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

Outline

Temperature Regulation During Exercise

Heat Stress & Physiological Temperature Regulation

Gisolfi & Wenger ESSR 1984

Thermal Command

Signal(Load Error)

Set Point Temperature

IntegrateAfferent Signals

Deep Body Temp.

HypothalamicTemp.

Skin Temp.

CoreTemps.

+_

Sweating

Vasodilation

Vasoconstriction

vascular pressures,ions & osmolality,exercise

Thermoregulatory Control Exercise

Sawka et.al. Handbook of Physiology, 1996

Sweating & “Active” Cutaneous Vasodilation

TCSSudomotor

Nerve Sweat Gland

Vasodilation

ACH, VIP

ACH, VIP

TCS

Sudomotor Nerve

Sweat Gland

Vasodilation

ACH

(?)

Non-Thermal

VasodilatorNerve

A.

B.

modified from Johnson & Proppe Handbook of Physiology, 1996

Sweating parallels vascular conductance

Sweating opposite of vascular conductance

(e.g., isometric exercise)

39.5

39.0

38.5

38.0

37.5

37.010 15 20 25 30 35

Core

Tem

pera

ture

(°C)

WBGT (°C)

1000W

500W

350W

200W

Exercise Intensity & Climate Effects on Core Temperature

UncompensableHeat 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

Summary: Temperature Regulation

• Metabolic Rate Dictates Required Heat Loss • Warmer Environment Greater Need for Evaporation• Core Temperature Increase

• Exercise Intensity• Climate

Exercise Performance & Exhaustion

100

90 80

70

60

Environmental Temperature (°C) 4 11 21 31

Tim

e t o

Exh

a ust

ion

(min

)

~10oC (50oF) Optimum Temperature

Heat Stress Reduces Endurance Exercise (~70-75% of VO2max)

Galloway & Maughan, MSSE, 1997

Heat Stress Reduces Maximal Aerobic Power(49oC; 20% RH)

Sawka et al. EJAP 1985

2 3 4 50

5

10

15

20%

Dec

reas

e A

erob

ic P

ower

, Hea

t

VO2 max ( L · min-1) , Temperate

% o

f Sub

ject

s Ex

haus

ted

From

Hea

t Str

ain

0

25

50

75

100

37.0 38.0 39.0 40.0 41.0

Core Temperature (°C)

CompensableHeat Stress(cool skin)

UncompensableHeat 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 StudiesOf Trained Athletes

(n = 123)

Core Temperature at Exhaustion: Champion Runners(maximal effort races)

modified from S. Robinson, Pediatrics 1963

0 2010 30Minutes of Running

40

38

39

41C

ore

Tem

pera

ture

(o C)

Ta= 30oCTa= 31oC

Ta= 31oC Ta= 10oC

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

Heat Stress Increases Circulatory Strain by Skin Blood Flow / Volume

Rowell Human Circulation 1986

SKBFmax= 7.8 L/min

Critical Core Temperature or Circulatory Strain?

Control Pre-cooling Pre-heating

Time to Exhaustion

(min)

46 63 28

Core Temp. (oC)

40.2 40.1 40.7

Skin Temp.(oC)

37.2 37.2 37.0

Heart Rate(bpm)

197 198 196

Gonzalez-Alonzo et.al., JAP 1999

(60% VO2 max; 40oC)

Brisson et al., JAP, 1991

(45 min cycling (65%VO2max)

Hyperthermia Increases Prolactin, Measure of Central 5-HT Activity

*

41C 10C

PRL

fr

om b

asel

ine

(ng/

ml)

50

40

20

10

0Ambient Temperature

Tc >38oC

Time (s)0 30 60 90 120

Volu

ntar

y ac

tivat

ion

perc

enta

ge

0

40

60

80

100 ControlHyperthermia

** *

* Significantly lower than control (P<0.05)

Hyperthermia Reduces Voluntary Muscle Force Activation

Nybo & Nielson JAP 2001

Exercise to exhaustion (60%VO2max) in hot or temperate;sustained MVC knee, voluntary activation by electrical stimulation to nervus femoris

(Control Tc = 38oC; Hyperthermia Tc = 40oC)

Time (s)0 20 40 60 80 100 120 140

IEM

G in

% o

f max

0

40

60

80

100ControlHyperthermia

*

* significantly lower than control (P<0.05)

83%

54%

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

37.0 98.6

39.0

40.0

41.0

42.0

38.0

?

CV

Pathobiology

CNS & CV

Mec

hani

sm(s

)?

100.4

102.2

104.0

105.8

107.6C 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?

Heat Acclimation

Actions of Heat Acclimation

• Thermal Comfort - Improved • Physiological Strain – Reduced• Exercise Performance - Improved

– Submaximal - Improved– Maximal - Same

Heat Acclimation Reduces Physiologic Strain

Days of Heat Exposure

0 2 4 6 8 10

Rec

tal T

empe

ratu

re, º

C

37

38

39

40

Hea

rt R

ate

100

120

140

160

180

Lind and Bass, Fed. Proc. 22: 704, 1963

Heat Acclimation Does Not Alter Core Temperature at Exhaustion

(60% VO2max, 40OC, 10% rh; VO2max = 49-74 ml/kg/min)

0 20 40 60

Time (min)

1 2 3 4 5 6 7 8 910

37.0

38.0

39.0

40.0

41.0

Esop

hage

al T

empe

ratu

re (°

C)

mean Tes = 39.7oCmean Tsk = 38.1oC

Nielson et.al. J. Physiol. (London) 1993

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

Time, h

0 1 2 3 4

Hea

rt R

ate,

b/m

in

80

100

120

140

160

180

Time, h

0 1 2 3 4

Rec

tal T

empe

ratu

re, º

C

36

37

38

39

40

Pre-Training11 wk Training; 4 d/wk8 Day HA

Aerobic Training Induces Partial Heat Acclimation

Cohen & Gisolfi, Med Sci Sports 14: 46, 1982

Physiology of Heat Acclimation

Core Temperature – Reduced Tolerance - UnchangedSweating - 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 - ImprovedFluid 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

Rectal Temperature (°C)

37.0 37.5 38.0 38.5 39.0

Sw

eatin

g R

ate,

ml/m

in

0

5

10

15

20

Day 1

Day 5

Day 9

Ladell Trans. Roy. Soc. Trop. Med. Hyg. 51: 189, 1957

38°C, 80% rh

Acclimation Improves Sweating ResponseDesert Climate

Tes oC37.0 37.5 38.0

Load

Sw

eat R

ate

mg/

(min

·cm

2 )

0.0

0.2

0.4

0.6

0.8Post-Heat

Post-Ex

Pre-Ex

Nadel et.al. JAP 1974

Acclimation Improves Sweating Responses More than Aerobic Training

(65% V02ma : 24o C )Lo

cal S

wea

t Rat

e

Acclimation

Training

Training

Tes oC37.0 37.5 38.0 38.5

Fore

arm

Blo

odflo

wcc

/(100

cc·m

in)

0

2

4

6

8

10

12

14

Post-Heat

Post-ExPre-Ex

Acclimation Improves Skin Blood Flow Response More than Aerobic Training

(75% V02max, 35oC, 75% RH )

Roberts et.al. JAP 1977

Acclimation

TrainingTraining

CA

RD

IAC

OU

TPU

T(1

min-1 )

11

12

13

14

Before AcclimatizationAfter Acclimatization

STR

OK

EVO

LUM

E(m

l)

60

80

100

TIME (min)0 10 20 30 40 50 60 70 80

HEA

RT

RA

TE(b

eats

min

-1)

130

150

170

190

210 Tr = 40oCTs = 39oC

Tr = 37.8oCT s= 37.5oC

Heat Acclimation Reduces Circulatory Strain(14 d;48oC)

Rowell et.al. JAP 1967

Better Maintained

Increased

Reduced

Day Of Heat Acclimation0 1 2 3 4 5 6 7 8 9

Sub

ject

s W

ith S

ynco

pe

0

5

10

15

20

Heat Acclimation Reduces Heat Syncope

Bean & Eichna, Fed. Proc. 1943

( n = 45)

( n = 38)

Days0 2 4 6 8 10

-8

-4

0

4

8

12

16

20

24

28

32

36

40

44

% Δ

Pla

sma

Volu

me

Days0 2 4 6 8 10

0

4

8

12

16

20

24

28

32

36

40 ABCDMEANVALUES

P < .05

Heat Acclimation & Plasma Volume Expansion(Hot/Wet; 40-50% VO2max)

% Δ

TC

P Ex

pans

ion

Senay et.al., JAP 1976

MeanMean

60

50

40

30

20

10

0.2 0.6 1.0 1.4 1.8 2.2

Alan & Wison, JAP 1971

Sweat Rate (mg / cm2 / min)

Swea

t Sod

ium

(meq

/ L)

UnacclimatedAcclimated

Heat Acclimation & Sweat Rate Effects on Sodium Loss

Met

abol

ic R

ate

(ml·k

g-1·m

in-1

)

0

15

16

17

18

19

50 min 110 min

Pre HAPost HA

49°C, 20% rh, 1.56 m/sec

Acclimation Lowers Metabolic Rate during Exercise

Sawka et al. ASEM , 1983

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•

Hydration / Blood Volume

0

10

20

30

40

50

60

32 38 43 49

90 100 110 120

oCoF

97

81

65

49

32

16

Km Miles

Dehydration Reduces Self-Paced Work

Adolph & Associates, Man in Desert, 1945

0

0L

10LFluid

Available

Montain et al. IJSM, 1998

(30ºC, 50% rh)

0 20 4036

37

38

39

0 20 400 20 40

n=9n=9 n=8

65% VO2max45% VO2max25% VO2max

EU3% BWL5% BWL

Cor

e Te

mp.

(ºC

)

Exercise Time (min)

Dehydration Increases Core Temperature

47% VO2max; 49ºC, 20% rh.

Dehydration Reduces Core Temperature Tolerance

Sawka et al. JAP. 1992

Eu 5% BWL

Cor

e Te

mp.

Tol

eran

ce (º

C)

38

40

39

*

0.2

0.6

1.0

1.4

Sw

eatin

g R

ate

(mg

/cm

/min

)

Core Temp. Core Temp.FB

F (m

l /10

0ml /

min

)36 37 38 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

7% BWL

37.0 38.0 39.0

20

30

40

50

0

0

0

0 Eu 3% BWL5% BWL

Who

le B

ody

Swea

ting

(g /

m2 / h

)

Rectal Temp. (ºC)

Sawka et al. JAP 1985

(25% VO2 max; 49 ºC, 20% rh)

Montain et al. JAP 1995

37 38 39

Eu 3% BWL

5% BWL

Loca

l Sw

eatin

g(m

g / c

m2 /m

in)

Esophageal Temp. (ºC)

(45% VO2 max;30ºC, 50% rh)

0.4

0.8

1.2

0.0

Dehydration Reduces Sweating Graded to Water Deficit

Car

diac

out

put

(L/m

in)

19

20

21

22

23

Time (min)0 20 40 60 80 100 120 140

Leg

Blo

od F

low

(L/m

in)

13

14

15

16

Heat & Dehydration Can Reduce Muscle Blood Flow(35°C, 45% rh; 61% VO2 max; 4%BWL)

-Gonzalez-Alonzo, et al, J. Physiol (London) 1998

Dehydration

Control

Does Hyperhydration Improve Thermoregulation?

Study Temperature SweatCore Skin Rate

Blyth & Burt (1961) nc Moroff & Bass (1965) Greenleaf & Castle (1971) nc nc ncLatzka et.al. (1997) nc nc ncLatzka et.al. (1998) nc nc ncNielsen (1971) Nielsen (1974) Gisolfi & Copping (1974) ncNadel et al. (1980) nc Grucza et al. (1987) nc Candas et al. (1988) nc nc ncLyons et al. (1990) nc

Montner et al. (1996) nc nc

Minutes of Exercise0 30 60 90

% P

lasm

a Vo

lum

e

0

5

10

15

20

Cor

e Te

mpe

ratu

re (°

C)

36

37

38

39

Acute Plasma Volume Expansion& Exercise - Heat Exposure

(45% VO2 max, 45 °C, 20% rh)

= msw

= Performance Time

Sawka et al. EJAP 1983

NS

Albumin Inf.

Control

UnacclimatedEuhydrated

1.0

0.8

0.6

0.4

0.2

36 37 38

Core Temperature ( ºC )

Loca

l Sw

eatin

g (m

g / c

m2 /

min

)Ss #1, EX ITS = 34.5

Erythrocyte Volume Expansion & Control of Sweating(45% VO2max ; 35º C 45% rh)

Sawka et al. JAP 1987

Pre-Infusion

Post-Infusion

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