THERMOREGULATION during Exercise in the Heat

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THERMOREGULATION during Exercise in the Heat
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Overview

• Physiological Responses
• Acclimatization
• Heat Stress and Treatment
• Prevention and Strategies

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Physiological Responses

• Cardiovascular
• Metabolic

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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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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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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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Estimated Caloric Heat Loss at Rest and During
Prolonged Exercise
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Metabolic Responses to Exercise in the Heat
w Body temperature increases.
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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EXERCISE IN HEAT AND COLD
Optimal temperature for endurance perfromance may
be about 10o C
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Metabolic water

• 2500 kcal training expenditure of 60 kg
  marathon runner
• 400 g carb 240 g H2O
• 100 g fat 110 g H20
• 320 g muscle glycogen 960 g H2O
• 3 of body mass lost with no effective loss in
  water

oxidation
oxidation
dissolution
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Why would the maximal oxygen consumption decrease
when exercising in the heat?
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Body Fluids and Exercise in the Heat
w Sweating increases.
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Fluids and Performance

• Performance declines when
• 1.5 pre-exercise water deficit (endurance)
• gt3 weight loss (endurance)
• gt5 weight loss (strength/power)
• Hypohydration has a greater detriment in the heat
  than cold
• Contribution of CNS to fatigue greater in heat
• Too much fluid causes hyponatremia
• Ultramarathoners
• Optimize vs. Maximize

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Non-Thermal, Exercise Induced Sweating

• Sweating increases with exercise independent of
  temperature increases
• Central command (motor cortex?SSNA)
• Exercise Pressor Reflex
• Mechanosensitive
• Metabosensistive

Afferents
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Sweating

• Both plasma osmolality and plasma volume may
  independently affect sweating
• ? osmolality ? ? sweat rate (threshold ?, slope
  ? )
• ? volume ? ? sweat rate
• ? volume ? ? sweat rate
• ? volume, ? osmol. ? ? sweat rate (? slope)

normal
Iso-osmo, hypovol.
? sweat
? temp
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Isoosmotic hypovolemia
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Exercise hyperosmotic
Deionized water
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Acclimatization

• Acclimatization adapt to a new climate
• Acclimation adapt to an artificial environment

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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 (1.4-gt 3 l/hr).
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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 (training itself
improves this)
w Heart rate increase is less (than
nonacclimatized)
w Stroke volume increases
w Muscle glycogen usage decreases lower lactate
production
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HEAT ACCLIMATIZATION
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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

• Target heart rate can used to control intensity
  during acclimatization
• HR body temp and RPE are ? for a given exercise
  intensity after acclimatization
• 10-14 days needed to maximize
• Magnitude of stimulus magnitude of adaptation
• ? requirement for fluid because of ? sweating
• Pre-exercise body temp ?, rate of ? the same
• Pre-cooling improves performance
• Sunburn reduces thermoregulation and exercise
  performance

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Heat Acclimatization

• Does heat acclimatization improve performance in
  temperate conditions?
• Why is there no physiological adaptation to
  dehydration (Maughan, 2004)?
• Is exercise induced dehydration different than
  heat induced dehydration?

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Heat Stress and Treatment
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WARNING SIGNS OF HEAT DISORDERS


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W
W
PW
W
C
PC
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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.
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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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Subjective Symptoms Associated With Overheating
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Prevention and Strategies
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Preventing Hyperthermia
McArdle, Katch, and Katch, Exercise Physiology,
2001

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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.
w Exercise with fans, breeze or indoors
w RHR gt 8 bpm increase indicates incomplete
recovery
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WBGT

• Integrates humidity, wind, radiation and air
  temperature
• Underestimates risk when humidity gt 70
• Does not account for clothing, exercise
  intensity, fitness, acclimatization, illness,
  body fat etc.

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College Station
May-September
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FLUID INTAKE AND EXERCISING IN THE HEAT
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Strategies to Prevent dehydration

• Pre-exercise hydration (gatorade?)
• Water ? urine production to maintain osmolality
• What are your strategies?
• Use body weight as a guide (16 oz 1 lb)
• Must standardize weighing
• Chilled beverage (absorb heat)
• Palatable beverage (increase consumption)
• Carbs only necessary when exercise gt60 minutes
• Electrolytes usually not necessary with normal
  diet
• Caffiene/alcohol volume vs. diuretic effect
• Monitor urine (color, volume, conductivity,
  specific gravity, osmolality)
• Caveats?

A tomato has more water than cola (95 vs 89)
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When you exercise on a hot humid day, do you
sweat more than when you exercise on a dry day of
the same temperature? How do you explain the
fact that in the high humidity the sweat drips
off you?
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Other considerations

• Hypertension, CVD, diabetes (esp. neuropathies),
  aging, and obesity impair cardiovascular
  function, skin blood flow, and sweat responses
  thus impair temperature regulation
• Diuretics, Beta blockers alpha agonists
  vasodilators and alcohol can alter blood flow and
  cardiovascular response and worsen dehydration
• Thirst mechanism impaired with aging

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Other lingering considerations

• What about potassium?
• Every person is different

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Apocrine Sweat gland

• Any of numerous sweat glands found primarily in
  the skin of the armpit, pubic region, and areolae
  of the breasts that produce a secretion that is
  more viscous than that formed by the eccrine
  glands. Secretions from these glands occur most
  frequently during periods of emotional stress or
  sexual excitement.

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Sudomotor

• Pertaining to the efferent nerves that control
  the activity of sweat glands

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