ADAPTATION TO HEAT STRESS

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ADAPTATION TO HEAT STRESS
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TYPES OF ADAPTATIONS TO HEAT STRESS

• ACCLIMATIZATION - ADAPTATIONS RESULTING FROM
  NATURAL CHANGES IN THE ENVIRONMENT.
• ACCLIMATION - ADAPTATIONS INDUCED BY UNUSUAL
  ALTERATIONS IN THE ENVIRONMENT.

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OTHER RELATED TERMS

• HABITUATION - DESENITIZATION OR DAMPENING OF A
  NORMAL RESPONSE TO A STRESSOR (e.g., decreased
  effects of hidromeiosis after heat adaptation).
• CONDUCTANCE - TENDENCY OF AN INDIVIDUAL TO LOSE
  BODY HEAT TO THE ENVIRONMENT.
• INSULATION - TENDENCY OF AN INDIVIDUAL TO RESIST
  LOSS OF BODY HEAT TO THE ENVIRONMENT.

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TYPES OF ADAPTATIONS TO HEAT STRESS

• PERIPHERAL ADAPTATIONS.
• CENTRAL ADAPTATIONS.
• CIRCULATORY ADAPTATIONS.
• METABOLIC ADAPTATIONS.

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PERIPHERAL ADAPTATIONS
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• SWEATING BEGINS AT A LOWER CORE AND SKIN
  TEMPERATURE (I.E., LOWER THRESHOLD FOR THE ONSET
  OF SWEATING).

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• GREATER SWEAT RATE FOR A GIVEN INCREASE IN CORE
  TEMPERATURE (TC) AND SKIN TEMPERATURE DUE TO
  INCREASED SENSITIVITY OF THE SWEAT GLANDS TO
  SYMPATHETIC NERVOUS SYSTEM STIMULATION, INCREASED
  SWEATING CAPACITY OF THE GLANDS, AND/OR DECREASED
  EFFECTS OF HIDROMEIOSIS (e.g., habituation).

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• REDUCED EFFECTS OF HIDROMEIOSIS (e.g.,
  habituation) AS WELL AS REDUCED GLYCOGEN
  DEPLETION IN SWEAT GLANDS, WHICH RESULTS IN
  INCREASED SWEAT RATE.
• MAINTENANCE OF HIGHER SWEAT RATES.

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• INCREASED SELECTIVE REGIONAL SWEATING AS WELL AS
  INCREASED WETTED SKIN SURFACE AREA, WHICH
  ENHANCES THE POTENTIAL FOR EVAPORATIVE HEAT LOSS
  AND MINIMIZES THE WASTE OF SWEAT BY DRIPPING.

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• GREATER SWEATING CAPACITY, PARTICULARLY IN A
  HUMID ENVIRONMENT. SWEATING CAPACITY INCREASES
  FROM 1.5 L/HOUR UP TO 2-3 L/HOUR WITH MAXIMUM
  DAILY SWEAT RATE OF 10-15 L.
• INCREASED VASODILATION OF CUTANEOUS VASCULATURE
  AND INCREASED SKIN BLOOD FLOW FOR A GIVEN
  INCREASE IN CORE TEMPERATURE, WHICH INCREASES
  CONVECTIVE HEAT TRANSFER TO THE SKIN AND
  EVAPORATIVE AND CONVECTIVE HEAT LOSS TO
  ENVIRONMET.

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• INCREASED SKIN BLOOD FLOW (I.E., TRANSFER OF
  HEAT) COMBINED WITH INCREASED SWEATING CAPACITY
  RESULTS IN PROPORTIONALLY GREATER RELIANCE ON
  EVAPORATIVE HEAT EXCHANGE, WHICH WILL DECREASE
  SKIN TEMPERATURE.
• NOTE DECREASED SKIN TEMPERATUE INCREASES THE
  POTENTIAL FOR CONVECTVE AND CONDUCTIVE HEAT
  TRANSFER FROM THE CORE TO THE SKIN SURFACE AREA
  AND CONVECTIVE HEAT LOSS TO THE ENVIRONMENT IF
  NEEDED DUE TO AN INCREASE IN THE TEMPERATURE
  GRADIENT BETWEEN THE SKIN AND AIR.

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• DECREASE IN SODIUM LOST IN SWEAT DUE TO AN
  INCREASE IN SWEAT GLAND SENSITIVITY TO
  ALDOSTERONE.
• NOTE SWEAT IS MORE DILUTE WHICH INCREASES THE
  POTENTIAL FOR EVAPORATIVE HEAT EXCHANGE.

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CENTRAL ADAPTATIONS
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• DECREASE IN HEART RATE,WHICH DECREASES
  CARDIOVASCULAR STRAIN.

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• LESS OF AN INCREASE IN CORE TEMPERATURE FOR A
  GIVEN HEAT
• STRESSOR, WHICH DECREASES THERMAL STRESS.

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• SYSTOLIC BLOOD PRESSURE IS BETTER MAINTAINED DUE
  TO INCREASED PLASMA VOLUME OR AT LEAST
  MAINTENANCE OF PLASMA VOLUME AT NORMAL BASELINE
  VALUE.
• INCREASE IN SODIUM AND WATER REABSORPTION BY THE
  KIDNEYS.

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CIRCULATORY ADAPTATIONS
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• INCREASE IN THE STABILITY OF THE CARDIOVASCULAR
  SYSTEM AND HENCE, A DECREASE IN CARDIOVASCULAR
  STRAIN.
• A. DECREASE IN HEART RATE.
• B. INCREASE IN STROKE VOLUME.

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INCREASE IN STROKE VOLUME

• INCREASE IN END-DIASTOLIC VOLUME DUE TO AN
  INCREASE IN PLASMA VOLUME AND AN INCREASE IN THE
  RETURN OF BLOOD FLOW DUE TO INCREASED
  VENOCONSTRICTION.
• DECREASE IN END-SYSTOLIC VOLUME DUE TO INCREASED
  CONTRACTILITY (STARLING LAW) AND DECREASED
  RESISTANCE TO BLOOD FLOW RESULTING FROM
  VASODILATION OF THE CUTANEOUS VASCULATURE AND
  DECREASED BLOOD VISCOSITY (OSMOLARITY).

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• PLASMA VOLUME INCREASES INITIALLY, BUT EVENTUALLY
  RETURNS BACK TO BASELINE VALUE (INCREASED PLASMA
  VOLUME DECREASES HEART RATE).

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• INCREASE IN EXTRACELLUAR FLUID AND TOTAL BODY
  WATER, WHICH DECREASES TENDENCY OF HYPOHYDRATION
  AND INCREASES POTENTIAL SWEAT CAPACITY.

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• INCREASE IN HEMODILUTION RESULTING IN BETTER
  TRANSFER OF HEAT FROM THE CORE OF THE BODY TO THE
  SKIN.
• A. NOTE BOTH TRAINING AND HEAT ADAPTATION
  INCREASE HEMODILUTION (INCREASE IN RELATIVE
  FLUID CONTENT OF THE BLOOD AS PROTEIN IS FLUSHED
  INTO THE VASCULAR SPACE).
• B. CONVERSELY, HEMOCONCENTRATION TENDS TO
  OCCUR AT ANAEROBIC THRESHOLD, WHICH TRAINING
  CAN INCREASE RESULTING IN THE TREND TOWARD
  GREATER HEMODILUTION.

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• INCREASE IN VASODILATION OF THE CUTANEOUS
  VASCULATURE FOR A GIVEN CORE TEMPERATURE.

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• DIASTOLIC BLOOD PRESSURE DECREASES DUE TO
  VASODILATION OF THE CUTANEOUS VASCULATURE.

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• SYSTOLIC BLOOD PRESSURE IS BETTER MAINTAINED DUE
  TO THE INCREASES PLASMA VOLUME OR AT LEAST
  MAINTENANCE OF PLASMA VOLUME AT NORMAL BASELINE
  VALUE.
• NOTE Increased vasodilation and plasma volume
  decreases resistance to blood flow which
  decreases end-systolic volume (ESV) thereby
  increasing stroke volume (SV) and decreasing
  heart rate (HR) and cardiovascular (CV) strain.

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METABOLIC ADAPTATIONS
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• DECREASE IN METABOLIC RATE TO PERFORM A GIVEN
  SUBMAXIMAL WORKLOAD.
• DECREASE IN THE RELATIVE CONTRIBUTION OF
  ANAEROBIC METABOLISM AND AN INCREASE IN THE
  RELATIVE CONTRIBUTION OF AEROBIC METABOLISM TO
  PERFORM A GIVEN SUBMAXIMAL WORKLOAD.

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• DECREASE IN BASAL METABOLIC RATE DUE TO A
  DECREASE IN SYMPATHETIC NERVOUS SYSTEM
  STIMULATION OF THE THYROID GLAND.
• DECREASE IN CHO UTILIZATION AND AN INCREASE IN
  FAT UTILIZATION AS AN ENERGY SOURCE.
• DECREASE IN LACTIC ACID ACCUMULATION.

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• RETURN OF MAXIMAL OXYGEN UPTAKE RATE TO
  NORMOTHERMIC, BASELINE VALUE ACUTE HEAT STRESS
  DECREASES
• MAXIMAL OXYGEN UPTAKE RATE.
• THE AFFECT OF HYPOHYDRATION WTH THERMAL STRESS IN
  REDUCING MAXIMAL OXYGEN UPTAKE RATE IS NOT
  AFFECTED BY ADAPTATION TO HEAT.

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TRAINING
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• MODERATE AEROBIC EXERCISE AT AN INTENSITY gt 50
  OF MAXIMAL OXYGEN UPTAKE RATE OR gt 60 OF MAXIMAL
  HEART RATE) FOR 8-12 WEEKS HAS BEEN SHOWN TO
  PRODUCE ADAPTATIONS TO THE HEAT.
• HIGH LEVELS OF FITNESS HAVE BEEN REPORTED TO
  ENHANCE THE HEAT ADAPTATION PROCESS. ALSO,
  PHYSICALLY FIT INDIVIDUALS HAVE BEEN SHOWN TO
  RETAIN HEAT ADAPTATION FOR LONGER PERIODS OF
  TIME.

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OTHER COMMENTS REGARDING HEAT ADAPTATION
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• MOST PEOPLE BELIEVE THAT HEAT ADAPTATION IS A
  RELATIVE
• PHENONMENON.
• ADAPTATIONS SUCH AS IMPROVEMENTS IN HEART RATE
  WHICH DEVELOP MOST RAPIDLY ARE ALSO LOST MORE
  RAPIDLY THAN SLOWER OCCURRING THERMOREGULATORY
  IMPROVEMENTS. GENERALLY TRUE FOR ALL
  ENVIRONMENTAL ADAPTATIONS.

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• SLEEP LOSS, INFECTIONS, ALCOHOL ABUSE, SODIUM
  DEPLETION, AND DEHYDRATION LESSEN THE BENEFITS OF
  HEAT ADAPTATION.
• BECAUSE SODIUM MAY BE LOST DURING HYPERTHERMIC
  CONDITIONS, IT IS RECOMMENDED THAT A
  UNACCLIMATIZED PERSON TAKE 10 G/DAY SODIUM
  SUPPLEMENT, UNLESS WATER SUPPLY IS LIMITED.
  ACCLIMATIZED PERSON REQUIRES NO SODIUM
  SUPPLEMENTATION BEYOND THAT IN THE NORMAL DIET.

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Physical Training, Cardiorespiratory Fitness, and
Heat Tolerance
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ADAPTATIONS FROM PHYSICAL TRAINING THAT IMPROVES
HEAT TOLERANCE
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PERIPHERAL RESPONSES TO TRAINING
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• FOREARM BLOOD FLOW.
• LOWER THRESHOLD FOR ONSET OF INCREASED FOREARM
  BLOOD FLOW.
• NO CHANGE IN FOREARM BLOOD FLOW RATE FOR A GIVEN
  CHANGE IN CORE TEMPERATURE (I.E., NO CHANGE IN
  THE SENSITIVITY OR THE SLOPE OF THE LINE).
• INCREASED HEAT TRANSFER BY CONVECTION FROM CORE
  TO SKIN.
• NOTE ADDITIONAL HEAT ACCLIMATION LOWERS
  THRESHOLD FOR ONSET OF FOREARM BLOOD FLOW (I.E.,
  SKIN BLOOD FLOW) AND INCREASES SENSITIVITY OF
  FLOW RATE (I.E., GREATER SKIN BLOOD FLOW FOR A
  GIVEN INCREASE IN CORE TEMPERATURE).

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• SWEAT RATE.
• TRAINING INCREASES SENSITIVITY OF SWEAT GLANDS
• (I.E., GREATER INCREASE IN SWEAT RATE FOR A
  GIVEN INCREASE IN CORE TEMPERATURE) AND SLIGHTLY
  LOWERS THE THRESHOLD FOR THE ONSET OF SWEATING.
• THE ADDITION OF HEAT ADAPTATION TO TRAINING
  LOWERS THRESHOLD FOR THE ONSET OF SWEATING
  WITHOUT CHANGING SWEAT GLAND SENSITIVITY IN
  COMPARISON TO TRAINING.
• MAXIMAL OXYGEN UPTAKE RATE IS THE SINGLE BEST
  PREDICTOR OF SWEAT RATE FOR A GIVEN INCREASE IN
  CORE TEMPERATUE.

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• DECREASED SUBCUTANEOUS FAT.
• INCREASED TRANSFER OF HEAT VIA CONDUCTION FROM
  THE BLOOD TO THE SKIN SURFACE.
• INCREASED EVAPORATIVE HEAT LOSS DUE TO
  INCREASED TRANFSER OF HEAT BY CONVECTION AND
  CONDUCTION FROM THE CORE TO THE SKIN, INCREASED
  SWEAT RATE FOR A GIVEN INCREASE IN CORE
  TEMPERATURE, AND LOWER THRESHOLDS FOR THE ONSET
  OF SWEATING AND SKIN BLOOD FLOW.

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• LOWER SKIN TEMPERAUTRE AS MORE HEAT IS
  DISSIPATED FROM THE SKIN SURFACE TO THE
  ENVIRONMENT.
• NOTE TRAINING IN COLD WATER DOES NOT IMPROVE
  HEAT TOLERANCE BECAUSE CORE TEMPERATURE IS NOT
  ELEVATED DUE TO THE FACT THAT THE THERMAL
  CONDUCTIVITY OF WATER IS FOUR TIMES GREATER THAN
  AIR.

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CENTRAL RESPONSES TO TRAINING
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• LOWER CORE TEMPERATURE.
• SWEAT RATE.
• SLIGHTLY LOWER THRESHOLD FOR THE ONSET OF
  SWEATING AND INCREASED SENSITIVITY OF SWEAT
  RESPONSE FOR A GIVEN INCREASE IN CORE TEMPERATRE.
• FOREARM BLOOD FLOW.
• SLIGHTLY LOWER THRESHOLD FOR THE ONSET OF
  FOREARM BLOOD FLOW.

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CARDIOVASCULAR RESPONSES TO TRAINING
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• LOWER THRESHOD FOR ONSET OF FOREARM BLOOD FLOW.
• INCREASED PLASMA VOLUME.
• CHANGES IN THE COMPONENTS OF FICK EQUATION
• VO2 CARDIAC OUTPUT (Q) X OXYGEN EXTRACTION.
• VO2 (SV X HR) X (A-V O2 DIFFERENCE).
• VO2 (EDV - ESV) X HR X (A-V O2 DIFFERENCE).
• INCREASED STROKE VOLUME, DECREASED HEART RATE,
  AND INCREASED OXYGEN EXTRACTION FOR A GIVEN
  WORKLOAD

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INCREASED STROKE VOLUME, DUE TO

• INCREASED END-DIASTOLIC VOLUME (EDV)
• 1. INCREASED PLASMA VOLUME.
• 2. INCREASED ANATOMICAL VOLUME OF THE HEART
  FROM ENDURANCE TRAINING.
• 3. INCREASED CENTRAL BLOOD VOLUME VENOUS
  RETURN.
• - DECREASED POOLING OF BLOOD IN THE
  CUTANEOUS VASCULATURE.
• - REDUCED MUSCLE BLOOD FLOW (Q) DURNG
  SUBMAXIMAL EXERCISE.

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INCREASED STROKE VOLUME, DUE TO

• DECREASED ESV
• INCREASED CONTRACTILITY.
• - INCREASED PRELOAD (STARLING LAW).
• - INCREASED VENTRICULAR WALL THICKNESS
  PRIMARILY A PHENOMENON OF STRENGTH
  TRAINING.

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INCREASED STROKE VOLUME DUE TO

• DECREASED END-SYSTOLIC VOLUME (CONTD)
• DECREASED PERIPHERAL RESISTANCE.
• - DECREASED VISCOSITY OF BLOOD DUE TO INCREASED
  HEMODILUTION AS PROTEIN IS FLUSHED INTO THE
  VASCULAR SPACE DRAWING WATER WITH IT
  HEMODILUTED BLOOD HAS BETTER POTENTIAL
  CONVECTIVE HEAT TRANSFER.
• (REMEMBER TRAINING INCREASES ANAEROBIC
  THRESHOLD AND THUS THE WORKLOAD AT WHICH
  HEMOCONCENTRATION OCCURS).
• - INCREASED CAPILLARIZATION.
• - INCREASED VASODILATION DUE TO DECREASED SNS
  ACTIVITY FOR A GIVEN WORKLOAD (REMEMBER THAT
  NOREPINEPHRINE FROM SNS CAUSES WIDESPREAD
  VASOCONSTRICTION).

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LOWER SUBMAXIMAL HEART RATE

• ENDURANCE TRAINING LOWERS HEART RATE AND THUS
  REDUCES THE MYOCARDIAL OXYGEN REQUIREMENT FOR A
  GIVEN SUBMAXIMAL WORKLOAD (WHICH DECREASES HEAT
  PRODUCTION BY THE HEART) DUE TO
• - INCREASED STROKE VOLUME.
• - INCREASED VAGUS DOMINANCE.
• THE INCREASED STROKE VOLUME AND DECREASED
  HEART RATE RESULTS IN LOWER CARDIOVASCULAR STRAIN
  IN A HOT, HUMID ENVIRONMENT

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INCREASED OXYGEN EXTRACTION DUE TO

• INCREASED MITOCHONDRIAL DENSITY.
• INCREASED OXIDATIVE ENZYMES.
• INCREASED MYOBLOGIN LEVELS.
• INCREASED CAPILLARIZATION.

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SUMMARY

• INCREASED SV AND LOWER HR TO MAINTAIN A GIVEN
  CARDIAC OUTPUT (Q) REDUCES CARDIOVASCULAR STRAIN
  (I.E., IMPROVED MYOCARDIAL EFFICIENCY).

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SUMMARY

• INCREASED OXYGEN EXTRACTION CAPABILITIES
  COMBINED WITH A LOWER CARDIAC (Q) FOR A GIVEN
  WORKLOAD MAY TRANSLATE INTO IMPROVED EFFICIENCY
  RESULTING IN LOWER SUBMAXIMAL OXYGEN UPTAKE
  REQUIREMENTS (I.E, METABOLIC RATE) FOR A GIVEN
  WORKLOAD.
• TRAINING HAS BEEN SHOWN TO DECREASE SUBMAXIMAL
  OXYGEN UPTAKE REQUIREMENTS AND CONSEQUENTLY, HEAT
  PRODUCTION.

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SUMMARY

• IMPROVED EFFICIENCY MAY ALSO RESULT IN GREATER
  RELIANCE ON SLOW-TWITCH (ST) MOTOR UNITS AS
  ANAEROBIC THRESHOLD IS INCREASED FOLLOWING
  TRAINING HENCE MORE OF THE CHEMICAL ENERGY FROM
  ATP BREAKDOWN WOULD GO TOWARDS MECHANICAL MOTION
  AND LESS TO HEAT PRODUCTION.

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SUMMARY

• INCREASED ANAEOBIC THRESHOLD WOULD ALSO
• - INCREASE RELIANCE ON AEROBIC METABOLISM
  AND FAT UTILIZATION.
• - DECREASE RELIANCE ON ANAEROBIC METABOLISM
  AND CARBOHYDRAE UTLIZATION AS WELL AS DECREASE
  LACTIC ACID PRODUCTION.
• - INCREASE TENDENCY TOWARD HEMODILUTED BLOOD
  WHICH HAS BETTER HEAT TRANSFER CAPABILITIES.

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TRAINING GUIDELINES FOR IMPROVING HEAT TOLERANCE
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• INTENSE TRAINING gt 50 OF MAXIMAL OXYGEN
  UPTAKE RATE OR HEART RATE RESERVE, OR gt OF 60
  OF MAXIMAL HEART RATE.
• DURATION 30-60 MIN/SESSION FOR 8-12 WEEKS.
• FREQUENCY 5 DAYS PER WEEK.

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• TRAINING WHICH INCREASES MAXIMAL OXYGEN UPTAKE
  BY 15 OR MORE MAXIMIZES IMPROVEMENTS IN EXERCISE
  HEAT TOLERANCE.
• HIGHER VOLUMES OF TRAINING (MILES/WEEK OR
  GREATER TRAINING BASE) MAY ALLOW INDIVIDUALS TO
  ADAPT MORE QUICKLY TO HEAT EXPOSURE.
• BOTH CORE AND SKIN TEMPERATURE MUST BE ELEVATED
  IN ORDER FOR TRAINING TO FULLY IMPROVE HEAT
  TOLERANCE.
• REMEMBER THAT HEAT ADAPTATION IS A RELATIVE
  CONCEPT AS HIGHER VOLUMES OF TRAINING AS WELL AS
  LONGER PERIODS AND/OR HIGHER DEGREES OF HEAT
  EXPOSURE ENHANCE ADAPTATION.

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QUESTIONS??
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• THATS ALL FOLKS!!