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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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