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Exercise and EnvironmentTemperature

•Brooks Ch 22•Astrand p 527-537

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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

25Advanced 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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• 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

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• 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 CLOSEBlood 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

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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