1. Exercise and Thermal Stress
Chapter 25
Exercise and Thermal Stress
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

2. Thermal Balance Core temperature Heat transfer Heat absorption
Heat loss
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

3. Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

4. Hypothalamic Regulation of Temperature
Hypothalamus — central coordinating center for temperature regulation
Activation of body’s heat-regulating mechanisms
Thermal receptors in the skin
Changes in blood temperature perfusing the hypothalamus
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

5. Thermoregulation in Cold Stress: Heat Conservation and Heat Production
Vascular adjustments
Cutaneous cold receptors constrict peripheral blood vessels.
Muscular activity
Shivering
Hormonal output
Epinephrine and norepinephrine (short term)
Thyroxine (long term)
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

6. Thermoregulation in Heat Stress: Heat Loss
Radiation
Electromagnetic heat waves
Conduction
Direct contact between molecules
Convection
Movement of adjacent air or water molecules
Evaporation
Vaporizing water
Evaporative heat loss at high ambient temperatures
Heat loss in high humidity
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

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McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

8. Integration of Heat-Dissipating Mechanisms
Circulation
Blood redirected to skin to dissipate heat.
Evaporation
Sweating begins within several seconds of the start of vigorous exercise.
Cooled blood returns to core to absorb additional heat.
Hormonal adjustments
Vasopressin and aldosterone help maintain blood volume.
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

9. Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

10. Effects of Clothing on Thermoregulation
Clothing insulation (clo units)
Wind speed
Body movements
Chimney effect – baggy clothes
Bellows effect – movement increases ventilation of air layers
Water vapor transfer
Permeation efficiency factor – clothes absorb sweat
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

11. Effects of Clothing on Thermoregulation
Cold-weather clothing
Layers trap air
Moisture properties
Warm-weather clothing
Light in color
Football uniforms – poor heat dissipation
Modern cycling helmet does not thwart heat dissipation.
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

12. Exercise in the Heat Circulatory adjustments
Vascular constriction and dilation
Maintenance of blood pressure
Core temperature during exercise
Temperature rises as intensity increases.
More fit individuals generate more heat at the same percentage of O2max, but maintain a lower core temperature.
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

13. Water Loss in the Heat: Dehydration
Magnitude of fluid loss
The more prolonged or intense the exercise, the greater the loss.
Significant consequences
Dehydration may threaten health.
Physiologic and performance decrements occur.
Diuretics
Cause greater fluid loss from plasma than sweating
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

14. Maintaining Fluid Balance: Rehydration and Hyperhydration
A benefit from exogenous glycerol?
When consumed with 1 to 2 L of water, glycerol facilitates water absorption and extracellular fluid retention.
Adequacy of rehydration
Thirst is a poor indicator.
Use changes in body mass as guide.
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

15. Electrolyte Replacement
Added sodium
Combining solid food with plain water
Drink 25 to 50% more fluid than that lost via sweating.
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

16. Whole-Body Precooling
Research shows mixed results.
Cooling skin 5 to 6°C improved cycling performance for 30 minutes.
Failed to improve triathlon or thermic responses to 90 minutes of soccer
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

17. Factors That Modify Heat Tolerance
Acclimatization
Physiologic changes that improve heat tolerance
Optimal acclimatization requires adequate rehydration.
Training status
Increased sensitivity and capacity of sweating response
Plasma volume increases
Greater skin and GI blood flow
Larger volumes of more dilute sweat
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

18. Factors that Modify Heat Tolerance
Age
Age-related differences in heat tolerance
Some age-related factors affect thermoregulatory dynamics.
Children
Lower sweating rate and higher core temperature
Sweat is more concentrated.
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

19. Factors that Modify Heat Tolerance
Gender
When studies control for fitness level and relative exercise intensity, no gender differences are observed.
Sweating – Women
Sweat less prolifically than men despite having more heat-activated sweat glands
Sweat smaller volumes
Begin sweating at higher skin and core temperatures
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

20. Factors that Modify Heat Tolerance
Compared to men, women tend to cool faster.
Menstrual cycle alters skin blood flow and sweating response.
Body fat insulates body, retards heat dissipation, and adds to metabolic cost of weight-bearing activities.
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

21. Complications from Excessive Heat Stress
Heat cramps
Imbalances in fluids and electrolytes
Heat exhaustion
Blood pools in periphery
Core temperature rises
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

22. Complications from Excessive Heat Stress
Heat stroke
Exertional heat stroke
Heat from exercise
Hot, humid environment
Signs and symptoms
Stop sweating, core temp >105°F, altered mental status
Requires immediate medical attention
Oral temperature unreliable
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

23. Exercise in the Cold Body fat, exercise, cold stress
Fat insulates
Higher body fat = greater cold tolerance
Children and cold stress
Large ratio of body surface area to mass
Don’t tolerate cold well
More effective peripheral vasoconstriction and increased energy metabolism
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

24. Acclimatization to Cold
Humans have less capacity to acclimate to cold than to heat.
The Ama
Korean and Japanese women divers
Tolerate cold for long periods of time
Other examples of cold adaptation
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

25. How Cold Is Too Cold? The wind-chill temperature index
Modified in 2001
Provides information on winds and freezing temperatures
Respiratory tract during cold-weather exercise
Cold air poses no danger, causes airway moisture loss
Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition

26. Copyright © 2007 Lippincott Williams & Wilkins.
McArdle, Katch, and Katch: Exercise Physiology: Energy, Nutrition, and Human Performance, Sixth Edition