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
Slide 5
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)
Slide 6
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
Slide 7
Summary: Temperature Regulation Metabolic Rate Dictates
Required Heat Loss Warmer Environment Greater Need for Evaporation
Core Temperature Increase Exercise Intensity Climate
Slide 8
Exercise Performance & Exhaustion
Slide 9
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
Slide 10
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
Slide 11
% 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)
Slide 12
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
Slide 13
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
Slide 14
Heat Stress Increases Circulatory Strain by Skin Blood Flow /
Volume Rowell Human Circulation 1986 SKBF max = 7.8 L/min
Slide 15
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)
Slide 16
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
Slide 17
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%
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
Slide 24
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
Slide 25
Aerobic Training Induces Partial Heat Acclimation Cohen &
Gisolfi, Med Sci Sports 14: 46, 1982
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
Slide 34
Acclimation Lowers Metabolic Rate during Exercise Sawka et al.
ASEM, 1983
Slide 35
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
Slide 36
Hydration / Blood Volume
Slide 37
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
Slide 38
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
Slide 39
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 *
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
Slide 45
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
Slide 46
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