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THERMOREGULATIONTHERMOREGULATION

ANDAND

EXERCISESEXERCISES

• Introduction

• Mechanisms of Body Temperature • Physiological Response to Exercise in the Heat• Health Risks During Exercise in the Heat• Acclimatization to Exercise in the Heat

• Exercise in the Cold• Physiological Responses to Exercise in the Cold• Health Risks During Exercise in the Cold• Cold Acclimatization

IntroductionIntroduction

• Homeostasis – Attempt to maintain a constant internal environment

• Body temp. – 36.10 C to 37.80 C97 F0 to 1000 F

• Room temp. – 210 C to 250 C 69.80 F to 770 F

• Converted formula: C to F → 9/5C + 32

F to C → 5/9(F-32)

Heat Exchange MechanismHeat Exchange Mechanism• Temp. variation 10C• Situations to change: heavy exer., illness, heat & cold (extreme

conditions)• Reason to change:

•• Transfer of Body Heat:Transfer of Body Heat:

Heat ProductionHeat Productionoror

Heat gainHeat gain==// Heat lossHeat loss

Heat form Heat form deep bodydeep body

BloodBloodSkinSkin

4 mechanism4 mechanismEnvironmentEnvironment11

1. Guyton AC, Hall JE. Textbook of medical physiology. Philadephia (PA): Saunders Company, 1996

• Inactive skeletal muscle temp. 330C and 350C• Onset of exercises:

• Rate of heat transfer between core body to skin is determined by

1. Temp. gradient between these two2. By overall skin conduction

↑muscle temp.↑muscle temp.Reversal of temp. Reversal of temp.

gradient betwngradient betwnmuscle & arterial muscle & arterial

bloodblood

Heat from Heat from muscle to muscle to

arterial bloodarterial blood

Body core Body core

SkinSkin

RadiationRadiation

ConductionConduction

ConvectionConvection

EvaporationEvaporation

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•• Conduction:Conduction:One material to another through direct molecular contact

•• Convection:Convection:One place to another by the motion of a gas or a liquid

across the heated surface Total heat loss:

Body to Air – 10% to 20%Body to Water – 26 times greater

Motion/movementMotion/movementof gas or liquidof gas or liquid Rate of heat removalRate of heat removalαα

•• Radiation:Radiation:

Heat loss occurs in the form of infrared rays (electromagnetic waves)

Thermogram

- Skin constantly radiates heat in all directions to theobjects around it (vice versa)

- At rest it is the primary method for discharging the body’s excess heat

- At rest, the nude body loses about 60% of its excess heat by radiation

ThermogramThermogram

•• Evaporation:Evaporation:Primary avenue for heat loss during exe.

During exercises – 80% of heat loss

At rest – 20% of heat loss

Insensible water loss: (without awareness)

- It occurs in skin, lungs & mucosa- It accounts 10% of heat loss

- No control mechanism

Mechanism:High body High body

temp.temp.Sweat Sweat

productionproductionSweat reachesSweat reaches

skinskin

Liquid sweat Liquid sweat converted converted

as vapor by heatas vapor by heat

1 L of sweat = 580 kcal (2,428kj)3.5L/h reported in trained athletes

Mechanism of Heat LossMechanism of Heat Loss (Summary)2

Mechanism of heat loss Rest Exercises (70% VO2 max)

% total Kcal/min % total Kcal/min

Conduction & convection

20 0.3 15 2.2

Radiation 60 0.9 5 0.8

Evaporation 20 0.3 80 12.0

1.5 15

1. Cheuvront SN, Haymes EM. Thermoregulation and marathon running. Sports Med 2001; 31: 743-622. Jack H. Wilmore, David L. Costill: Physiology of Sport and Exercises (2nd Edi.); Human Kinetics: 1998

• By the four mechanism heat loss/gain defined by heat balance equation1

Metabolism ± Radiation ± Conduction ± Convection –Evaporation = Heat storage

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•• The role of hypothalamusThe role of hypothalamus

•• Cutaneous VasodilationCutaneous Vasodilation

•• SweatingSweating

Physiology of Temperature RegulationPhysiology of Temperature Regulation• Body’s thermostat• It has large number of heat sensitive & ⅓ of cold sensitive

neurons1

Anterior hypothalamus – dealing with ↑ in body heatPosterior hypothalamus – responsible for ↓ in body temp.2

• It is sensing core body temp. & temp of throughout the body• Neurons compare & integrate the central & peripheral temp.

information3

• It has “set-point”4 & sensitive of change as small as 0.010C (0.0180F)

A. The Role of the HypothalamusA. The Role of the Hypothalamus

1. Cooper KE. Some historical perspectives on thermoregulation. J Appl Physiol 2002; 92: 1717-242. Scott K. Powers, Edward T.Howley, Exercise Physiology, 3rd Edition, McGraw-Hill 3. Boulant JA. Role of the preoptic-anterior hypothalamus in thermoregulation and fever. Clin Infect Dis

2000; 31 Suppl. 5: S157-614. Benzinger TH. Heat regulation: homeostasis of central tempera ture in man. Physiol Rev 1969;

49: 671-759

HYPOTHALAMUS HYPOTHALAMUS Control of Heat ExchangeControl of Heat Exchange

• Body temp – 98.60F/370C

• During exe internal temp exceed 400C & active muscle temp exceed 420C

Energy system – efficientNerves system – in efficient

• Temperature regulation: Thermoreceptors – 1. Central

2. PeripheralCentral – located in hypothalamus (monitor temp of blood)

Highly sensitive (0.010C enough)Peripheral – located in skin (monitor temp around body)

Information to Hypothalamus & cerebralcortex

HyperthermiaHyperthermia HyporthermiaHyporthermia

Impulses go to hypothalamusImpulses go to hypothalamus

Vasodilation occurs in skinVasodilation occurs in skinblood vessels, more heat lossblood vessels, more heat loss

across the skin across the skin

Sweat glands become more activeSweat glands become more active↑ evaporation heat loss↑ evaporation heat loss

↓ blood and/or skin temp↓ blood and/or skin temp

Impulses go to hypothalamusImpulses go to hypothalamus

Vasoconstriction occursVasoconstriction occursIn skin blood vessels In skin blood vessels –– lessless

Heat lossHeat loss

Skeletal muscles activated,Skeletal muscles activated,causing shivering, which causing shivering, which ↑↑

metabolism & generates heatmetabolism & generates heat

Body temperature increasesBody temperature increases

↑ blood and/or skin temp↑ blood and/or skin temp

Heat loadHeat load

CoreCore

SkinSkin

HypothalamusHypothalamus

CutaneousCutaneousVasodilationVasodilation

SweatingSweating

EffectorsEffectors

IntegratorIntegrator

Thermal receptorsThermal receptors

Physiological response to an increase in “heat load”Physiological response to an increase in “heat load”

Scott K. Powers, Edward T.Howley, Exercise Physiology, 3rd Edition, McGraw-Hill

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Physiological responses to cold stressPhysiological responses to cold stress

ColdCold

SkinSkin

CoreCore

HypothalamusHypothalamus

IntegratorIntegrator

ReceptorsReceptors ShiveringShivering

CutaneousCutaneousvasoconstrictionvasoconstriction

CatecholamineCatecholaminereleaserelease

ThyroxinThyroxinreleaserelease

EffectorsEffectors

Scott K. Powers, Edward T.Howley, Exercise Physiology, 3rd Edition, McGraw-Hill

B. Cutaneous VasodilationB. Cutaneous Vasodilation

• Cutaneous circulation controlled by 2 branch of SNS- Noradrenergic active vasoconstrictor system- Active vasodilator system of uncertain

neurotransmitter1

• Release of an unknown co-transmitter from cholinergic nerves is the primary mechanism of cutaneous active vasodilation2

• Under stress – SkBF approximately 8L/m or 60% of cardiac output

1. Kenney WL, Johnson JM. Control of skin blood flow during exercise. Med Sci Sports Exerc 1992; 24: 303-12

2. Kellogg DL, P´ergola PE, Kosiba WA, et al. Cutaneous active vasodilation in humans ismediated by cholinergic nerve co-transmission. Circ Res 1995; 77: 1222-8

C. SweatingC. Sweating

• Activation of eccrine sweat glands causes sweat

• These glands covers most of the body & innervated by sympathetic cholinergic nerve fibres

• SNS stimulation on sweat gland elicits secretion of a so-called precursor fluid

• Precursor fluid: composition of which resembles that of plasma, except plasma proteins

SweatSweat• Sweat production mechanism• Reabsorbtion – Na+ & Cl-1• During light sweating – complete reabsorption• ↑ rate of sweating (during intense exe) – less time for

reabsorb• Na+ & Cl- concentration in sweat:

1. Dani¨el Wendt, Thermoregulation during Exercise in the Heat, Sports Med 2007; 37 (8):669-682

Subjects Na+ (mmol/L) Cl- (mmol/L) K+ (mmol/L)

Untrained ♂ 90 60 4Trained ♂ 35 30 4

Untrained ♀ 105 98 4Trained ♀ 62 47 4

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Sweat mechanism with well trainedSweat mechanism with well trained• Release of hormones

1. Aldosterone2. Antidiuretic hormone (ADH)

• Aldosterone ( release from Adrenal cortex)

1. In response to stimuli- ↓ blood Na+ content- ↓ blood volume- ↓ BP

2. FunctionsStrong stimulation of sweat glands- more reabsorbtion of Na+ & Cl-- limits Na+ excretion from kidneys

3. Uses- more Na+ retention → water retention- maintaining blood Na+ level

• Antidiuretic hormone

• Stimulation of water reabsorbtion from the kidneys• It promotes fluid retention → compensate mineral &

water loss

• Loss of sweatingLoss of sweating

In hot condition & heavy exercises →

1L / hr / sqm of body surface area

That is 2 – 4% of body weight

It may be 6 – 10% for long distance runners

Role of Kidney & Hypothalamus in thermoregulationRole of Kidney & Hypothalamus in thermoregulation

Rose SyndromeRose Syndrome

• Ross syndrome is described as a rare clinical disorder of unknown cause

• It is described by Ross in 1958• Characterized by the triad of tonic pupil,

hyporeflexia/areflexia and segmental anhidrosis

1. Maria Nolano et all., Ross syndrome: a rare or a misknown disorder ofthermoregulation? A skin innervation study on 12 subjects; Brain (2006), 129, 2119–2131

Physiological Response to Exercises in the HeatPhysiological Response to Exercises in the Heat

• Exercises at 210 – 260C

• Producing heat burden to the heat controlling mechanism

• Occurring physiological changes when body exposed to heat stress

• Heat stress: Any environmental condition that causes an ↑ in body temp & jeopardizes homeostasis

1.Cardiovascular Function2.Energy Production

1. Cardiovascular Function:1. Cardiovascular Function:• Exercises + Heat → Burden + Additional burden

• Heat transmission from muscle skin

• CO (Q) must be shared to - skin &- working muscle

• Exercises in hot environment sets up a competition between

• Redistribution of blood - ↓ cardiac input

• ↓ End-diastolic volume

• ↓ SV, ↓ Arterial pressure (systemic & pulmonary)

Circulatorysystem

Active muscleActive muscle VsVs Skin (for the limited blood supplySkin (for the limited blood supply

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• Cardiovascular Drift:An increase in HR during exercise to

compensate for a decrease in SV. This compensation helps maintain a constant ‘Q’

Best endurance performance are achieved in cool conditions

2. Energy Production:2. Energy Production:

• ↑ O2 uptake• Working muscle use more glycogen & to produce

more lactate compared with exe in cold

Glycogen depletion & ↑ muscle lactate

Sensation of fatigue & exhaustion

VO2 maxVO2 maxHRHR

Blood LactateBlood LactateHigh in Hot environmentHigh in Hot environment

Exercises Vs EnvironmentExercises Vs Environment

Factors Influencing ThermoregulationFactors Influencing Thermoregulation

• The Environment (Humidity factors)

• Dehydration

1. Humidity (water vapor) & Heat Loss1. Humidity (water vapor) & Heat Loss

• Major role in heat loss – evaporation

• Concentration gradient:Changes in the concentration of a substance from one

area to another

• High humidity – limits sweat evaporation & heat loss1

• Low humidity – more sweat, high H2O loss & dehydration1

1. Cheuvront SN, Haymes EM. Thermoregulation and marathon running. Sports Med 2001; 31: 743-62

Consider two situationsConsider two situations

Temp Temp –– 32.232.200C/90C/9000FF

Profused sweatProfused sweat

High evaporationHigh evaporation

Minimal / no awareness Minimal / no awareness of sweatof sweat

Large amount of heat removalLarge amount of heat removal

1 litter seat evaporation results1 litter seat evaporation resultsloss of 580 kcal(2428 kj) loss of 580 kcal(2428 kj)

Exposure to ExerciseExposure to Exercise

Temp Temp -- 32.232.200C/90C/9000FF

Profused sweatProfused sweat

Low evaporationLow evaporation

Sweat drips off the skinSweat drips off the skin

Little heat removalLittle heat removal

Humidity Humidity –– 10% 10% Humidity Humidity –– 90%90%

Risk of developing heat illnessRisk of developing heat illness

2. Dehydration2. Dehydration• Sweat loss must be matched by fluid consumption to avoid

dehydration• The stimulus to drink is not initiated until an individual has

incurred a water deficit of approximately 2% of body mass1

• The physical work capacity for aerobic exercise is reduced when a person is dehydrated by marginal (1–2% total body water [TBW]) water deficits

• Dehydration reduces both skin blood flow and sweating responses during exercise1

• Thermal dehydration results in a hyperosmotic, hypovolaemic condition & this ↓ heat loss effector response2

1. Sawka MN, Montain SJ. Fluid and electrolyte supplementation for exercise heat stress. Am J Clin Nutr 2000;72: S564-72

2. Sawka MN. Physiological consequences of hypohydration: exercise performance and thermoregulation. Med Sci Sports Exerc 1992; 24: 657-70

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Effectors that alter body temperature:Effectors that alter body temperature:

Hot environmentHot environment

1.1. Sweat glandsSweat glands2.2. Smooth muscles around Smooth muscles around

the arteriolesthe arterioles

Cold environmentCold environment3. Skeletal muscles3. Skeletal muscles4. Several endocrine glands4. Several endocrine glands

-- Thyroxine from thyroid Thyroxine from thyroid glandgland

-- CatecholaminesCatecholamines

Assessment Assessment -- Mean Body TemperatureMean Body Temperature• Tbody – Weighted average of skin temp & internal

body temp

• Skin temp measurement:Placing sensors (thermistors) on the skin in diff

areas of the body (arm, trunk, leg & head)• Tskin =(0.1x Ta)+(0.6XTt)+(0.2XTl)+(0.1XTh)• Internal body temp:

It can be measured following areas- Rectum (Tr)- Tympanic membrane in the ear- Esophagus

Tbody = (0.4 X Tskin) + (0.6 X Tr)

Heat Content of the BodyHeat Content of the Body• It is total calories of heat contained in the body tissue

• 0.83 – Average specific heat of the body (kcal/kg/C0)

• Specific heat – Amount of heat required to ↑ temp of the substance by 10C

• Kcal is the unit of measure for heat energy

• 1kcal is equal ‘the amount of heat energy needed to raise 1kg of water 10C at 150C

HC = 0.83 (Wtb X Tbody)

Heat IllnessHeat Illness

1.1. Heat CrampsHeat Cramps

2.2. Heat ExhaustionHeat Exhaustion

3.3. Heat StrokeHeat Stroke

HIGH TEMPERATURE + HIGH HUMIDITY + PHYSICAL WORKHIGH TEMPERATURE + HIGH HUMIDITY + PHYSICAL WORK= HEAT ILLNESS= HEAT ILLNESS

1. Heat Cramps1. Heat Cramps• Least serious & characterized by severe cramping of skeletal

muscle

• First reported by Talbot1

• Heat cramps are painful, involuntary muscle spasms that usually occur during heavy exercise in hot environments

• Muscles most often affected - calfs, arms, abdominal wall and back

•• Causes:Causes: 1.The exact cause of heat cramps is unknown, 2. Probably related to electrolyte problems (mineral loss) 3. Dehydration 4. High rate of sweating

1. Talbot HT. Heat cramps. Medicine 1935; 14: 323-76

•• Symptoms: Symptoms: First signal that the body is having trouble with theheatMuscle spasms

- Painful- Involuntary- Brief- Intermittent - Usually self-limited

•• Treatment:Treatment:- Firm pressure on cramping muscles - Gentle massage to relieve spasm - Sips of water. - If nausea occurs, discontinue use

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2. Heat Exhaustion2. Heat Exhaustion• A heat disorder resulting from an inability of the CV system to

meet all the body tissues needs while shifting blood to the periphery for cooling

• It typically occurs when blood volume ↓, by either excessive fluid loss or mineral loss from sweating

•• SymptomsSymptoms: 1. Extreme fatigue, 2. Breathlessness,3. Dizziness, 4. Vomiting, 5. Fainting, 6. Cold & Clammy or Hot and dry skin, 7. Hypotension 8. A weak, rapid pulse9. Elevated temperature

Development of Heat ExhaustionDevelopment of Heat ExhaustionHeatHeat

ExcessiveExcessiveSweatingSweating

Cutaneous Cutaneous Arteriolar dilationArteriolar dilation

HypovolemiaHypovolemia

Decreased Cardiac OutputDecreased Cardiac OutputDecreased Mean Arterial PressureDecreased Mean Arterial Pressure

Circulatory ShockCirculatory Shock

•• Treatment:Treatment:1. Rest in cooler environment 2. Feet elevated to avoid shock3. Administration of salt water (if conscious)4. IV – saline (if unconscious)

• If allowed to progress, heat exhaustion can deteriorate to heat stroke

3.Heat Stroke3.Heat Stroke• The most serious heat disorder, resulting from failure of the

body’s thermoregulatory mechanisms• A life-threatening illness & characterized1 by

- A rise in internal body temperature to a valueexceeding 400C (1040F)

- Cessation of sweating- Hot & dry skin- Rapid pulse & Respiration- Usually Hypertension- Confusion & Delirium- Unconsciousness- Convulsions or Coma

• Redistribution of blood & loss of fluids due to sweating leads to a decline in central blood volume2

1. Bouchama A, Knochel JP. Heat stroke. N Engl J Med, 2002; 346: 1978-882. Hales JRS. Hyperthermia and heat illness. Pathophysiological implications for avoidance and treatment.

Ann NY Acad Sci, 1997; 813: 534-44

Development of Heat StrokeDevelopment of Heat StrokeStrenuous ExerciseStrenuous Exercise

Hot, Humid EnvironmentHot, Humid Environment

Inadequate TemperatureInadequate TemperatureRegulationRegulation

Severe Elevation of Severe Elevation of Core TemperatureCore Temperature

Impaired CNSImpaired CNSFunctionFunction

Organ and Tissue Damage Organ and Tissue Damage

DeathDeath

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Heat CrampsHeat Cramps

Muscle CrampsMuscle Cramps

Heat ExhaustionHeat Exhaustion Heat StrokeHeat Stroke

WeaknessWeakness

Pale & Cool skinPale & Cool skin

FatigueFatigue

Profuse SweatingProfuse Sweating

ThirstThirst

ConfusionConfusion

Hot & Dry skinHot & Dry skin

Strong & Rapid PulseStrong & Rapid Pulse

Faintness & DizzinessFaintness & Dizziness

Cessation of sweatingCessation of sweating

Chills or goose bumpsChills or goose bumps

Headache & nauseaHeadache & nausea

Increasing SeverityIncreasing Severity

WARNING SIGNS OF HEAT DISORDERSWARNING SIGNS OF HEAT DISORDERS Measuring Heat StressMeasuring Heat Stress• Wet Bulb Globe Temperature (WBGT):

A measurement of temp that simultaneously accounts for conduction, convection, evaporation, & radiation, providing a single temp reading to estimate the cooling capacity of surrounding environmentThe apparatus for measuring WBGT consists of a dry bulb, a wet bulb, &

a block globe

• The dry bulb measures the actual air temp (TDB)

• The wet bulb is kept moist, & tmep (TWB) is lower than the dry bulb

• The diff between wet & dry bulb temp indicates the environment ‘s capacity for cooling by evaporation

• The black globe absorbs radiated heat & its temp (TG) is good indicator for the environment’s capacity for transmitting radiated heat

• Overall atmospheric challenge to body temp in a specific environment

WBGT = 0.1TWBGT = 0.1TDBDB + 0.7T+ 0.7TWBWB + 0.2T+ 0.2TGG

Intervention Strategies for Exercise in the Intervention Strategies for Exercise in the HeatHeat• Whole-Body Precooling

• Hyperhydration

• Clothing

• Heat Acclimatisation

• RehydrationProven that particularly

effective

Other basic strategies

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WholeWhole--Body PrecoolingBody Precooling• The basis of whole-body precooling is to reduce heat content of the body

by cooling the periphery before exercise

• It is achieved by a variety of methods:- Cold air cooling- Cold water immersion- Water cooled garment1

• Current evidence2:It is able to ↑ capacity for prolonged exercises at various ambient temperature

• Disadvantage:- Practical application is limited- More time required to achieve sufficient body cooling

1. Daanen HA, van Es EM, de Graaf JL. Heat strain and gross efficiency during endurance exercise after lower, upper or Whole body precooling in the heat. Int J Sports Med 2005; 26: 1-10

2. Marino FE. Methods, advantages, and limitations of body cool-ing for exercise performance. Br J Sports Med 2002; 36: 89-94

HyperhydrationHyperhydration• Dehydration influence thermoregulation

• Hyperhydration induced by having subjects over drink plain water1

• Glycerol solutions are used as a hyperhydration agent

• Glyceral reduce the rate of elimination of water1

• Lyons et al2. gave subjects 2L of water with & without glycerol over a 2.5 hr period prior to treadmill walking in a hot, dry environment

Result: Glycerol ingestion showed a substantially lower core temp (0.70C), a reduction in urine output & a higher sweating rate (33%)

1. Sawka MN, Montain SJ, Latzka WA. Hydration effects on thermoregulation and performance in the heat. Comp Biochem Physiol 2001; 128: 679-90

2. Lyons TP, Riedesel ML, Meuli LE, et al. Effects of glycerol-induced hyperhydration prior to exercisein the heat on sweating and core temperature. Med Sci Sports Exerc 1990; 22: 477-83

• Montner et al.1 reported that glycerol hyperhydration increased time-to exhaustion in temp climates, but found no significant thermoregulatory advantages

• Latzka et al.2 found that glycerol hyperhydration extended endurance time (from 30 to 34 mins) in subjects exposed to uncompensable heat stress, but that it had no beneficial effect on thermoregulaton

• Conclusion:There are some indicaations that hyperhydration reduces

thermal strain during exercise, but data supporting this notion are not very robust

1. Montner P, Stark DM, Riedesel ML, et al. Pre-exercise glycerol hydration improves cycling endurance time.Int J Sports Med 1996; 17: 27-33

2. Latzka WA, Sawka MN, Montain SJ. Hyperhydration: tolerance and cardiovascular effects during uncompensable exercise heat stress. J Appl Physiol 1998; 84: 1858-63

ClothingClothing• The type and amount of clothing worn can have a major

impact on heat dissipation during exercise1

• It interferes with the evaporation of sweat & ↑ skin and core tempe, as well as reduction in cooling efficiency are observed

• Clothing that poses the least amount of resistance to evaporation may prove beneficial

• In practice, this would mean relatively minimal clothing, which can range from a basic swimsuit to a short-sleeve t-shirt and mid-thigh shorts2

1. Maughan RJ, Shirreffs SM. Exercise in the heat: challenges and opportunities. J Sports Sci 2004; 22:917-27

2. Gavin TP. Clothing and thermoregulation during exercise. Sports Med 2003; 33: 941-7

Heat AcclimatisationHeat Acclimatisation• Regular exposure to hot environments results in a number of

physiological adaptations that reduce the negative effects associated with exercise in the heat

• These adaptations1 includes- ↓ body core temperature at rest- ↓ heart rate during exercise- ↑ sweat rate and sweat sensitivity- ↓ sodium losses in sweat and urine- An expanded plasma volume (PV)

1. Armstrong LE, Maresh CM. The induction and decay of heat acclimatisation in trained athletes. Sports Med 1991; 12: 302-12

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Practical Recommendations for Heat AcclimatisationPractical Recommendations for Heat Acclimatisation• Process of acclimatisation to exercise in the heat begins

within a few days, and full adaptation takes 7–14 days

Adaptation Days of heat Acclimatisation

↓ HR during exercise 3 – 6

Plasma volume expansion 3 – 6

↓ in sweat Na+ & Cl- concentrations 5 – 10

↑ in sweat rate & sweat sensitivity 7 – 14

↑ in cutaneous vasodilation 7 – 14

Range of days required for different adaptations to occur Range of days required for different adaptations to occur during heat acclimatisationduring heat acclimatisation11

1. Armstrong LE, Maresh CM. The induction and decay of heat acclimatisation in trained athletes. Sports Med 1991; 12: 302-12

• It is not necessary to train every day in the heat

• Exercising in the heat every third day for 30 days results in the same degree of acclimatisation as exercising every day for 10 days1

• It has been recommended that strenuous interval training or continuous exercise should be performed at an intensity exceeding 50% of an athlete’s maximal oxygen uptake2

• Evidence says that exercise bouts of about 100 minutes are most effective3

1. Fein LW, Haymes EM, Buskirk ER. Effects of daily and intermittent exposure on heat acclimation of women. Int J Biomet 1975; 19: 41-52

2. Armstrong LE, Maresh CM. The induction and decay of heat acclimatisation in trained athletes. Sports Med 1991; 12: 302-12

3. Lind AR, Bass DE. Optimal exposure time for development of heat acclimation. Fed Proc 1963; 22: 704-8

• It is a transient process and will gradually disappear if not maintained by repeated exercise-heat exposure

• It appears that the first physiological adaptations to occur during heat acclimatisation are also the first to be lost1

• Most studies show that dry-heat acclimatisation is better retained than humid-heat acclimatisation2

1. Armstrong LE, Maresh CM. The induction and decay of heat acclimatisation in trained athletes. Sports Med 1991; 12: 302-12

2. Pandolf KB. Time course of heat acclimation and its decay. Int J Sports Med 1998; 19: S157-60

Overview of principles regarding heat Overview of principles regarding heat acclimatisationacclimatisation

• Full adaptation takes 7–14d to be completed

• Heat acclimatisation is best achieved by strenuous interval training or continuous exercise at ≥50% of maximal oxygen uptake for at least 1h every 3d

• Exercise bouts of about 1.5–2.0h seem most effective for the induction of heat acclimatisation

• Acclimatisation responses are maintained for at least 1wk, but probably <1mo

RehydrationRehydration

• Heat acclimatisation actually increase the requirement for fluid replacement because of the earlier onset of sweating1

• Core temperature responses after dehydration are the same for unacclimatised and acclimatised individuals

• Indicating that the advantages conferred by heat acclimatisation are abolished by dehydration2

• Rehydration during exercise in the heat should therefore be made a clear priority

1. Armstrong LE, Maresh CM. The induction and decay of heat acclimatisation in trained athletes. Sports Med 1991; 12: 302-122.

2. Sawka MN, Montain SJ. Fluid and electrolyte supplementation for exercise heat stress. Am J Clin Nutr 2000; 72: S564-72

• Rehydration can be achieved with beverages

• Factors that influence the effectiveness of a beverage1:- Rate of gastric emptying- Intestinal absorption- How well the fluids are retained in the intra- & extracellularfluid compartments

• Rate of gastric emptying:

• The volume effect can be overruled by chemical composition of a drink

1. Gisolfi CV, Summers RD, Schedl HP, et al. Effect of sodium concentration in a carbohydrate-electrolyte solution on intestinal absorption. Med Sci Sports Exerc 1995; 27: 1414-20

Emptying is closely related to gastric volumeVolume α Emptying

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• CHO + Sports drinks – Slow rate of gastric emptying by its effects on

- Energy content- Osmolality1

• Studies shows that the greater importance of energy content in the control of gastric emptying2

• Gastric emptying & Fluid absorption in small intestine

• CHO & Na+ content in sports drinks

1. Brouns F. Gastric emptying as a regulatory factor in fluid uptake. Int J Sports Med 1998; 19: S125-82. Vist GE, Maughan RJ. The effect of osmolality and carbohydrate content on the rate of gastric emptying

of liquids in man. J Physiol 1995; 486: 523-31

Practical Recommendations for RehydrationPractical Recommendations for Rehydration• Rate of consume fluid should closely matches loss of water

through sweating and urine losses1

• This generally requires the ingestion of 200–300mL of fluid every 10–20 minutes2

• Beneficial effects of fluid intake during events lasting <20–30 minutes may be small3

• Events lasting >30 minutes are advised to drink 200–300mL of their preferred sports drink just before exercise

1. Convertino VA, Armstrong LE, Coyle EF, et al. American College of Sports Medicine position stand: exercise and fluid replacement. Med Sci Sports Exerc 1996; 28: i-vi

2. Casa DJ, Armstrong LE, Hillman SK, et al. National athletic trainers’ association position statement:fluid replacement for athletes. J Athl Train 2000; 35: 212-24

3. Maughan RJ, Leiper JB, Vist GE. Gastric emptying and fluid availability after ingestion of glucose and soy protein hydrolysate solutions in man. Exp Physiol 2003; 89: 101-8

• To continue drinking the same sports drink throughout the event until there are 20 minutes remaining

• Maintaining 400–600mL of fluid in the stomach will optimise gastric emptying4

• >7% carbohydrate are associated with a delay in gastric emptying and reduced intestinal absorption5

• There is evidence that solutions with multiple forms of carbohydrate can produce a greater absorption of solute and water than solutions with only a single form of carbohydrate6

4. Convertino VA, Armstrong LE, Coyle EF, et al. American College of Sports Medicine position stand: exercise and fluid replacement. Med Sci Sports Exerc 1996; 28: i-vi

5. Murray R. Rehydration strategies: balancing substrate, fluid, and electrolyte provision. Int J Sports Med1998; 19: S133-5

6. Leiper JB, Brouns F, Maughan RJ. Effects of variation in the type of carbohydrate on absortion fromhypotonic carbohydrate-electrolyte solutions (CES) in the human jejunal perfusion model. J Physiol1996; 495: 128

Overview of principles regarding RehydrationOverview of principles regarding Rehydration

• Consumption of fluids should closely match the rate of water loss

• It takes 20–30 min for ingested fluids to be evenly distributed throughout the body

• The use of sports drinks with a 7% carbohydrate content improves intestinal water absorption

• Water retention can be optimised by the ingestion of solutions containing at least 50 mmol/L sodium in a volume & >1.5 times the amount of sweat lost

Exercises in the ColdExercises in the Cold

• The world’s lowest recorded temperature is -88.3°C (-126.9°F) at Vostok, approximately 960 km east of the South Pole, at an elevation of about 3,420 m1

• Tropical man does not tolerate or adapt well to the cold2

• Greater survival (adaptive) range with exposure to cold than to heat3

• The outer core temperature (Tc) survival limits approximate 27°C and 42°C (47), with the possibility of deathaccompanying a decrease of approximately 10°C, but an increase of only about 5°C4

1. Wilson C. Climatology of the cold regions: Southern hemisphere. Hanover, NH: U.S. Army Cold Regions Research & Engineering Laboratory; 1968. Monograph I-A3c

2. Bangs C, Hamlet MP. Hypothermia and cold injuries. In: Auerbach PS, Geehr EC, eds. Management of wilderness and environmental emergencies. New York: Macmillan Publishing Co.; 1983:27–63

3. Jodie M. Stocks Human Physiological Responses to Cold Exposure. Aviation, Space, and Environmental Medicine • Vol. 75, No. 5 • May 2004

4. Robinson S. Temperature regulation in exercise. Pediatrics 1963; 32(part II):691–702.

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Vostok

• Cold stress agents:- Air- Water

• Hypothermia occurs when core temperature decreases to 35°C (95°F) or less1

• Methods to maintain the Homeostasis:1. Shivering - four to five fold ↑ body’s resting rate of heat

production2. Nonshivering thermogenesis - stimulation of

metabolism by the SNS 3. Peripheral vasoconstriction - stimulation to the

smooth muscle, surrounding the arterioles in the skin

1. Pozos RS, Iaizzo PA, Danzl DF, et al. Limits of tolerance to hypothermia. In: Fregly MJ, Blatteis CM, editors.Handbook of physiology: environmental physiology. Bethesda (MD): American Physiology Society, 1996: 557-75

&&ShiveringShivering

ShiveringShivering

• A rapid, involuntary cycle of contraction & relaxation of skeletal muscles and is aided by catecholamine secretion1

• Shivering thermogenesis is estimated to provide the heat up to one-third of the total heat production during cold exposure2

• The contractile force thus generated may be 15–20% of maximal voluntary muscle activation

1. Banet M, Hensel H, Liebermann H. The central control of shivering and non-shivering thermogenesis inthe rat. J Physiol (Lond) 1978; 283:569–84

2. FOSTER, D. O., AND M. L. FRYDMAN. Tissue distribution of cold induced thermogenesis in conscious warm- or cold-acclimated rats reevaluated from changes in tissue blood flow: the dominant roleof brown adipose tissue in the replacement of shivering by nonshivering thermogenesis. Can. J. Physiol.Pharmacol. 57: 257–270, 1979.

Shivering causes the muscle to contractShivering causes the muscle to contract

Constricts the arteriolesConstricts the arterioles

↓ blood flow to the shell of the body↓ blood flow to the shell of the body

Prevents unnecessary heat lossPrevents unnecessary heat loss

Metabolic rate of the skin Metabolic rate of the skin ↓↓

↓ Skin’s temperature↓ Skin’s temperature

Factors Affecting Body Heat LossFactors Affecting Body Heat Loss• The body’s ability to meet the demands of thermoregulation

is limited when exposed to extreme cold

• Anatomical factor:1. Fat2. Insulating Shell3. Area/mass ratio

• Environmental factor:1. Windchill2. Clothing

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• Fat : - Subcutaneous fat is excellent source of insulation- The thermal conductivity of fat is relatively low - Subcutaneous fat thickness are a good indicator of anindividual’s tolerance for cold exposure

• Insulating shell:1. Superficial skin together with subcutaneous fat (15%)2. Underlying muscle (85%)

•• Area/mass ratio:Area/mass ratio:Area/mass ratio α loss of body heat

Body Surface Area (BSA) m2 =

•• Windchill: Windchill: - ↑ rate of heat loss via convection & conduction- humid air & more physiological stress

Person Weight (kg) Height (cm) Surface area (cm2)

Area/mass ratio

Adult 85 183 210 2.47

Child 25 100 79 3.16

Cm x kg3600

Heat Loss in Cold WaterHeat Loss in Cold Water• Conduction is the primary mechanism

• It has thermal conductivity about 26 times >than air

• Body generally loses heat four times faster in water than air of the same temp.

• At 320C – maintain constant internal temp.

• Below 320C – hypothermic state at a rate proportional to either duration of exposure / thermal gradient

• Rectal temp: Water temp. ↓ rate of temp/hr150C 2.10C40C 3.20C

It may accelerate if the water is moving

• Fat vs. Cold water1:

Body composition

Temp. Rectal temp. Exercise Duration

30% Body fat 11.80C No change 6hr 50min

10% Body fat 11.80C Decreasing 30min

Rest & cold waterExercises & cold water Vs. Heat transfer

1. Pugh, L.G., & Edholm, D.G., The physiology of channel swimmers. Lancet. 1955; 2: 761-767

Physiological Response to Exercises in the ColdPhysiological Response to Exercises in the Cold• Muscle function:

- nerves system responds by altering the normal muscle fiber recruitment patterns

- significantly ↓ muscle shortening velocity & power

• Metabolic response:

1. Prolonged exe ↑ lipid metabolism

2. This is achieved by release of catecholamines

Cooling muscle → weakness → Fatigue

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3. In cold environment- triggers a marked ↑ in catecholamines- peripheral vasoconstriction- ↓ blood supply to the subcutaneous skin- ↓ or no mobilization of FFA

4. Blood Glucose- hypoglycemia → suppressing shivering mechanism &significantly ↓ rectal temp.

5. Muscle Glycogen- higher usage in cold water exercises1

1. Young at al., Thermoregulation during cold water immersion is unimpaired by low muscle glycogenlevels. Journal of Applied Physiology. 1989; 66: 1809-1816

Health RiskHealth Risk1. Hypothermia2. Frostbite

• Hypothermia:- Temp. below 34.50C (94.10F) → beginning point

to lose the hypothalamus function- Below 29.50C → complete loss of function

Cardiac effects:1. Death from hypothermia has result of cardiac

arrest2. Cooling influences the SA node3. ↓ core tmp. & HR → decline in Q

Respiratory effects:- No threat- Air rapidly warmed when passes mouth to trachea

- Mouth respiration may irritate the passage- Excessive cold exposure may lead to ↓ RR & volumes

Air entry level Air tempNose 00C

Nasal passage 150CThroat 200CLungs 300C

Treatment:Treatment:• Mild hypothermia → protection, providing dry clothing & warm

beverages

• Moderate to severe → treatment to avoid cardiac arrhythmias & slowly re-warming

• Severe → hospitalization & medication

Frostbite:Frostbite:1. Exposed skin can freeze when temp below 00C2. It occurs as a consequence of the body’s attempts to prevent

heat loss

• Treatment:

Early intervention & hospitalization

Vasoconstruction in the skin vesselsVasoconstruction in the skin vessels

↓ blood flow ( lack of 0↓ blood flow ( lack of 022 & nutrients)& nutrients)

Skin cools rapidlySkin cools rapidly

Tissue deathTissue death

Cold AcclimatizationCold Acclimatization• Limited studies

• Chronic daily exposure to cold water may ↑ subcutaneous body fat1

• Repeated exposure may alter peripheral blood flow & skin temp.

• It allows greater cold tolerance

1. Kang, B.S., Song, S.H., Suh, C.S., & Hong, S.K., Changes in body temperature and basal Metabolic rate of the ama. Journal of Applied Physiology; 1963; 18: 483-488