Maintenance of the human body requires numerous chemical
reactions that operate best at a particular body core
described as homoeothermic or warm blooded and need to
maintain a relatively constant core temperature for these
chemical reactions, called metabolism, to occur optimally to
produce efficient mechanical movement.
engaged in exercise the body temperature is elevated as the
metabolic rate increases to meet the energy demands of the
efficiency view approximately 30% of metabolism is converted
to mechanical work while the remaining 70% is expressed as
heat. Further heat, may be gained from exposure to the
sun in the form of radiant heat or convection currents.
order to preserve health and function optimally the body must
find ways of dissipating this heat.
Normally resting core temperature is about 98.6°F ( -/+ 1°F
) and may reach 102°F and higher in some individuals
during exercise with or without adverse effects.
are various homeostatic mechanisms employed by the body in an
attempt to maintain the core temperature within narrow limits.
During, exercise there is increase blood flow to the
skin—vasodilatation—to maximise heat loss. At the same time
blood is diverted to the working muscles.
is therefore an inherent competition between mechanisms that
maintain a large blood flow to exercising muscles and
those that provide adequate thermoregulation.
heat is lost by radiation, conduction, convection and
transfer via electromagnetic heat waves through the air.
Transfer of heat by direct
air molecules around body
warmed by body heat.
loss via air currents moving warm air away from body.
Evaporation of sweat from the skin surface is the
most effective way to remove heat from the body.
the sweat reaches the skin it evaporates and cools the surface
causing a heat transfer from the blood. The cooled blood
returns to the internal body environment where it absorbs heat
from the tissues and the process is repeated.
is described as a hypotonic saline solution meaning that in
comparison to body fluids it is a less concentrated salt
contains over 99% water and only 0.2 to 0.4% sodium
chloride—mainly sodium. When the body sweats the relative
concentration of salt left in the body fluids is
during exercise under extreme adverse conditions
over prolonged periods are significant amounts of electrolytes
lost. Normally after exercise sufficient electrolytes
replenished at meal times.
fluid in sweat comes from the extra cellular fluid of
the blood plasma and that bathing the cells. Sweating
therefore places demands on the body's fluid reserves and
creates a relative state of dehydration.
does not cool the skin.
Thermoregulation important ?
primary focus of thermoregulation is of course the continued
health of the individual.
failure to control body heat will also work to the detriment
of sports performance.
fluid loss exceeds replacement, dehydration can occur. This
may result in decreased blood pressure, increased heart rate,
decreased blood to working muscles and the skin, and
subsequent decrement in exercise performance.
individual may also be at risk of developing heat
illnesses, such as heat cramps, heat exhaustion and heat
kg weight lost — 1 litre body fluid lost.
Dehydration during exercise in the heat occurs primarily
from sweating which causes a decrease in circulating blood
volume. An associated decrease in blood pressure places stress
on the cardiovascular system trying to distribute adequate
blood to the working muscles and at the same time diverting
blood to the skin for heat exchange.
decrease in blood volume causes the heart rate to increase in
an attempt to provide blood flow needed to meet the energy
demands of the exercise.
dehydration continues exercise performance and the sweat
mechanism is compromised and the body temperature increases
juggling of circulatory adjustments in an attempt to cool the
body also causes early accumulation of lactic add with
subsequent premature use of glycogen stores and early
fatigue during exercise. Decreased blood flow to working
muscles reduces the body's capacity to buffer and oxidise
lactic acid. In an attempt to maintain blood to these muscles,
hepatic (liver) blood flow is
lactate uptake and
oxidation by the liver.
factors are interrelated and contribute to early fatigue
during even moderate exercise in the heat.
and dehydration are not synonymous and thirst is a poor
indicator of dehydration.
well documented and accepted ( Hawely, et al, 1998, Cross et
al, 1991; Murray, 1996, Reher, 1996). It is important
therefore not to rely on voluntary drinking.
commencement of thirst a player may already be dehydrated 2 to
3%. A decrease in performance will occur at all levels of
dehydration. As the level of dehydration increases,
performance levels will continue to drop..
Fluid loss equal to 3% of body weight can cause a significant
reduction in aerobic work capacity ranging/row 6% to 15%.
Further fluid losses of 4% or 5% can cause declines in
performance by 20% to 30% ( Wilmore JH et al, 1994; Nadel et
al, 1987, cited in Meir et al., 1995; Murray R, 1996). Note
that these decrements in performance may be heightened
depending upon the fitness of the individual and relative
conditions on the day.
losses from the body can be expressed in terms of weight lost.
That is for every one kilogram of weight lost during exercise
one litre of fluid has been lost—adjusted for any intake of
fluid or urine voided.
% values of body weight lost for rugby league:
Forwards: 2.4% 20-24°C 67.73% humidity
Backs: 1.4% 20-24°C 67-73% humidity
player x 2.4% weight loss = 1.92 litres, ie. approximately 2
litres of fluid lost.
most important issues of thermoregulation concern the health
of the individual in avoidance of hyperthermia and associated
heat illness. The major forms of heat illness in order of
severity are heat cramps, heat exhaustion and heat stroke.
is often no clear-cut demarcation between overlap. When heat
illness does occur however, immediate action must be taken to
reduce the heat stress and rehydrate the person until medical
help is available "(McArdle W. et al, 1996).
Cramps or involuntary muscle spasms usually occur in the
specific muscles exercised after prolonged intense work. Body
temperature may or may not be elevated. They are thought
to occur as a result of water loss rather than salt depletion,
causing an imbalance in the body's electrolyte concentrations.
exhaustion can develop in unacclimatised individuals early in
the season during hot humid weather. Exercise-induced heat
believed to occur due to ineffective circulatory adjustments
and depletion of extracellular fluid—particularly blood plasma
volume—from excess sweating.
tends to pool in the periphery due to the dilated
blood vessels markedly reducing central blood volume and
cardiac output (McArdle et al, 1996).
Therefore the cardiovascular system struggles to
adequately meet the body's needs.
and symptoms include extreme fatigue, weak, rapid pulse, low
blood pressure in an upright position, headache,
and dizziness, Sweating may be reduced and body
temperature elevated to 39°C but not above dangerous levels
player suffering these symptoms should cease exercise and
rest in a cool environment. Heat exhaustion can deteriorate
into heat stroke if allowed to progress (Wilmore JH et al,
1994), Summon medical assistance. If conscious, fluids should
be administered orally but if unconscious an intravenous feed
of saline is recommended.
Stroke is the most serious and complex heat stress malady and
requires immediate medical attention. Heat stroke is caused by
failure of the in excessively high body temperature which is
sweating response normally shuts down and the skin is dry and
hot, body temperature rises to 4l.5°C or higher and
severe strain is placed on the circulatory system. However
there have been instances reported where some individuals body
temperature has only reached 39°C during heat stroke and in
others where sweating has continued to occur so the skin
presents as wet and not dry.
may occur particularly when the relative humidity is
high which prevents the evaporation of sweat from the skin.
highlights the importance of recognising the
peculiarities amongst individuals and not fall prey to recipe
diagnosis and treatment.
heat stroke is a medical emergency aggressive steps need to be
taken whilst awaiting medical treatment.
elevated core temperature needs to be rapidly reduced as
mortality is related to both the magnitude and duration of
hyperthermia. Immediate treatment may include alcohol
rubs, fans, cool baths and ice packs but do not promote
shivering—shivering promotes an unwanted increase in body
temperature is often highly inaccurate due to the heightened
pulmonary ventilation—increased breathing—during and
immediately post exercise. The oral temperature is therefore
influenced by the evaporative cooling effect in the mouth from
passing air currents. Rectal temperature is far more accurate.
exercising in the heat, if you suddenly feel chilled and goose
bumps form on your skin, stop exercising, get into a cool
environment and drink plenty of cool fluids. The body's
thermoregulation system has become confused and thinks that
the body temperature needs
to increase even more! Left untreated this condition can lead
to heat stroke and death. (Wilmoye JH et al, 1994)
are the risk factors?
Relative Humidity and Heat:
amount of relative humidity and heat on the day are extremely
important. The humidity is the amount of water vapour carried
in the air relative to its carrying capacity at a given
bulb temperature and is
most significant with regard to thermoregulation. Playing or
training when the wet bulb temperature is high—hot humid
conditions— reduces the temperature gradient between skin and
air and the sweat tends to pool on the surface of the body.
there is little chance for the sweat to evaporate and the body
and Colour of Clothing:
fitting clothing does not allow air flow to assist in
evaporative or convective cooling. Similarly clothing that
does not "breathe" for example, nylon and polyester, does not
allow absorption and removal of sweat.
they promote a warm
layer of fluid between the garment and body similar to the
colours attract more heat especially black jerseys.
Black reflects no colour and therefore absorbs all light and
heats up significantly. On the other hand white reflects all
the colours and is much cooler. Spare a thought for Ourimbah
and the 'All Blacks'.
Players Susceptible to Hyperthermia.
Larger players as they have a decreased surface area to body
volume ratio and thus a decreased heat transfer from the body.
players for the above reason and because fat is a good
insulator, reducing heat loss.
players as heat produced is dependent on the intensity of
exercise, Unfit individuals work harder at a higher relative
intensity to keep up with fitter players.
players or those recently unwell who have had fever, diarrhoea
or vomiting, will commence playing in an already dehydrated
Players, unacclimatised to heat will not sweat or supply blood
to the skin as effectively. Therefore, ensure there has been
adequate time for acclimatisation to the effects of heat
especially new players from cooler areas. About 7-10 training
sessions specific to the environmental conditions of playing
are recommended. Consider training in the same conditions as
alcohol the night before and
caffeine based beverages
(cola drinks) to avoid their diuretic effects.
Children have a reduced sweat mechanism, sweat less and sweat
less precisely, and they have a higher core temperature during
heat stress. They usually require longer to acclimatise to
heat. Children also have a larger surface area to body volume
rations which can help heat loss but may also allow greater
area for radiant heat gain.
Hydration is the absorption of fluid into the body and water
is the most popular medium. The aim is to replace the level of
body fluids lost mainly through sweat although this is seldom
possible during exercise in adverse conditions. It is
important therefore that all attempts are made to replace as
much fluid as possible and not to rely on voluntary drinking.
On hot days hydration should commence well before the game
starts and continue during and after cessation of play.
extreme heat, hydration should commence the night prior. On
relatively cool or cold days hydration can be commenced 5-10
minutes prior to warm-up, provided the individual is hydrated—ie
Consideration should be given to pre-game ingestion of fluid
and anxiety causing diuresis and this fluid loss should be
accounted for in the overall fluid balance. Clear urine is a
broad indicator of an adequate level of hydration but fluid
should be replaced after voiding.
rate of gastric emptying from the stomach to the small
intestine is important as very little water is absorbed via
the stomach itself. The gastric emptying rate of plain water
is about I litre per hour for adults at rest—ie 250ml/15
minutes— but can vary. Cool to cold fluids tend to empty more
rapidly, may help reduce core temperature and are generally
the stomach prior to playing with a large single ingestion of
emptying and stretching the stomach wall.
increased stomach volume allows the player to comfortably
accommodate more fluid during ingestion on the field. Care
should be taken not to overload and distend the stomach to
uncomfortable lengths as this will hinder expansion of the
diaphragm muscle during exercise.
Individuals vary in their gastric emptying rates as well as
their tolerance to gastric volumes during varying exercise
intensities. Gastric upset can occur with high exercise
intensities— above 70% to 80% VO max—but again this varies
unclear whether complaints of gastrointestinal symptoms by
players are a function of an unfamiliarity of exercising with
a full stomach or because of delays in gastric emptying.
therefore recommended that / individuals learn their tolerance
limits for maintaining high gastric fluid levels for various
exercise intensities and duration whilst at training. Often
gastric distress is more associated with dehydration or
attempts at re-hydration from a dehydrated state.
order to prevent dehydration and gastric distress a
basic guideline, for adult players would be the consumption of
400 to 600 ml of fluid either immediately before or 10 to 15
minutes prior to play and then regular ingestion of 150 to 250
ml of fluid at 15 minute intervals during play may be required
depending upon the conditions on the day.
Determining Sweat Rate:
the dehydration/thirst anomaly a hydration plan should be
adopted and calculated on individual sweat rates during a game
and not on ad libitum fluid intake. Under different exercise
conditions sweat rates can vary from 1 litre per hour to as
much as 3 to 4 litres per hour under extreme conditions. The
rates can also vary widely amongst
estimate of sweat loss can be made by comparing the pre and
post training body weights over time. Ensure corrections are
made for ingested fluid during training and any urine
Pre-training wt, Post-training wt. Change in body wt. Add
Corrected change in body weight, less urine voided
(85kg) 83500 g
lost per 60 minutes, (ie. 2 litres or 2 kg body weight)
example a player demonstrated a sweat loss rate of 2.0 litre
hydration plan for this individual should be modelled
on replacing this amount of fluid each hour to remain well
hydrated. 'Unfortunately under playing conditions this would
prove difficult but it provides guidelines from which to
Thermoregulation on game day:
practical adopt a planned hydration, programme calculated at
training from sweat rates and known fluid holding capacities
and modified according to environmental conditions.
shade at half-time taking advantage of any air currents
available for convective and evaporative cooling.
jerseys and wet down body to maximise surface area for
evaporative cooling or use wet towels to promote conductive
promote shivering as this causes muscle contraction of hair
follicles resulting in unwarranted generation of heat.
head gear as nearly 30 to 40% of body heat can be lost through
the highly vascularized head region even though it only
represents about 8% of the body's total surface area (McArdle
et al., 1996). Wet down head with water.
relatively low humid conditions do not replace wet jersey's
with dry ones to take advantage of evaporative heat loss and
the fact water conducts heat faster than air. However in high
humid conditions consider a change of jersey— if practical—to
promote heat loss. Because sweat stays on the skin in humid
weather it forms a warm layer beneath the jersey. As there is
little or no evaporation of sweat from the jersey itself
transfer of heat through the wet jersey does not occur.
or sports drink?
is the universal medium and is usually hypotonic.
mentioned earlier sweat is also hypotonic so that
during exercise as dehydration progresses the concentration
of electrolytes in the body fluids become progressively more
Therefore the need to replace body water is greater than
the need for electrolytes because, only by replacing water is
the concentration of electrolytes returned to normal.
that ingested fluid in the gut is technically outside the body
an isosmotic or hypotonic solution would be
the favoured hydration
drinks offer dual advantages of hydration and carbohydrate
feeding to delay fatigue. Past scientific findings did not
support exogenous carbohydrate feedings to sustain mixed
anaerobic and aerobic activities such as rugby league.
studies however have supported delayed onset of fatigue with
supplementation of such feeding's during exhaustive repetitive
bouts of anaerobic activity. Enhanced performance has
been linked to maintenance of blood glucose and sparing of
liver glycogen levels, increased muscle glucose and possible
increase in muscle glycogen levels.
research suggests that a sports drink with a low concentration
of an electrolyte and carbohydrate solution has a low
isosmotic count and has advantages over plain water as a
Commercially prepared sports drinks containing 6%
carbohydrate, preferably a glucose polymer, with 20mm of
sodium per litre are typically described as isosmotic.
fluids are absorbed relatively quickly from the
gut—assisted via active
pump—and promote greater
post exercise through reduced urine production (Burke, L„
current recommendation is
to avoid solutions of carbohydrate greater than
8% concentration with high
reduce the temporary shift of fluid into the gut and decreased
rate of gastric emptying.
Similarly electrolyte concentration should be kept to a
minimum to avoid further temporary dehydration due to the
lower concentration of sweat relative
to body fluids—0.5-0.7
grams per litre of water should be sufficient to enhance
palatability and promote fluid retention (Murray, R., 1996).
improved palatability of sports drinks tends to enhance
voluntary drinking compared to water both during and post
exercise and varies between individuals. When assessing
palatability one should appreciate that perception of taste
changes at rest compared to exercise.
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© Ken Gow,