C H A P T E R 10

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C H A P T E R 10
EXERCISE IN HOT AND COLD ENVIRONMENTS
THERMOREGULATION
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Learning Objectives
w Find out how the body gets rid of excess body
heat to maintain homeostasis at rest and during
exercise.
w Discover how the body adapts to exercise in a
hot environment.
w Learn why humidity, wind, and cloud cover are
important factors when exercising in the heat.
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Learning Objectives
w Learn how the body minimizes excessive heat
loss during exposure to cold.
w Find out the dangers of cold-water immersion.
w Discover how to exercise safely in the cold.
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BODY HEAT GAINED AND LOST
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Modes of Heat Transfer
Radiationinfrared rays given off by any
molecular mass as a function of the temperature
of the mass
Conductionthrough direct molecular contact heat
is transferred down a heat gradient (?H) from a
hotter medium (like skin) to a cooler medium
(like air or water) Convection is movement of
the air or water (e.g., from a fan), which
facilitates conduction by removing the warmed air
or water next to the skin so a greater ?H is
maintained
Evaporationas fluid evaporates, heat is lost
(580 kcal/L of water evaporated) Convection
facilitates evaporation by removing the
humidified air next to the skin so a greater
difference in water vapor pressure can be
maintained
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Evaporation
w As body temperature rises, sweat production
increases.
w Sweat reaches the skin and evaporates.
w Evaporation accounts for 80 of heat lost
during exercise, but only for about 20 at rest.
w Insensible (non-sweating) water loss removes
about 10 of heat.
w Dehydration is a potential problem with
sweating. It is essential that water be replaced
when exercising, particularly in a hot humid
environment when one is sweating profusely.
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HEAT REMOVAL FROM THE SKIN
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Estimated Caloric Heat Loss at Rest and During
Prolonged Exercise
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Humidity
w Plays a major role in heat loss
w Affects our perception of thermal stress
w When high (regardless of temperature), humidity
limits evaporation of sweat because it lowers the
water vapor pressure difference between the sweat
on the skin and the water in the surrounding air
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Key Points
Heat Balance Homeothermy
w Humans maintain a relatively constant internal
temperature of 36.1 to 37.8 C (97.0 to 100.0 F)
during normal rest.
w Body heat is transferred by radiation,
conduction, and evaporation, the latter two
facilitated by convection.
w During exercise, evaporation is the main means
of heat loss during rest, radiation is.
w Higher humidity reduces potential evaporation
and thus attenuates heat loss.
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Internal Body Temperature
w Can exceed 40C (104F) during exercise
w May be 42C (107.6F) in active muscles
w Small increases can make muscles' energy
systems more efficient by increasing the rates
of enzyme reactions.
w Above 40C can adversely affect the nervous
system and reduce the ability to unload excess
heat (leading to heat-related disorders)
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Regulators of Heat Exchange
Hypothalamus Thermostat
Central (in the hypothalamus) and peripheral
(primarily in the skin) thermoreceptors
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HYPOTHALAMUS AND HYPERTHERMIA

1. Hypothalamus integrates the input comparing it
   with the set point then initiates cooling
   mechanisms
2. Skin arterioles vasodilate under control of the
   sympathetic nerves, allowing elevated blood flow
   in the skin, thus heating the skin
3. Sweat glands increase sweat secretion onto the
   surface of the skin under control of the
   sympathetic nerves, allowing increased
   evaporative heat loss
4. Elevated core temperature sensed by
   thermoreceptors in the hypothalamus

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HYPOTHALAMUS AND HYPOTHERMIA

• 1. Hypothalamus integrates the input from skin
  and hypothalamic thermoreceptors comparing it
  with the set point then initiates warming
  mechanisms
• 2. Skin arterioles vasoconstrict under control
  of the sympathetic nerves, decreasing blood flow
  in the skin, thus cooling the skin and decreasing
  the difference in temperature between skin and
  air
• Skeletal muscles are activated to shiver, thus
  increasing heat production
• With prolonged exposure to the cold, thyroid
  hormone and catecholamine secretion increase,
  which also stimulates muscle metabolism

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Rate of Heat Exchange
w Heat produced by average body at rest is 1.25
to 1.5 kcal per minute.
w Heat produced during exercise can exceed 15
kcal per minute.
w This heat must be dissipated by the bodys
thermoregulatory systems to avoid dangerously
high body temperatures.
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Homeostasis and Steady State
Steady State a constant internal environment
that may be different than rest, for example
during steady state exercise
Powers and Howley, Exercise Physiology, 2004
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Cardiovascular Response to Exercise in the Heat
w Active muscles and skin compete for blood
supply.
w Blood pools in the skin, reducing venous return
so that stroke volume decreases.
w Heart rate gradually increases to compensate
for lower SV (cardiovascular drift) so that
cardiac output is maintained. However, maximal
cardiac output (and therefore VO2max) is reduced
in the heat because of the attenuated stroke
volume.
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Metabolic Responses to Exercise in the Heat
w Body temperature increases, which stimulates
enzyme reactions (Q10 effect 2-fold increase for
each 10 degrees).
w Oxygen uptake increases at a given submaximal
level, but VO2max is reduced.
w Glycogen depletion is hastened.
w Muscle lactate levels increase.
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Body Fluids and Exercise in the Heat
w Sweating increases.
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EXERCISE IN HEAT AND COLD
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Sodium, Chloride, and Potassium Concentrations in
the Sweat of Trained and Untrained Subjects
During Exercise
Thus, with training there is a decreased loss of
electrolytes in the sweat, along with an
increased sweat rate.
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Variables Affecting Environment Heat Load
w Air temperature
w Humiditydesert versus tropics
w Air velocity (convection)still air versus
moving air
w Amount of thermal radiatione.g., cloud cover,
ground
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Preventing Hyperthermia
w Avoid exercising in hot and humid conditions
above a WBGT index of 28C (82.4 F).
w Schedule practices or events in early morning
or at night.
w Wear light-weight, light-colored, loosely-woven
clothing.
w Drink plenty of fluids!!
w Know the symptoms of heat stress.
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Subjective Symptoms Associated With Overheating
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Treatment of Heat Disorders
Heat crampsmove to cooler location and
administer fluids or saline solution (0.9 NaCl
in water)
Heat exhaustionmove to cooler environment,
elevate feet give saline if conscious or
intravenous saline if unconscious
Heat strokerapidly cool body in cold water, ice
bath or wet towels immediately seek medical
attention
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Key Points
Heat Stress
w Heat cramps appear to be caused by loss of
fluids and minerals due to sweating.
w Heat exhaustion results from the cardiovascular
system being unable to meet the needs of muscles
(metabolism) and skin (cooling) due to lower
blood volume (from sweating).
w Heat stroke is caused by failure of the body's
thermoregulatory system, i.e., neural dysfunction.
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FLUID INTAKE AND EXERCISING IN THE HEAT
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Heat Acclimatization
w Ability to get rid of excess heat improves
w Sweat sooner, sweat glands produce a greater
volume of sweat, and the sweat is more dilute
(less concentrated because of reduced
electrolytes)
w Reduced blood flow to skin more available to
muscle
w Blood volume increases
w Heart rate increase is less (than
non-acclimatized)
w Stroke volume increases
w Muscle glycogen usage decreases lower lactate
production
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Heat Acclimatization
You can achieve heat acclimatization by
exercising in the heat for 1 hour or more each
day for 5 to 10 days. Cardiovascular adaptations
occur within the first 3 to 5 days while changes
in sweating mechanisms may take up to 10 days.
Reduce exercise intensity to 60 to 70 the
first few days before resuming more intense
workouts.
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HEAT ACCLIMATIZATION
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Responses to Exercise in the Cold
w Muscles weaken and fatigue occurs more rapidly
w Susceptibility to hypothermia increases
w Exercise-induced FFA mobilization is impaired
due to vasoconstriction of subcutaneous blood
vessels
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Health Risks of Exercise in the Cold
w Ability to regulate body temperature is lost if
Tbody drops below 34.5 C (94.1 F).
w Hypothermia causes heart rate to drop, which
reduces cardiac output.
w Vasoconstriction in the skin reduces blood flow
to skin, eventually causing frostbite.
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How Does the Body Conserve Heat?
Shivering thermogenesisrapid involuntary cycle
of contraction and relaxation of muscles
Nonshivering thermogenesisstimulation of
metabolism, e.g. through increased thyroid
hormone release (brown fat?)
Peripheral vasoconstrictionreduces blood flow to
skin, so effectively increases the layer of
insulation
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Factors That Affect Body Heat Loss
w Body size and composition
w Air temperature
w Wind chill
w Water immersion
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WARMING OF INSPIRED AIR
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Exercise Capacity in the Cold

•  VO2 max is usually decreased 6-8 in the cold
  environment.
• Also running speed is slower in the cold
  environment 
• Performance is actually improved in mild weather
  (cold for Texans) 55 degrees.