chapter
11
Exercise in Hot
and Cold
Environments:
Thermoregulation
To Maintain a Steady-State Core
Temperature, the Body Must Balance Heat
Gain With Heat Loss
Modes of Heat Transfer
Conduction involves direct molecular contact.
Convection involves motion of gas or liquid across
heated surface.
Radiation involves infrared rays. *
Evaporation involves loss of heat as fluid evaporates.
* Primary means of heat loss at rest
Removal of Heat From the Skin
MECHANISMS FOR HEAT BALANCE
Evaporation
• As body temperature rises, sweat production increases.
• Sweat reaches the skin and evaporates. (up to 3-4 L/h)
• Evaporation accounts for 80% of heat lost during
exercise.
Humidity
• Plays a major role in heat loss
• Affects perception of thermal stress
• When high (regardless of temperature), limits
evaporation of sweat
Key Points
Heat Balance
• Humans maintain a constant internal
temperature of 36.1 to 37.8 ºC (97.0 to
100.0 ºF).
• Body heat is transferred by conduction,
convection, radiation, and evaporation.
• During exercise, evaporation is the main
means of heat loss; during rest, radiation is.
• Higher humidity reduces potential evaporation
and thus affects heat loss.
Internal Body Temperature
• Can exceed 40 ºC (104 ºF) during exercise
• May be 42 ºC (107.6 ºF) in active muscles
• Small increases can make muscles’ energy
systems more efficient
• Above 40 ºC can affect the nervous system and
reduce the ability to unload excess heat
Regulators of Heat Exchange
Hypothalamus
Central and peripheral thermoreceptors
Effectors
•
•
•
•
Sweat glands
Smooth muscle around arterioles
Skeletal muscles
Endocrine glands
ECCRINE SWEAT GLAND
Rate of Heat Exchange
• Heat produced by an average body at rest is 1.25 to 1.5
kcal per minute.
• Heat produced during exercise can exceed 15 kcal per
minute.
• This heat must be dissipated by the body’s
thermoregulatory systems.
Key Points
Control of Heat Exchange
• The hypothalamus monitors the body’s temperature
and speeds up heat loss or heat production as needed.
• Peripheral thermoreceptors in the skin relay
information back to the hypothalamus.
• Central thermoreceptors in the hypothalamus transmit
information about internal body temperature.
• Sweat gland activity increases to lower body
temperature by evaporative heat loss.
(continued)
Key Points (continued)
Control of Heat Exchange
• Smooth muscles in the arterioles dilate or constrict
to allow the blood to dissipate or retain heat.
• Skeletal muscle activity increases temperature by
increasing metabolic heat.
• Metabolic heat production can also be increased
by actions of hormones.
• Mean body temperature is a weighted average of
skin and core temperatures.
Cardiorespiratory Exercise
in the Heat
• Active muscles and skin compete for blood supply.
• Stroke volume decreases.
• Heart rate gradually increases to compensate for lower
SV (cardiac drift).
Metabolic Responses to Exercise in
the Heat
•
•
•
•
Body temperature increases.
Oxygen uptake increases.
Glycogen depletion is hastened.
Muscle lactate levels increase.
Body Fluid and Exercise in the Heat
• Sweating increases (up to 3-4 L/hr or 10-15 L/d)
• High volumes of sweat cause
 blood volume to decrease,
 loss of minerals and electrolytes, and
 release of aldosterone and ADH and water
reabsorption in kidneys.
Variables of Environmental
Heat Load
•
•
•
•
Air temperature
Humidity
Air velocity
Amount of thermal radiation
Warning Signs and Symptoms of Heat
Cramps, Heat Exhaustion, and Heatstroke
Adapted by permission of All Sport, Inc.
Treatment of Heat Disorders
Heat cramps—Move to cooler location and administer
fluids or saline solution.
Heat exhaustion—Move to cooler environment, elevate
feet; give saline if conscious or intravenous saline if
unconscious.
Heatstroke—Rapidly cool body in cold water or ice bath
or with wet towels; seek medical attention.
Preventing Hyperthermia
• Avoid exercising in humid conditions above a WBGT
index of 28 °C (82.4 °F).
• Schedule practices or events in early morning or at
night.
• Wear lightweight, light-colored, loosely woven clothing.
• Drink plenty of fluids.
• Know the symptoms of heat stress.
Key Points
Heat Stress
• WBGT measures air temperature and accounts
for heat exchange via conduction, convection,
evaporation, and radiation.
• Heat cramps appear to be caused by loss of fluids
and minerals due to sweating.
• Heat exhaustion results from the cardiovascular
system being unable to meet the needs of
muscles and skin due to lower blood volume (from
sweating).
• Heatstroke is caused by failure of the body’s
thermoregulatory system.
Heat Acclimation
• Ability to get rid of excess heat improves.
• Sweating becomes more efficient.
• Blood flow to skin is reduced; more blood is available to
muscles.
• Blood volume increases.
• Heart rate increase is lower.
• Stroke volume increases.
• Muscle glycogen usage decreases.
Effects of Fluid Intake on Core Body
Temperature During 2-Hour Run
Archives of Environmental Health "Fluid ingestion during distance running," D.L. Costill, 1: 520-525, 1970.
Adapted with permission of the Helen Dwight Reid Educational Foundation. Published by Heldref
Publications, 1319 18th Street, NW, Washington, DC 20036-1802. www.heldref.org. Copyright © 1970.
Rectal Temperature and Heart Rate
Responses to Exercise Before and After
Heat Acclimation
Adapted, by permission, from D.S. King et al., 1984, "Muscle metabolism during exercise in the heat in unacclimatized and
acclimatized humans," Journal of Applied Physiology 59: 1350-1354.
Did You Know . . . ?
You can achieve heat acclimation 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, but 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.
Key Points
Heat Acclimation
• Repeated exposure to heat stress during exercise
improves your ability to get rid of excess heat.
• Sweating increases in well-exposed areas to
promote heat loss.
• Stroke volume increases to aid in delivery of more
blood to the muscles and skin.
• Muscle glycogen use is reduced to delay onset of
fatigue.
• Amount of heat acclimation depends on
environmental conditions and duration of exposure
and intensity of exercise.
How Does the Body Conserve
Heat?
Shivering involves rapid involuntary cycle of contraction
and relaxation of muscles.
Nonshivering thermogenesis is stimulation of
metabolism.
Peripheral vasoconstriction reduces blood flow to
skin.
WARMING OF INSPIRED AIR
Factors That Affect Body Heat Loss
•
•
•
•
Body size and composition
Air temperature
Windchill
Water immersion
Windchill Equivalent Temperature
Chart
Responses to Exercise in the Cold
• Muscles weaken and fatigue occurs more rapidly.
• Susceptibility to hypothermia increases.
• Exercise-induced FFA mobilization is impaired due to
vasoconstriction of subcutaneous blood vessels.
Health Risks of Exercise in the Cold
• Ability to regulate body temperature is lost if Tbody drops
below 34.5 °C (94.1 °F).
• Hypothermia causes heart rate to drop, which reduces
cardiac output.
• Vasoconstriction in the skin reduces blood flow to skin,
eventually causing frostbite.