replacement after exercise is not only important
from a recovery perspective, but also for match preparations or practice sessions.
In a healthy tennis player, the kidneys excrete excess body water; therefore,
consuming excess fluids typically does not induce hyperhydration. Researchers
Hydration in Competitive Tennis 419
have tried to develop methods to hyperhydrate athletes to see whether this practice
can enhance performance. One method that aids greater fluid retention is supplementing with glycerol,38 but the numerous side-effects have precluded glycerol
supplementation as a practical option.39 It is important, however, to make sure the
tennis athlete is euhydrated before play. The American College of Sports Medicine recommendation that athletes to consume between 400 to 600 mL of water 2
hours before exercise (to allow the kidneys time to regulate total body water
volume), should be used as a minimum standard for tennis players to help promote euhydration.40 This volume would need to be higher if the athlete has lost a
substantial amount of fluid in the previous match or practice session on the same
day, or if the environmental conditions are hot or humid. Prepractice and match
hydration should include some carbohydrate, as well as sodium supplementation.
In warm-to-hot conditions, tennis players should consume salty foods (pasta
sauces, salted pretzels, soups, etc) and add salt to fluids. Approximately 1.5 g∙L−1
of sodium should be included in sports drinks or water in the hours leading to
practice or competition.18
Fluid Intake and Requirements
During a Practice or Match
Most hydration education and implementation is focused on fluid, and to a lesser
extent electrolyte, ingestion during practice and competition. Despite a favorable
osmotic gradient for absorption of water, there is still debate as to which is the
best type of fluid to consume, as CHO-electrolyte drink promotes fluid absorption
better than plain water.8,12,41–43
Fluid volume ingested during practice and play is one area that needs to be
individualized for an athlete’s sweat rate, environment, acclimation, and training
status. A competitive tennis player who has a normal sweat rate of 2.0 L·hr−1
would need to drink 0.25 L (approximately 8.5 ounces) on each changeover
(assuming five changeovers per hour) to replace just 62.5% of the hourly lost
fluid.12 If the player was trying to remain euhydrated (2.0 L·hr−1), then 0.40 L
(approximately 13.5 ounces) is needed on each changeover. Alternatively, 0.30 to
0.40 L of fluid should be consumed every 15 minutes of exercise (1.2 to 1.6
L·hr−1).35 These figures are chosen because they are equal to, or slightly higher
than the approximate gastric emptying rate 1.2 L·hr−1.19,26 Any amounts larger
than this would be a physiological challenge for the athletes and may produce
gastrointestinal discomfort.
Palatability of the fluid ingested is important in the amount of voluntary fluid
an athlete will consume.44 The greater the palatability, the greater the volume of
fluid an athlete consumes. Athletes consume calorie, or calorie-free, flavored
liquid during practice and competition to enhance fluid consumption rates to avoid
hypohydration.
As voluntary drinking often leads to involuntary hypohydration,13 a hydration
schedules can be developed by the trainer, coach, and athlete by measuring fluid
loss. The easiest method is to weigh the athlete before a practice session and then
measure how much water is consumed during the session, followed by postexercise weighing of the athlete. After adjusting for any urine losses, the water intake
420 Kovacs
is added to the change in body weight providing the approximate sweat rate and
fluid volume change for the athlete. Addition of Na+ concentrations above 20
mmol∙L−1 are recommended since urine production is much lower and more
ingested fluid is retained. Hydration protocols should be tested in practice, as a
full bladder could hamper concentration during competition. Likewise, a higher
osmolality prolongs the thirst and results in greater voluntary hydration.43
Fluid Intake and Requirements
After a Match or Practice
Postpractice or match hydration is not only important for immediate recovery, but
also for performance during play in a subsequent session on the same or the following day. Rehydration after exercise has three major purposes: 1) to replace
fluid volume to an equal or greater extent than the volume lost while sweating, 2)
to ingest liquid and/or solid carbohydrates to aid in glycogen resynthesis,45 and 3)
to replace electrolytes lost during sweating. Water cannot be the only fluid consumed after tennis play because the athlete is typically in a hypohydrated state and
an increase in plain water will dilute the lowered electrolyte concentration in the
blood and plasma even further. This fall in plasma osmolality and Na+ concentration reduces the athlete’s drive to drink and stimulates urine output, which could
lead to serious consequences (ie, hypohydration and hyponatremia).43,46 The addition of Na+ in postexercise beverages has been supported by multiple position
stands.40,47 Sodium supplementation after tennis play should be consumed at a
rate of ~1.5 g∙L−1.18 Although the incidence of hyponatremia is rare in tennis, it
does occur and the consequences are life threatening. Athletes and coaches should
be educated on the need to consume sodium-rich fluid and food postmatch to limit
the possibility of hyponatremia.
Conclusions and Practical Applications
Maintaining appropriate fluid levels before, during, and after tennis practice and
competition is vital for performance, as well as health and safety of the athlete. As
voluntary fluid ingestion does not provide enough fluid to offset the losses during
play, tennis players should employ a structured fluid intake program during practice and match sessions. Tennis players can sweat more than 2.5 L·h−1, yet it is
difficult for athletes to comfortably drink substantially more than 1.2 L·h−1. This
large difference makes consuming adequate fluids during tennis play difficult.
Athletes consume fluids at approximately 1.2 to 1.6 L·h−1, the rate of gastric emptying. Tennis players typically consume fewer fluids during matches than practice.
Subsequently, the later stages of matches and tournaments are when athletes are
more susceptible to hydration problems. Preventive measures are more effective
than reactive responses to hydration-related performance and safety decrements.
Electrolyte balance is an area of interest for tennis, due to the possibility of
large electrolyte losses in sweat. The major electrolyte lost during exercise is Na+,
not K+, and this reduction in Na+ has been linked to muscle cramping. A typical
tennis player who may sweat more than 10 L of sweat with an average sodium
concentration (50 mmol·L−1) will need to replace at least 29 g of sodium chloride
Hydration in Competitive Tennis 421
per day.33 Therefore, Na+ supplementation is recommended in regular diet (eg,
sodium-rich foods), or sodium supplementation to fluid ingested before, during,
and after tennis play. Tennis athletes should drink more than 200 mL every changeover in mild temperatures, <27°C wet bulb globe temperature (WBGT), but equal
to or greater than 400 mL every changeover in hot and humid conditions (>27°C
WBGT). Each athlete should employ a specific hydration routine developed
through a deliberate and specifically designed program throughout practice and
match sessions. A CHO and electrolyte drink promotes fluid absorption to a
greater degree than water alone. However, water consumption has been shown to
be sufficient for tennis practice and matches lasting less than 90 minutes. The
athlete should consume a CHO-electrolyte drink if matches or practices are longer
than 90 minutes, not only for performance enhancement possibilities, but also to
aid in fluid palatability. Future research on tennis hydration should investigate
methods to increase fluid consumption during matches to limit the losses associated with excess sweat rates, without increasing gastrointestinal distress. It may
also be beneficial to monitor changes of ad libitum fluid consumption rates during
close matches as opposed to easier matches, and if the hydration rate changes
from the beginning to the end of matches. One area of research that should be
explored is whether hydration guidelines should be based on gender. Because
males typically sweat at a higher rate than females, it would be reasonable to suggest that they consume larger amounts of fluids than females.
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