Transcript Heat Adaptations, Fitness, and Training

ADAPTATION TO HEAT
STRESS
TYPES OF ADAPTATIONS
TO HEAT STRESS
• ACCLIMATIZATION - ADAPTATIONS
RESULTING FROM NATURAL CHANGES
IN THE ENVIRONMENT.
• ACCLIMATION - ADAPTATIONS INDUCED
BY UNUSUAL ALTERATIONS IN THE
ENVIRONMENT.
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.
TYPES OF ADAPTATIONS
TO HEAT STRESS
• PERIPHERAL ADAPTATIONS.
• CENTRAL ADAPTATIONS.
• CIRCULATORY ADAPTATIONS.
• METABOLIC ADAPTATIONS.
PERIPHERAL
ADAPTATIONS
• SWEATING BEGINS AT A LOWER
CORE AND SKIN TEMPERATURE
(I.E., LOWER THRESHOLD FOR THE
ONSET OF SWEATING).
• 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).
• 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.
Open circles = unacclimated
Dark circles = acclimated
• 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.
• 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.
• 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.
• 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.
CENTRAL ADAPTATIONS
• DECREASE IN HEART RATE,WHICH
DECREASES CARDIOVASCULAR
STRAIN.
Open circles = unacclimated
Dark circles = acclimated
LESS OF AN INCREASE IN CORE
TEMPERATURE FOR A GIVEN HEAT
STRESSOR, WHICH DECREASES
THERMAL STRESS.
• 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.
CIRCULATORY
ADAPTATIONS
• 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.
Open circles = unacclimated
Dark circles = acclimated
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).
• PLASMA VOLUME INCREASES INITIALLY,
BUT EVENTUALLY RETURNS BACK TO
BASELINE VALUE (INCREASED PLASMA
VOLUME DECREASES HEART RATE).
• INCREASE IN EXTRACELLUAR FLUID
AND TOTAL BODY WATER, WHICH
DECREASES TENDENCY OF
HYPOHYDRATION AND INCREASES
POTENTIAL SWEAT CAPACITY.
• 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.
• INCREASE IN VASODILATION OF THE
CUTANEOUS VASCULATURE FOR A
GIVEN CORE TEMPERATURE.
• DIASTOLIC BLOOD PRESSURE
DECREASES DUE TO VASODILATION
OF THE CUTANEOUS VASCULATURE.
• 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.
METABOLIC
ADAPTATIONS
• 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.
• 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.
• 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.
TRAINING
• MODERATE AEROBIC EXERCISE AT AN
INTENSITY > 50% OF MAXIMAL OXYGEN
UPTAKE RATE OR > 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.
OTHER COMMENTS
REGARDING HEAT
ADAPTATION
• 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.
• 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.
Physical Training,
Cardiorespiratory Fitness, and
Heat Tolerance
ADAPTATIONS FROM PHYSICAL
TRAINING THAT IMPROVES HEAT
TOLERANCE
PERIPHERAL RESPONSES TO TRAINING
•
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).
• 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.
• 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.
• 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.
CENTRAL RESPONSES TO TRAINING
• 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.
CARDIOVASCULAR RESPONSES TO
TRAINING
• 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”
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.
INCREASED STROKE VOLUME, DUE TO:
• DECREASED ESV:
• INCREASED CONTRACTILITY.
-
INCREASED PRELOAD (STARLING LAW).
- INCREASED VENTRICULAR WALL THICKNESS;
PRIMARILY A PHENOMENON OF
STRENGTH
TRAINING.
INCREASED STROKE VOLUME DUE TO:
•
DECREASED END-SYSTOLIC VOLUME (CONT’D)
• 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).
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”
INCREASED OXYGEN EXTRACTION DUE TO:
• INCREASED MITOCHONDRIAL DENSITY.
• INCREASED OXIDATIVE ENZYMES.
• INCREASED MYOBLOGIN LEVELS.
• INCREASED CAPILLARIZATION.
SUMMARY
• INCREASED SV AND LOWER HR TO MAINTAIN A
GIVEN CARDIAC OUTPUT (Q) REDUCES
CARDIOVASCULAR STRAIN (I.E., IMPROVED
MYOCARDIAL EFFICIENCY).
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.
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.
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.
TRAINING GUIDELINES FOR
IMPROVING HEAT
TOLERANCE
•
INTENSE TRAINING > 50% OF
MAXIMAL OXYGEN UPTAKE RATE OR
HEART RATE RESERVE, OR > OF 60% OF
MAXIMAL HEART RATE.
•
DURATION: 30-60 MIN/SESSION FOR
8-12 WEEKS.
•
FREQUENCY: 5 DAYS PER WEEK.
• 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.
QUESTIONS??
THAT’S ALL FOLKS!!