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Temperature Regulation During Exercise Exercise Performance & Exhaustion Heat Acclimation Hydration / Blood Volume Outline

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Temperature Regulation During Exercise

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Heat Stress & Physiological Temperature Regulation Gisolfi & Wenger ESSR 1984

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Thermal Command Signal (Load Error) Set Point Temperature Integrate Afferent Signals Deep Body Temp. Hypothalamic Temp. Skin Temp. Core Temps. + _ Sweating Vasodilation Vasoconstriction vascular pressures, ions & osmolality, exercise Thermoregulatory Control Exercise Sawka et.al. Handbook of Physiology, 1996

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Sweating & Active Cutaneous Vasodilation TCS Sudomotor Nerve Sweat Gland Vasodilation ACH, VIP TCS Sudomotor Nerve Sweat Gland Vasodilation ACH (?) Non-Thermal Vasodilator Nerve A. B. modified from Johnson & Proppe Handbook of Physiology, 1996 Sweating parallels vascular conductance Sweating opposite of vascular conductance (e.g., isometric exercise)

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39.5 39.0 38.5 38.0 37.5 37.0 1015 20 25 30 35 C o r e T e m p e r a t u r e ( C ) WBGT (C) 1000W 500W 350W 200W Exercise Intensity & Climate Effects on Core Temperature Uncompensable Heat Stress Compensable Heat Stress adapted from, Lind et.al JAP 1963 (20 g/min, 1.2 L/h) (10 g/min, 0.6 L/h) (7 g/min, 0.4 L/h) (4 g/min, 0.25 L/h) Prescriptive Zone

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Summary: Temperature Regulation Metabolic Rate Dictates Required Heat Loss Warmer Environment Greater Need for Evaporation Core Temperature Increase Exercise Intensity Climate

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Exercise Performance & Exhaustion

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100 90 80 70 60 Environmental Temperature (C) 4 11 21 31 Time to Exhaustion (min) ~10 o C (50 o F) Optimum Temperature Heat Stress Reduces Endurance Exercise (~70-75% of VO 2max ) Galloway & Maughan, MSSE, 1997

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Heat Stress Reduces Maximal Aerobic Power (49 o C; 20% RH) Sawka et al. EJAP 1985 2345 0 5 10 15 20 % Decrease Aerobic Power, Heat VO 2 max ( L min -1 ), Temperate

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% of Subjects Exhausted From Heat Strain 0 25 50 75 100 37.038.039.040.041.0 Core Temperature (C) Compensable Heat Stress (cool skin) Uncompensable Heat Stress (hot skin) Core Temperature at Exhaustion from Heat Strain modified from, Sawka et.al. ESS 2000; Sawka et.al. MSSE 2002 (n = 747) Maron et.al. EJAP 1977 Cheuvront & Haymes Spts. Med. 2001 (n= 776) Many Studies Of Trained Athletes (n = 123)

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Core Temperature at Exhaustion: Champion Runners (maximal effort races) modified from S. Robinson, Pediatrics 1963 0 20 10 30 Minutes of Running 40 38 39 41 Core Temperature ( o C) T a = 30 o C T a = 31 o C T a = 10 o C

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Why Does Heat Stress Reduce Exercise Performance? Circulatory Strain High Skin Blood Flow, Peripheral Pooling Stroke Volume, Cardiac Output, Blood Pressure Central Nervous System (Critical Core Temperature) Brain EEG & Motor EMG Activity, Voluntary Force Activation Serotonin (5-HT) Accumulation (Plasma Prolactin) Metabolic Skeletal Muscle Blood Flow Substrate Utilization / Metabolite Accumulation

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Heat Stress Increases Circulatory Strain by Skin Blood Flow / Volume Rowell Human Circulation 1986 SKBF max = 7.8 L/min

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Critical Core Temperature or Circulatory Strain? ControlPre-coolingPre-heating Time to Exhaustion (min) 466328 Core Temp. ( o C) 40.240.140.7 Skin Temp. ( o C) 37.2 37.0 Heart Rate (bpm) 197198196 Gonzalez-Alonzo et.al., JAP 1999 (60% VO 2 max ; 40 o C)

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Brisson et al., JAP, 1991 (45 min cycling (65%VO 2max ) Hyperthermia Increases Prolactin, Measure of Central 5-HT Activity * 41 C 10 C PRL from baseline (ng/ml) 50 40 20 10 0 Ambient Temperature T c >38 o C

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Hyperthermia Reduces Voluntary Muscle Force Activation Nybo & Nielson JAP 2001 Exercise to exhaustion (60%VO 2max ) in hot or temperate; sustained MVC knee, voluntary activation by electrical stimulation to nervus femoris (Control T c = 38 o C; Hyperthermia T c = 40 o C) 83% 54%

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Muscle Metabolism & Heat Stress Muscle Blood Flow - Unchanged Muscle Glycogen Utilization Increased (not all) Fink et.al. EJAP 1975 Febbraio JAP 1994 Jentjens JAP 2001 Muscle Lactate Accumulation Increased Young et.al. JAP 1985 Febbraio JAP 1994

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37.0 98.6 39.0 40.0 41.0 42.0 38.0 ? CV Pathobiology CNS & CV Mechanism(s)? 100.4 102.2 104.0 105.8 107.6 C F Summary: Heat Stress & Exercise Performance Reduction modified from: Cheuvront & Sawka JKLES 2001 Heat Stress Reduces Aerobic Performance Core Temperature Tolerance is Climate & Population Specific Multiple Mechanism Involvement CV Strain Important Role of CNS Shutdown?

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Heat Acclimation

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Actions of Heat Acclimation Thermal Comfort - Improved Physiological Strain Reduced Exercise Performance - Improved Submaximal - Improved Maximal - Same

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Heat Acclimation Reduces Physiologic Strain

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Heat Acclimation Does Not Alter Core Temperature at Exhaustion (60% VO 2max, 40 O C, 10% rh; VO 2max = 49-74 ml/kg/min) 0204060 Time (min) 12345678910 37.0 38.0 39.0 40.0 41.0 Esophageal Temperature (C) mean T es = 39.7 o C mean T sk = 38.1 o C Nielson et.al. J. Physiol. (London) 1993

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Heat Acclimation is Induced by: Repeated Heat Exposure Over Many Days Heat Stress Sufficient to Increase Body Temperature & Profuse Sweating Duration - 100 min / day Exposure - 4 to 14 days Specific to Heat Stress Exercise / Rest Intensity / Duration Desert / Tropic

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Aerobic Training Induces Partial Heat Acclimation Cohen & Gisolfi, Med Sci Sports 14: 46, 1982

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Physiology of Heat Acclimation Core Temperature Reduced Tolerance - Unchanged Sweating - Improved Earlier Onset Higher Rate Redistribution (Tropic) Hidromeiosis Resistance (Tropic) Skin Blood Flow - Improved Earlier Onset Higher Rate (Tropic) Metabolic Rate Lowered Lactate Lowered Muscle Glycogen UC or Reduced Cardiovascular Stability - Improved Heart Rate - Lowered Stroke Volume Increased Cardiac Reserve - Increased Blood Pressure - Better Defended Myocardial Compliance Increased Myocardial Efficiency - Improved Fluid Balance - Improved Thirst- Improved Electrolyte Loss - Reduced Total Body Water - Increased Plasma Volume - Increased & Better Defended Thermal Comfort - Improved Exercise Performance - Improved modified from Sawka et.al. Exercise & Sport Science 2000

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Acclimation Improves Sweating Response Desert Climate

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Nadel et.al. JAP 1974 Acclimation Improves Sweating Responses More than Aerobic Training (65% V0 2ma : 24 o C ) Local Sweat Rate Acclimation Training

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Acclimation Improves Skin Blood Flow Response More than Aerobic Training (75% V0 2max, 35 o C, 75% RH ) Roberts et.al. JAP 1977 Acclimation Training

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Heat Acclimation Reduces Circulatory Strain (14 d;48 o C) Rowell et.al. JAP 1967 Better Maintained Increased Reduced

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Heat Acclimation Reduces Heat Syncope Bean & Eichna, Fed. Proc. 1943 ( n = 45) ( n = 38)

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% Plasma Volume Heat Acclimation & Plasma Volume Expansion (Hot/Wet; 40-50% VO 2max ) % TCP Expansion Senay et.al., JAP 1976 Mean

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60 50 40 30 20 10 0.20.61.01.41.82.2 Alan & Wison, JAP 1971 Sweat Rate (mg / cm 2 / min) Sweat Sodium (meq / L) Unacclimated Acclimated Heat Acclimation & Sweat Rate Effects on Sodium Loss

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Acclimation Lowers Metabolic Rate during Exercise Sawka et al. ASEM, 1983

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Summary: Heat Acclimation & Acquired Thermal Tolerance Acclimation Reduces Strain & Improves Performance Acclimation is Specific to Type of Heat Strain Improved Evaporative Cooling is Critical to Acclimation Cardiovascular, Fluid, Metabolic Adaptations Support Acclimation

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Hydration / Blood Volume

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0 10 20 30 40 50 60 32384349 90 100 110 120 oCoFoCoF 97 81 65 49 32 16 Km Miles Dehydration Reduces Self-Paced Work Adolph & Associates, Man in Desert, 1945 0 0L 10L Fluid Available

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Montain et al. IJSM, 1998 (30C, 50% rh) 02040 36 37 38 39 0204002040 n=9n=9n=8 65% VO 2 max 45% VO 2 max25% VO 2 max EU 3% BWL 5 Core Temp. (C) Exercise Time (min) Dehydration Increases Core Temperature

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47% VO 2 max; 49C, 20% rh. Dehydration Reduces Core Temperature Tolerance Sawka et al. JAP. 1992 Eu 5% BWL Core Temp. Tolerance (C) 38 40 39 *

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0.2 0.6 1.0 1.4 Sweating Rate (mg /cm /min) Core Temp. FBF (ml /100ml /min) 363738 39 Eu- 5% BWL Dehydration Reduces Sweating & Skin Blood Flow During Exercise-Heat Stress Sawka etal. AJP 1989 Kenney etal. JAP 1990 36 37 38 3 9 4 8 12 16 20 Eu- 5% BWL

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7% BWL 37.038.039.0 20 30 40 50 0 0 0 0 Eu 3% BWL 5% BWL Whole Body Sweating (g / m 2 / h) Rectal Temp. (C) Sawka et al. JAP 1985 (25% VO 2 max; 49 C, 20% rh) Montain et al. JAP 1995 3738 39 Eu 3% BWL 5% BWL Local Sweating (mg / cm 2 /min) Esophageal Temp. (C) (45% VO 2 max;30C, 50% rh) 0.4 0.8 1.2 0.0 Dehydration Reduces Sweating Graded to Water Deficit

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Cardiac output (L/min) 19 20 21 22 23 Time (min) 020406080100120140 Leg Blood Flow (L/min) 13 14 15 16 Heat & Dehydration Can Reduce Muscle Blood Flow (35C, 45% rh; 61% VO 2 max; 4%BWL) -Gonzalez-Alonzo, et al, J. Physiol (London) 1998 Dehydration Control

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Does Hyperhydration Improve Thermoregulation? StudyTemperatureSweat CoreSkin Rate Blyth & Burt (1961) nc Moroff & Bass (1965) Greenleaf & Castle (1971) nc nc nc Latzka et.al. (1997) nc nc nc Latzka et.al. (1998) nc nc nc Nielsen (1971) Nielsen (1974) Gisolfi & Copping (1974) nc Nadel et al. (1980) nc Grucza et al. (1987) nc Candas et al. (1988) nc nc nc Lyons et al. (1990) nc Montner et al. (1996) nc nc

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Acute Plasma Volume Expansion & Exercise - Heat Exposure (45% VO 2 max, 45 C, 20% rh) = m sw = Performance Time Sawka et al. EJAP 1983 NS Albumin Inf. Control Unacclimated Euhydrated

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1.0 0.8 0.6 0.4 0.2 363738 Core Temperature ( C ) Local Sweating (mg / cm 2 / min) Ss #1, EX I T S = 34.5 Erythrocyte Volume Expansion & Control of Sweating (45% VO 2max ; 35 C 45% rh) Sawka et al. JAP 1987 Pre-Infusion Post-Infusion

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Summary: Hydration / Blood Volume Dehydration Increases Thermal & Cardiovascular Strain Dehydration Reduces Physical Work Performance Plasma Hyperosmolality & Plasma Hypovolemia Contribute Hyperhydration & Plasma Volume Expansion Do Not Improve Thermoregulation or Performance Erythrocyte Volume Expansion Improves Thermoregulation & Performance