Download HEAT ACCLIMATIZATION

TRAINING AND RACING IN
HEAT AND HUMIDITY
Michael Turnbull
8th October 2005
INTRODUCTION
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Physiology in the heat
Performance in the heat
Heat acclimatization
Adaptations
Gender and age differences
Dehydration
Heat injuries
Loss of acclimatization
Summary
Questions and Answers
PHYSIOLOGY IN THE
HEAT
• Exercise in Asia poses a particular stressful
challenge to the maintenance of normal body
temperature and fluid homeostasis
• Heat and humidity reduce the body’s ability to
lose heat resulting in a greater core
temperature and a higher sweat rate compared
to the same exercise in a moderate
environment.
PERFORMANCE IN THE
HEAT
• Triathlon performance is impaired in hot and
humid environments
• Contributory factors include:
– Hyperthermia – Impairs Central Nervous System.
– Muscle blood flow changes – Muscle blood flow is
reduced due to competition between the working
muscles and the skin.
– Metabolism – Muscle glycogen usage is increased
and lactate production is elevated
• Leads to an increased risk of heat injury
HEAT ACCLIMATIZATION
• Acclimatization by repeated exercise in the heat
(either strenuous interval training or continuous
exercise above 50% VO2 max) promotes
higher core temperatures.
• This results in physiological adaptations that
increase heat tolerance:
– Increased plasma volume
– Earlier onset of sweating
– Higher sweat rate
– Reduced sodium chloride loss in sweat
– Reduced skin blood flow
HEAT ACCLIMATIZATION
• After acclimatization, the increased sweat rates
improve the ability to control body core
temperature, leading to an improved
performance capacity.
GENDER DIFFERENCES
• Most women appear to be less heat tolerant
than men.
• Contributory factors include:
– Lower sweat rates
– Generally higher percentage of body fat
• However, when men and women are matched
for the same acclimatization and similar body
compositions – gender differences in the
physiological responses to thermal stress are
small.
AGE DIFFERENCES
• Does ageing impair your ability to
thermoregulate and exercise in the heat?
– Previously controversial in earlier studies
• More recent studies have shown that heat
tolerance does not appear to be compromised
by age in healthy and physically active older
subjects
– Physical deconditioning and a lack of heat
acclimatization are more likely causes of a decline
in performance
DEHYDRATION
• An earlier onset of sweating at a higher rate can
lead to potential dehydration
• Sweat loss can reach up to 3 litres per hour in
some athletes
• A 5% loss in body weight through dehydration
can lead to a 30% loss in exercise capacity
• Even if hydration techniques are correct, it has
been reported that some marathon runners can
lose 8% of their body weight in one race
HYDRATION
RECOMMENDATIONS
• Dehydration can be avoided through correct hydration
techniques:
– Be adequately hydrated prior to the exercise (400 – 800ml of
fluid 2 – 3 hours before)
– During the exercise, most athletes should consume between
150 – 300ml of fluid every 15 to 20 mins (PRACTICE!!!)
– Following the exercise, replace what you have lost
• Drink cold drinks, they are absorbed by the body
faster
• Fluid replacement during exercise reduces heart rate,
body temperature and perceived exertion levels
• For triathlons, water alone is not enough
• Electrolytes lost through sweating need to be replaced
HEAT INJURIES
• Symptoms include:
– Nausea, dizziness, reduction in sweat rate and general
ability to think rationally
• To prevent overheating:
– A maximum surface area should be exposed for evaporation
– Frequent water breaks couple with rest periods to allow the
core temperature to decline
• If the symptoms have already developed:
– Stop exercising
– Drink water or a balanced electrolytic drink
– Submerge the body in cool water
LOSS OF
ACCLIMATIZATION
• The rate of decay of heat acclimatization is
rapid
• Reductions in heat tolerance can occur within a
few days of inactivity
• Heat tolerance can decline significantly within 7
days of no heat exposure.
• A complete loss of heat tolerance can occur
following 28 days of no heat exposure.
• Repeated exposure is required to maintain
acclimatization.
PROMOTING HEAT
ACCLIMATIZATION
• Training in sweat clothing in a cool environment
can promote heat acclimatization
• The magnitude of acclimatization is generally
less than training in a hot and humid
environment
• BUT artificial heat training appears to be better
than attempting no heat acclimatization
measures.
• Important for those of you who travel to Europe
SUMMARY
• Heat and humidity have a negative effect on
performance.
• Heat acclimatization can improve performance
• A side effect of acclimatization is a greater risk
of dehydration
• Maintain hydration!
• When moving to a cooler climate for a short
period of time, train in the heat