Exercise Physiology and Heat Tolerance

Thermal Stress Lecture Objectives

• Explain how thermal balance is maintained in the body
• Describe four physical factors contributing to heat balance
• Understand cardiac output, heart rate, and stroke volume response during physical activity in hot weather
• Explain how acclimatization, training, age, gender, and body fat modify heat tolerance during physical activity

Heat Cramps, Exhaustion, and Stroke Lecture Objectives

• Understand symptoms, causes, and treatments for heat cramps, heat exhaustion, and heat stroke
• Understand the purpose of the wind-chill temperature index and factors comprising it
• Discuss immediate and long physiologic adjustments to cold stress

Temperature

• Temperature represents mean kinetic energy of a substance’s atoms
• Functional definition of temperature: Potential for heat exchange between substances (e.g., blood to capillary walls) or objects (e.g., playing surface to participant's body).
• Humans can tolerate a decline in deep body temperature of -10°C but only an increase of +5°C

Body Temperature Measurement

• Core temperature (T_c): 36.1 – 37.8°C (97 – 100°F)
• Skin temperature (T_sk): 33.3°C (91.4°F)

Thermoregulation

• The process whereby the body temperature is maintained or controlled under a wide range of environmental conditions

Assessing High Environmental Temperatures

• Relative Humidity: The moisture in the air relative to how much moisture (water vapor) the air can hold at a given condition
• Heat Stress Index: A measure used to estimate the risk of heat stress based on ambient temperature and relative humidity
• Wet Bulb Globe Temperature (WBGT): A scale, developed by the military, used to assess thermal heat load imposed by the environment. Commonly used in industrial settings and athletic situations

Thermal Balance

• Core temperature represents a dynamic equilibrium between factors that add and subtract body heat
• Core temperature rises if heat gain exceeds heat loss (e.g., during vigorous exercise in warm, humid environments)
• Core temperature declines in cold when heat loss exceeds heat production

Heat Exchange During Exercise

• 60-minute exercise performed at 900 kpm/min^-1 at different ambient temperatures

Effect of Clothing (Uniforms)

• Football equipment and clothing seal off 50% of the body from evaporative cooling benefits
• Football gear exacerbates increases in rectal and skin temperatures with exercise

What Happens When You Get Overheated?

• The increase in body temperature that occurs with exercise
• Physiological mechanisms used to dissipate heat
• Increase in sweating rate (evaporation)
• Vasodilation of the cutaneous (skin vessels)

Thermoregulation

• Hypothalamus coordinates temperature regulation; acts as a thermostat set at 37°C ± 1°C
• Initiates responses to protect from buildup or loss of heat
• Activation of heat-regulating mechanisms
• Thermal receptors in skin provide input to central control center
• Temperature of blood perfusing hypothalamus directly stimulates

Role of Skin and Subcutaneous Tissues

• Water vapor pressure gradient between skin and air
• Evaporation, convection, and conduction transfer heat out of the body
• Blood vessels in dermis; arrector muscles of hair react to heat
• Increased blood flow to the skin dissipates heat; decreased blood flow conserves heat

Blood Flow Redistribution

• Thermoneutral (25.0°C) and Heat (43.3°C)

Exercise in the Heat

• Cardiac output, heart rate, and stroke volume increases

OLD vs Young; Men vs women

• Comparison of heart rate with exercise intensity

Exercise Intensity and Body Temp

• Core temperatures, sweat rates, and heat stress in various exercises (uncompensable/compensable/prescriptive zones)

Example of a Heat Stress Study

• The Impact of Heat Exposure and Repeated Exercise on Circulating Stress Hormones

Core Body Temperature Response

• Rectal temperature response during room temperature and heat exposure at different exercise levels and times

Heart Rate

• Heart rate response during room temperature and heat exposure at different exercise levels and times

Plasma and Blood Volume

• Plasma and blood volume response in room temperature and heat exposure conditions

Catecholamine Response

• Epinephrine and norepinephrine response in room temperature and heat exposure conditions

Cortisol and HGh Response

• Cortisol and hGH responses in room temperature and heat exposure conditions

Factors Affecting the CV Response to Heat

• Age and gender
• Hydration: Dehydration increases physiological strain
• Fitness level: Fitness improves thermoregulatory function and heat tolerance
• Body composition
• Acclimatization: Adaptive changes occur when exposed to stressful environments; reduces strain. (Adipose tissue interferes with heat dissipation and increases metabolic cost of activity)

Acclimatization to Heat

• Improved cutaneous blood flow, effective redistribution of cardiac output, lowered threshold for sweating, and increased sweat output
• Lowered salt concentration in sweat, lower skin and core temperatures and heart rate during standard exercise, less reliance on carbohydrate catabolism during exercise

Acclimatization Effects

• Core temperature, heart rate, and sweat rate response over time in unacclimatized/acclimatized individuals

Hydration Level in Various Sports

• Sweat rate and voluntary fluid intake in various sports

Fluid Ingestion During and after Exercise

• The ingestion and absorption of fluids during and after exercise and associated issues

Heat Illness

• Probably the greatest stress on the human cardiovascular system is the combination of exercise and hyperthermia
• This stresses can present life-threatening challenges, especially for highly motivated athletes exercising in hot environments

Minor Exertional Heat Illness

• Heat cramps: acute disorder characterized by brief, recurrent, and excruciating pain in voluntary muscles of legs, arms, or abdomen
• Heat syncope: temporary circulatory failure due to pooling of blood in peripheral veins and reduced ventricular filling; leading to a decrease in cardiac output

Major Exertional Heat Illness

• Heat exhaustion: rapid and weak pulse, fatigue, weakness, profuse sweating, psychological disorientation, and fainting
• Heat stroke: serious medical emergency characterized by elevated skin and core temperatures, tachycardia, vomiting, diarrhea, hallucinations, and coma

Exertional Heat Illness

• Symptoms of exertional heat illness (mild, moderate, and severe signs/symptoms)

Treatment of Heat Illness

• Treatment procedures for each type of heat illness

Cold Exposure

• Human exposure to extreme cold produces significant physiologic and psychological challenges
• Further compromise of core temperature in chronic exertional fatigue, sleep loss, inadequate nourishment, reduced tissue insulation, and reduced/depressed shivering heat production

Hypothermia

• Decrease in body’s temperature (3 stages)
• Stage 1: Body temp drops 1-2°C below normal (35-34°C)
  • Loss of complex motor tasks, breathing becomes rapid & shallow
  • Loss of neuromuscular function at subsequent lower temps
• Stage 2: Body temp drops 2-4°C below normal (34-32°C)
• Stage 3: Body temp below 32°C
  • Impaired organ function
  • Death may occur

Physiological Changes with Reduced Core Temperature

• Physiological changes in body temperature across stages (mild, moderate, and severe)

Body Fat, Physical Activity, and Cold Stress

• Differences in body fat influence cold function
• Increased subcutaneous fat improves/increases cold insulation in swimmers and exercisers

How Cold is "Too Cold"?

• Increased peripheral vasoconstriction during cold exposure
• Dangerously low skin and extremity temperatures
• Factors that contribute to frostbite and other cold injuries: alcohol use, low fitness, and fatigue, dehydration, poor peripheral circulation

Wind Chill Index

• Provides a useful way to understand dangers from winter winds and freezing temperatures and provides frostbite threshold values

Cold Stress

• Physiological thermoregulation in the cold, monitored by cold receptors in the skin, abdominal viscera, spinal cord, change/rate of decrease in temperature, in numbers compared with heat receptors (differing actions), and signal to other parts of body

What Happens When You Get Cold?

• Decrease in environmental temperature
• Decrease in skin temperature
• Piloerection (hair stands up)
• Peripheral vasoconstriction
• Shivering
• Increased metabolic rate (BMR)
• Increased heat production

Acute Adaptations to Cold Stress

• Physiological responses to cold; cessation of sweating, increased shivering, peripheral vasoconstriction, thyroid hormone release, and sympathetic stimulation

Exercise in the Cold

• Decreased nerve conduction velocity and neuromuscular activity
• Reduction in force production, diminished power output, decreased time to peak power, and associated injuries (exercise-induced asthma/increased ventilation/higher oxygen consumption/metabolic changes/increased reliance on CHO)

Cold Acclimatization

• natural/artificial environment, behavioral changes, psychological, and physiological adaptations

Cold Stress Study

Exercise and Cold Stress

• Various processes and data used in an experiment/case study examining the effects of exercise and cold

Rectal Temperature

• Monitoring rectal temperature changes during different procedures and conditions

Signs/Symptoms of Lowered Body Temperature

• Constant shivering, blue lips/fingers, confused behavior, poor coordination, decreased mental capacity, and decreased physiological performance

Risk Factors for Cold Injury

Cold Stress and the Skin - Nonfreezing/Freezing Injuries

• Types of cold injuries to the skin

Mechanism of Frostbite

• Frostbite mechanism and conditions

Cold Weather Clothing and Thermoregulation

• Factors affecting clothing's clo (insulation value)

Questions?

• Blank page for questions