Exercise in the Cold and Hot
Piokilotherms vs Homeotherms
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Piokilotherms: at mercy of the elements
Homeotherms: can function independent
of the environment
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Ability to maintain constant temperature
Normal body temperature: 36.537.5°C
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During exercise can increase to 40°C with no ill
effects
Core temp: temperature of the hypothalamus,
temperature regulator of the body
Thermal gradients: temperature differences from
one point to another the lead to movement of
heat
Temperature always equilibrates from hot to cold
Temperature Regulation
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Metabolism
Shivering
Nonshivering thermogenesis
Metabolic Rate
Heat loss
Metabolism
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Heat is produced naturally during normal
metabolic reactions
Most reactions lose ~75% of energy as
heat
At BMR heat loss is ~100 kcal/hr
BMR is porportional to ¾ power of body
weight (surface rule)
Shivering
Main mechanism for increasing heat
during negative heat balance
 Involuntary muscle contraction
 Maximum shivering can increase
body’s heat production 5X
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Preshivering tone can increase heat
production 50-100%
 An effective way to increase body
temperature b/c no work is done by
the muscles and most of the energy
is expended is heat
 Increases Q by increasing SV via
increased venous return
 Limits: glycogen depletion,
hypoglycemia, fatigue, ex., hypoxia,
drugs (alcohol and barbiturates)
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Nonshivering thermogenesis
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Increased thyroxin secretion (thyroid) and
catecholamine secretion (adrenals)
increase metabolic rate
Thyroxin increases the rate of all cells
Cats., esp norepi. release FFA, increasing
metabolic rate
Metabolic Rate (Q10 and food)
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Q10 is rate of a physiologic process at a
particular temperature to the rate at a
temperature 10°C lower
Increased metabolic rate can be selfperpetuating (dangerous)
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At high temperatures, hypothalamus loses
ability to cool the body
Rate of temperature increases faster at
higher temperatures
Metabolic rate temporarily increases
following food intake, esp. proteins
Heat Loss
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Radiation
Conduction
Convection
Evaporation
Radiation
Loss or gain in the form of
electromagnetic waves
 At rest, in a comfortable environment,
radiation accounts for 60% of total
heat loss
 Varies with body position and clothes
 Human skin, regardless of color,
absorbs ~97% of radiant energy that
strikes it
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Conduction
Transfer of heat from a body to an
object
 Or, heat transfer within an organism
down a thermal gradient
 ~3% of total heat loss at room
temperature occurs this way
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Convection
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Conduction of heat to air or water
Amounts to ~12% of all heat lost at room
temperature
Heat is conducted to water or air, moves
so that other molecules can be heated
Faster in water or air?
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Greater in the wind (air movement)
Wind chill effect
Can also occur in the circulatory system
Heat moves with the blood from the
core to the periphery
As skin temp. incr., heat loss to
environment increases
Rate is affected by blood flow and
temperature gradient (core/periphery)
Evaporation
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~25% of heat is lost this way in a
comfortable environment
Quantity of heat absorbed by sweat as it
evaporates: latent heat of vaporization
Body loses 0.58 kcal/gm H2O
evaporates
Only means of cooling at high
environmental temperatures
Critical for exercise
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If body cannot lose heat this way, body
temp. increases rapidly
Sweat is only effective if it evaporates
High humidity: evaporation reduced or
prevented
Effective evaporation also hampered by
little air movement
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Women have a lower sweating capacity
than men do
Process occurs by sweating and
insensible water loss: ventilation, diffusion
through skin, does not include sweat,
urine, and feces
Sweat rates are at 0 when temperature is
low
Hypothalamus and temperature
regulation
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Temperature regulatory center
Set-point it tries to keep
Sweating normally occurs at 37°C
Set-point can change in response to
dehydration, starvation, and fever
Responds to heat primarily through
heat-sensitive neurons in the preoptic
area of anterior portion
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More cold than heat receptors in the skin
Thermoregulators transmit impulses to the
spinal cord, to hypothalamus, initiates
response
Ant. Hypothalamus stimulates the sweat
glands, evaporative heat loss
“Hunting Reflex” primarily in hands and
feet
Exercise in the Cold
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Clothing
Oxygen Consumption
Ventilation
Heart
Muscle Strength
Metabolic Changes
Oxygen Consumption
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Maximum uptake is unaffected by the cold
Submax. VO2 increases in the cold
Why?
Ventilation
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Increases in the cold, particularly if
exposure is sudden
Abrupt exposure can lead to gasping
reflex
Heart
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Changes in cardiac performance more
common in men
Incidence of arrhythmias increase in
cold
Muscle Strength
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Strength decreases with lower muscle
temp.
Metabolic Changes
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Increase use of CHO as substrate
Light ex: glycogen depletes faster
Max ex: depletion is independent of
temp.
Prolonged exposure to cold:
hypoglycemia, suppresses shivering,
core temp drops, lactate higher
Fat metabolism suppressed, even
though cat. response is higher
Acclimatization and Habituation to
Cold
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Acclimatize: physiologic compensation
to environmental stress over time
Habituation: lessening of the sensation
associated with an environment
Shivering threshold
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First test of acclimatization
Cold-acclimatized people maintain heat
production with less shivering, more
nonshivering thermogenesis
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Hand and Feet temperature
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Second test for acclimatization
Acclimatized: maintain almost normal
temperature
Habituation also plays a role
Ability to sleep in the cold
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Third and final test
Seems to depend on extent of
nonshivering thermogenesis induced by
increased secretion of norepi
Hypothermia
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Depresses the CNS, lose ability to shiver,
sleepiness, coma, death
Lower temp: lower cellular met. rate,
further lowering temp.
Profound effects on the CV system
Central BV decreases; plasma
sequestration, inadequate fluid intake, cold
diuresis
 Risk
factors
 cold
exposure
 Lack of protective clothing
 Leanness
 Inadequate fluid intake
 High wind chill
 Use of alcohol &/or drugs
 Use of snow to relieve thirst
 Glycogen depletion
Frostbite
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Caused by ice crystal formation within the
tissue
Occurs in exposed skin
Can lead to tissue death
Exercise in the Heat
Plasma volume decreases
during exercise in the heat
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becomes acute at intensity increases
decrease of plasma volume is made worse
by loss of body fluids through sweating
may not be enough blood to adequately
perfuse all areas during exercise in the
heat
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central blood volume may decrease and
cause a decrease in cardiac filling
pressure
results in increased HR in attempt to
compensate for lower SV
submax. HR increases also
@ max levels, skin vessels
vasoconstrict to help maintain blood
pressure and Q
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negative response on heat transfer
circulatory regulation takes precedence
over temperature regulation in this case
VO2 max is not impaired in the heat unless
the person was experiencing thermal
imbalance before beginning the exercise
Sweating Response
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primary means of heat dissipation during
exercise (evaporation)
in heat, sweating is very important b/c
body tends to gain rather than lose heat by
radiation, conduction, and convection
during exercise, sweating is related more
to intensity than environmental
temperature
Acclimatization to heat
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first two weeks of heat exposure results
in lower heart rate, core temperature,
RPE, and skin temperature at rest and
during exercise
primary physiological adjustments are
increased peripheral heat conductance,
plasma volume, and sweating,
decreased core temperature at onset of
sweating, and improved distribution of
sweat over the skin
CV Adaptations
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acclimatization induces a 3-27% increase
in plasma volume, if acclimatization is
done with exercise training
increased plasma volume helps to
maintain SV, central blood volume, and
sweating capacity
also increases in vasopressin, renin, and
aldosterone in early days
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blood flow to the skin decreases
decreased skin blood flow helps to
maintain central blood volume, which is
vital for maintaining BP, SV, and muscle
blood flow during exercise
core temperature is lower during
exercise
decrease in skin blood flow is
accompanied by a large increase in
sweating and evaporative cooling
capacity
Sweating Response
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acclimatization increases this response
almost 3X, from ~1.5 l/hr to 4 l/hr
accompanied by a more even distribution
of sweating
sweat losses of sodium chloride decrease
b/c of increased secretion of aldosterone
Thermal Distress
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includes dehydration, heat cramps, heat
exhaustion, heat syncope, and heat stroke
hyperthermia is caused by an imbalance
between heat gain and heat loss
Dehydration
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loss of fluid from the body
can decrease sweat rate, plasma volume,
Q, VO2 max, work capacity, muscle
strength, and liver glycogen
at fluid deficit of 5% of BW, symptoms
include discomfort, and alternating states
of lethargy and nervousness
level >7% is extremely dangerous
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at levels >10%, ability to walk is
impaired, and is accompanied by
discoordination and spasticity
as 15% is neared, the person
experiences delirium, shriveled skin,
along with decreased urine volume, loss
of ability to swallow food, and difficulty
swallowing water
>20% the skin bleeds and cracks
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thirst does not keep up with fluid
requirements
physical fitness helps prevent this
during prolonged exercise, can develop
hyponatremia, caused by excessive
sodium loss in the sweat without adequate
replacement
Heat cramps
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characterized by involuntary cramping and
spasm in muscle groups used during
exercise
it occurs in people who have exercised
and sweated heavily
often the individual is conditioned and
acclimatized
fluid and electrolyte replacement, rest,
some say magnesium helps
Heat exhaustion
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rapid, weak pulse, hypotension, faintness,
profuse sweating and psychological
disorientation
results from acute plasma volume loss and
inability of the circulation to compensate
for the concurrent vasodilation in the skin
and the active working muscles
Treatment
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have person lie down in cool area
administer fluids
rest and drink plenty of fluids for next 24
hr.
not allowed to participate for rest of the
day
Heat syncope
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related to heat exhaustion, but can occur
without major sweat loss
typically, it occurs after exercise when the
individual stops moving and blood pools
can occur secondary to heat exhaustion or
independently
Heat stroke
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failure of hypothalamic temperature
regulatory center; represents a major
medical emergency
principally caused by failure of the
temperature regulatory center in
hypothalamus, which causes failure of the
body’s heat loss mechanisms
characterized by a high core
temperature, hot, dry skin, and
extreme CNS dysfunction
 associated with increases in plasma
norepinehprine, epi, and endotoxin
levels that have been associated with
vascular collapse
 risk is greatest in high temperatures,
humidity, hottest time of the day, and
on days with little wind
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Treatment
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cool person with tepid water
ice packs on the groin, neck, and axilla
send to hospital ASAP
Prevention
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ensure athletes are well-conditioned
avoid overheating
be aware of early symptoms of heat
stress: thirst, fatigue, lethargy, and visual
disturbances
athletes should not train harder than
normal intensity
should not compete if have an illness
accompanied by a fever
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schedule practice during the cooler times
of the day
modify or cancel sessions when the wet
bulb temperature is 25.5°C or greater
plan regular fluid breaks
supply a drink that is cold and contains
some CHO and electrolytes
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hyperhydrate before activity
fluid replacement should be encouraged
during the early stages of practice and
competition
athletes should be weighed every day
before and after practice. If a 2-3%
decrease in weight, should consume
more fluid, if 4-6%, should decrease
activity levels, 7% loss, consult a
physician
Rhabdomyolysis
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Myoglobin excretion is increased
Myoglobin is cleared by the kidneys
Kidneys shut down, especially with heat
Can lead to death
Occurs with excessive exercise
Rare
May be a predisposition, latent metabolic
disorder
Diuretics contribute to this
CHO as well, but a rare possibility