Heat acclimatization occurs when repeated exercise-heat exposures are sufficiently stressful to invoke profuse sweating and elevate body temperatures. Generally, about 1-2-wk of daily exposures of 90 min are required; but highly aerobic fit athletes can heat acclimatize in half that time.

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Exercise and Environment
Temperature

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• Environmental Heat
– Body temp and Heat Transfer
– Exercise in Heat
– Acclimatization to Heat
• Environmental Cold
– Exercise in Cold
– Heart, muscle and metabolic responses to
cold
– hypothermia
Outline

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• People have an ability to live and work in very hot and
very cold environments
– Able to tolerate these variable environments by tightly
regulating our internal (core) temperature - homeotherms
– We utilize behavioral and physiological means to regulate
our core temperature
• normal core temp 36.5-37.5oC
• core - defined as temperature of the hypothalamus
• Experimentally - rectal and esophageal temperatures are used
for core temperature
– Oral temperature is influenced by breathing cold air
– Tympanic temperature is influenced by head skin
temperature
Environmental Temperature

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• At temperatures > 41oC cells begin to deteriorate
• At temperatures < 34oC cellular metabolism slows
greatly, leading to unconsciousness and cardiac
arrhythmias
• During exercise core temperature (Tc) can exceed
40oC - rise is proportional to intensity
– Pregnant women should not allow temp to rise above
38.9 oC - fetal hyperthermia
• While the Tc remains ~ constant at rest
– skin temperature is influenced by the environment,
metabolic rate, clothing and hydration state - fig 22-2
Core Temperature (Tc)

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• Heat balance - fig 22-3
– directed by
hypothalamus-thermostat
– hot and cold receptors in
skin and hypothalamus
• Clothing and air movement
across the skin affect
capacity for heat loss
Core Temperature

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• Hypothalamic
control initiates
heat production
(shivering) or
dissipation
(evaporation) to
regulate core
temperature - fig
22-7
Core Temperature (Tc)

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• Radiation
– heat in form of electromagnetic radiation - 60% of heat
loss at room temperature at rest
– Sun is largest source of radiant heat
• Conduction
– transfer through direct contact
– Rate of transfer depends on temperature gradient and
conductive properties of surface
• Convection - conduction to/from air or water
– Depends on body surface area exposed to surrounding
medium and the flow of that medium
– more rapid in water (~25 times)
– heat loss is much greater in wind and moving water
• Wind-chill index fig 22-4
Heat Transfer

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• When core temp above set point (37 ºC) anterior
hypothalamus elicits physiological cooling mechanisms
• Evaporation
– 70% of heat loss in the heat
– heat absorbed by sweat as it evaporates from the skin
– 1gm sweat = 2411.3 Joules(.58kcal)
• sweat is only effective for cooling if it evaporates
– max sweat rate is ~ 1.5 L/hr in sedentary untrained
individual
– Max rate improves to ~4 L/hr with exercise acclimatization
to hot humid environments
– Eccrine glands - cooling (forehead, back, palms)
– Appocrine glands - odours (axillary and pubic regions)
Physiological responses to Heat Gain

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• Humidity - heat exchanged with environment by
vapour transfer
– Driving force is differences in humidity
• Relative humidity - given as percentage
• Ratio of water vapour in the air to saturated vapour
pressure
• Saturated vapour pressure is the vapour pressure at
which no more water can be held
• As air temperature increases, water content limit
increases
• Vasodilation - inc peripheral blood flow in the heat, inc heat
loss by convection and radiation
Physiological responses to Heat Gain

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• When core temperature drops below set-point (37 ºC)
• Posterior hypothalamus elicits physiological warming
– Increased Nor-Epinephrine
– Mobilization of FFA
– metabolic heat production (thyroxin)
• Anterior hypothalamus elicits physiological warming
– Shivering
• Increases metabolic heat production by up to 5 times
• Onset of shivering is determined by skin temperature
– Vasoconstriction
• Constriction of vascular smooth muscle cells reducing peripheral blood flow and
heat losses via convection and radiation
– Piloerection
• Hairs stand on end in order to trap still air layer against skin
• Arrector pili muscles attached to the hair follicle involuntarily contract -fig 22-5
Physiological responses to Heat Loss

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• Core temp
increase with
exercise
intensity
– Fig 22-9 - high
environmental
temp adds to
metabolic heat
stress of
exercise
Exercise in Heat

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• Cardiovascular effects
– depends on bodies ability to dissipate heat and maintain
blood flow to active ms
• during exercise (acute)
– plasma vol dec
• Due to increased BP and loss of fluid from sweat
– decreased central blood volume results in dec filling
pressure and a compensatory increased HR
• HR increase not viable at higher intensities
– near max - vasoconstrict periphery
• To maintain BP and Q - triage
– No change in VO2 max unless subject started with a
thermal imbalance
Exercise in Heat

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• Sweating Response
– Primary means of heat dissipation during exercise
– Sweat rate related more to exercise intensity than
environmental temperature
– Pre-cooling body prior to exercise may improve
performance
• When water lost though sweating is not replaced
dehydration occurs
• Dehydration process of body fluid loss
– Results in hypohydrated state
• Moderate levels of dehydration will impair CV and
temperature regulation - impact performance - fig 22-10
• A fluid loss of 5% of body weight will cause irritability,
fatigue and discomfort. This level of dehydration is common
in football and distance running.
Exercise in Heat

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• Goal of drinking during exercise is to prevent
excessive dehydration (> 2 % body weight loss)
• Hypohydration > 7% is extremely dangerous
– salivating and swallowing are difficult.
• Hypohydration > 10%
– coordination problems and spasticity.
• > 15% - delirium and shriveled skin.
• > 20% dehydration death will occur.
• Sweating results in loss of Na+, Cl-, urea, lactate
and K+ as well
• electrolytes need to be replaced after exercise
with excessive sweating
Fluid Balance

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• Making weight in a variety of sports results in voluntary
hypohydration
• Anaerobic - impact of 5% hypohydration on performance is
inconclusive
• Aerobic - VO2 max declines at 5% weight loss, even without
thermal stress
• 2% weight loss observe higher resting Tc and negation of
the benefits of acclimation and short term training
• Time to fatigue reduced in submax treadmill protocol
– Euhydration + water replacement - 106 min
– Euhydration + inadequate water - 97 min
– Hypohydration + water - 87 min
Hypohydration and Performance

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• Hyponatremia
– With excessive water replacement, + elecrolyte loss +
reduced renal blood flow due to exercise
– A severe decrease in extra-cellular sodium [ ] can result -
hyponatremia
– Can cause swelling of brain cells and death if not treated
properly
• New rehydration recommendations
– Rehydrate .4 to .8 L / hour for marathon events
– Large individuals who sweating heavily in heat at higher end of
range
– Adjust for clothing, duration and extreme conditions
– Post exercise 1.5 L for each Kg of weight loss
Fluid Balance

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• Acclimatization
– adaptations produced by a change in the natural
environment
• Acclimation
– adaptations to laboratory environment
• heat and exercise are both required for
optimal adaptational responses
• Acclimation in first 2 weeks
– dec HR, core temp, perceived exertion, skin temp at rest
and submaximal exercise
– reduce losses of minerals (sweat and urine)
Acclimatization

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Advanced Exercise Physiology, ACSM, 2006

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• Table 22-2
– increased plasma volume (range 3 - 27 %)
– increased sweat capacity (1.5 - 4 L per hour)
– decreased core temp at onset of sweating
– decreased skin blood flow - improved skin distribution of
sweat
– reduce losses of minerals (sweat and urine)
Acclimatization

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Advanced Exercise Physiology, ACSM, 2006

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• Majority of physiological adjustment in 4-6 days
– does not occur without exercise
– intense exercise most effective
– humidity specific adaptation
– athletes a little faster in acclimatizing
– sweat rate and mineral changes take up to two weeks to
adapt
• Loss of Acclimatization occurs in absence of heat
and physical fitness
• physically fit retain benefits longer than sedentary
individuals
• Adaptations to dry heat last longer than adaptation
to humid heat
Individual Variation

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• If ambient temperature is above skin temperature, heat loss
must be nearly all through evaporation
• If ambient temperature is above average skin temperature
(35 ºC) heat loss through convection is lost.
– Increased air velocity actually warms you up.
• Rate of heat storage determines how long a worker can be
exposed to a hot environment
• Wet bulb Globe Temperature (WBGT) index is most
common index for heat stress to protect workers
• WBGT = 0.7tnwb +0.2tg + 0.1ta
– tnbw = temp of naturally ventilated wet bulb thermometer
– tg = 150mm diameter black globe temperature
– ta = air temperature
Work in the Heat

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WBGT index is adjusted for clothing insulation:
Clo value WBGT correction (ºC)
0.6 – Summer work uniform 0
1.0 – Cotton overalls -2
1.4 – Winter work uniform -4
1.2 – Impermeable layer -6
Critical WBGT index (prescriptive zone) also
adjusted for metabolic heat production

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• cold core temperature during exercise is rare except
when survival is at stake
– Protective clothing and high metabolism of exercise
usually prevents drop in core temp
• Work, however, has a lower metabolic output, long
hours and may create an increased risk in cold
environments
• Movement in cold
– Numbing of exposed flesh
– Cumbersome protective clothing
• Manipulation with hands difficult
Exercise In Cold

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• Clothing-layers and breath ability important
– Must balance insulation value with heat production of
exercise
– Additional clothing after exercise important
– Metabolism drops, heat loss remains high
• Shivering increases metabolic costs
– Increases perception of effort
– May also impair movement patterns
– Agonist and antagonist contract
Exercise In Cold

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• O2 consumption
– VO2 max - unaffected by cold
• Submax VO2 increases at lower intensities (*work*)
– Due to higher heat loss
• Inc skin and ms blood flow during exercise in any temperature
• Greater thermal gradient in the cold results in greater heat loss
– Wet clothing in wind - 15-20% higher VO2 requirements
• Table 22-1 - exercise in cold
CV responses in Cold

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• Shivering - Inc VO2 utilization
– May also be increase in non-shivering thermogenesis -
due to inc catecholamines (stress) and leptin
• Swimming in cold water
– Reduced VO2 peak at higher intensities- fig 22-8
CV responses in Cold

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Exercise in Cold
Advanced Exercise Physiology, ACSM, 2006

38. 38
• Exercise can partially replace heat production of shivering
during cold
– Peripheral vasodilation with exercise - reduces insulation in body
– Exercise followed by cold exposure - higher threshold for
vasoconstriction and shivering
• Ventilation
– Inc ventilation - especially with sudden exposure - gasping reflex
– Hyperventilation, tachycardia, peripheral vasoconstriction,
hypertension
– Reduced blood CO2 - vasoconstriction in brain - confusion,
unconsciousness
Exercise in Cold

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• Heart
– Cold - peripheral vasoconstriction - Inc central blood
volume - inc BP
– Arrythmias - increase in cold
– Inc afferent impulses to hypothalamus and
cardiovascular control center
– Increase adrenal epinephrine
– Ventricular fibrillation - leading cause of death in people
with hypothermia
Exercise in Cold

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What happens to our hands in the cold
In the hand, blood flow is regulated
by the AVA’s (Arteriovenous
Anatomoses)
– Body is warm: AVA’s OPEN
Blood flows in large quantities
from the arteries through the
AVA’a to the superficial veins
– Body cools: AVA’s CLOSE
Blood flow is drastically reduced
due to increased sympathetic
activity
The remaining flow will return to the
body core through deep veins, which
are located close to the arteries

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Physiological Amputation
• Due to
reduced blood
flow, very little
heat input to
the hand
remains
• Essentially the
same effect as
occlusion

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• Muscle strength
• Muscle strength and peak power decrease as
muscle temp decreases - dec enzyme activity
– May require increase motor unit recruitment to
compensate for reduced output
– May also see reduced muscle blood flow
– Combine to increase lactate production and reduce its
clearance - early fatigue
Cold Exposure

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Cold Exposure
Advanced Exercise Physiology, ACSM, 2006

44. 44
Cold Exposure
Advanced Environmental Exercise Physiology, Human Kinetics, 2010

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• Endurance work with the hands show the best performance
at a muscle temp of 28 ºC - decrements below this temp
• Max power and contraction speed requires an optimal
temperature of 38 ºC
• Manual dexterity
– Reduced with decreased temperature
– Dec nerve conduction velocity (afferents and efferents)
– joint stiffness - plays major role
– Observe decrease in manual dexterity below skin temp of 20 ºC (27
ºC intra-articular temperature
– Strong decrease below 15 ºC skin (24 ºC intra-articular temperature
Cold Exposure

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• Metabolic Changes
• Increased use of carbohydrates during exercise in the cold
– Muscle glycogen reduced faster in light exercise - augments
increase in lactate
– Prolonged exposure - hypoglycemia
• Suppresses shivering (threshold -.5 Celsius lower
• Accelerates hyopthermia
• Fat metabolism depressed even with catecholamine rise
– may be due to reduced subcutaneous circulation
• These problems are compounded by fatigue, sleeplessness
and underfeeding
Cold Exposure

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• Shivering threshold
• Maintain temperature without shivering, or with less shivering
– Experiment - three week exposure - inc thyroid hormone - tissues more
sensitive to Nor epinephrine and Epinephrine
– Uncoupled ox phosphorylation
– Heat released without ATP formed
– Leptin - released from Adipose
– Stimulates Sympathetic NS
• Sleeping Ability in the cold - Depends on non shivering thermogensis
– Aborigines (Australia) - vasoconstriction of periphery - sleep in cold
without covering
• Temperature of hands and Feet
– Unacclimatized - temperature decreases progressively
– Acclimatization - temperature maintained
– Intermittent vasodilation in periphery
• Hunting response - CIVD (cold induced vasodilation)
– Habituation as well, become more tolerant
Acclimatization to cold

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At a certain skin temperature, AVA’s in the hand open and blood flows through
the hand increasing hand temperature. Once hand temperature increases, AVA’s
close
- cyclic behaviour
Onset of CIVD at a skin temperature of approx. 20ºC
Warm core = greater levels of CIVD (cold induced vasodilation)
Cold core = eliminates effects of CIVD

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• Core Temperatures
• Mild - 32-35 ºC
• Moderate - 28 - 32 ºC
• Severe - below 28 ºC
• Hypothalamus ceases to control body temp at
extremely low temperatures (< 30 ºC)
– CNS depressed
– Lose ability to shiver
– Sleepiness - - -> coma
– Reduced metabolic rate --> dec temperature
Hypothermia

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• Cardiovascular
– Central blood volume decrease
– Exacerbated by
• inadequate fluid intake
• Plasma sequestration
• Cold diuresis
• Hypothermia is possible during endurance exercise events
– Heat loss greater than production
– Glycogen depletion - blood glucose declines, CNS functioning
declines
Hypothermia