Thermoregulation and Heat Exchange

• Internal body temperature is regulated and nearly constant even with environmental temperature changes

Body Heat Exchange

• Heat is transferred between solid materials through direct molecular contact through conduction
• Heat is transferred via gas or liquid across a surface through convection
• Increased movement accelerates skin surface heat exchange
• Heat is transferred through electromagnetic energy via radiation
• Dry heat exchange occurs through convection, conduction, and radiation
• Heat loss occurs through the phase change from liquid to gas via evaporation
• During exercise, the primary form of heat loss is evaporation (~80%)
• Clothing can resist evaporation
• Less than 25% of ATP breakdown leads to cellular work
• More than 75% of ATP breakdown leads to metabolic heat production

Heat Balance Equation

• M - W ± R ± C ± K - E = 0 means heat balance is achieved
• M - W ± R ± C ± K - E < 0 indicates heat loss
• M - W ± R ± C ± K - E > 0 indicates heat gain

Humidity and Heat Loss

• Evaporation is affected by water vapor pressure (humidity)
• Increased humidity decreases evaporation
• Decreased humidity increases evaporation
• Prolonged evaporation through sweat causes dehydration
• The cooling capacity of sweat is limited when air temperature is greater than or equal to skin temperature, as convection is ineffective
• 1.5 L of evaporated sweat cools 400 W

Measuring External Heat Stress

• The wet bulb globe temperature (WBGT) includes convection, evaporation, and radiation
• The WBGT equation factors in:
• Dry-bulb temperature as the actual air temperature
• Wet-bulb temperature reflecting evaporative potential
• Globe temperature measuring radiant heat load
• WBGT = 0.1T + 0.7T + 0.2T

Body Temperature Regulation

• Thermoregulatory control is how body temperature is regulated
• The preoptic-anterior hypothalamus (POAH) acts as the body's thermostat in the brain
• The POAH receives input from sensory thermoreceptors and activates mechanisms when body temperature deviates
• Sensory receptors include peripheral thermoreceptors in the skin

Effects of Exercise in Heat

• Exercise in the heat increases:
• Internal body temperature
• Heart rate
• Fatigue
• Exercise in the heat decreases:
• Cardiac output
• Stroke volume
• Exercise performance
• Increased risk of heat illness is a possible effect of exercise in heat

Heat Exhaustion

• Heat exhaustion includes fatigue, dizziness, nausea, vomiting, fainting, and a weak, rapid pulse
• Thermoregulatory mechanisms are functional but overwhelmed

Exertional Heat Stroke

• Exertional heat stroke is thermoregulatory mechanism failure
• Characterized by core temperature greater than 40°C (or 40.5°C), confusion, disorientation, and unconsciousness (CNS dysfunction)
• Cold water immersion is the first response to treating exertional heat stroke

Heat Cramp

• Heat cramps involve severe, painful cramping of large muscles
• Triggered by Na+ losses and dehydration
• Most common in heavy sweaters

Effects of Exercise in the Heat

• Blood is distributed to the peripheral areas in the heat
• In cold conditions, blood is more internal to conserve heat

Impact of Heat Mitigation Strategy

• Fluid ingestion is a heat mitigation strategy
• Pre-event cooling and heat acclimation/acclimatization are also heat mitigation strategies
• Aerobic fitness aids in heat mitigation in exercise

Heat Acclimation and Acclimatization

• Heat acclimation occurs in an artificial environment
• Heat acclimatization occurs in a natural environment

HA Adaptations

• Adaptations occur through systematic and repeated heat exposures
• Adaptations include:
• Decreased heart rate
• Increased plasma volume
• Decreased perceived exertion and thermal sensation
• Decreased internal body temperature
• Decreased sweat electrolyte concentration
• Decreased skin temperature
• Increased sweat rate
• Increased exercise performance

HA Exercise Performance

• Heat acclimation improves exercise performance in both heat and cool conditions
• Hydration interacts with heat acclimation

Internal Body Temperature

• Internal body temperature is key
• Hyperthermia and sweat are controlled by intensity, duration, and environmental conditions

HA Decay

• When discontinuing heat exposure, physiological and performance adaptations fade
• HR and internal body temperature diminish at 2.5% per day without continued heat exposure
• Performance enhancements after HA persist for approximately 1 to 2 weeks

HA and Recovery

• HA indices fatigue
• A need to recover to maximize adaptations
• Intermittent heat exposures can maintain adaptations for a long time, and can lessen fatigue around competitions

Heat Acclimation vs Heat Acclimatization

• Heat acclimation advantages: easy to control variables, monitor adaptations, high dose, easy plan-making
• Heat acclimation disadvantages: accessibility, sports specificity
• Heat acclimatization advantages: high cost and time efficiency, balance between HA and training
• Heat acclimatization disadvantages: need to travel, less controlled, maybe less adaptations

Heat Mitigation Strategies

• The goal of heat mitigation strategies is decreasing internal body temperature

Considerations of Cooling

• Modality, timing, and availability must be considered

Cooling Modalities

• Dousing
• Cooling vest
• Cold-water immersion
• Ice towels

Timing of Cooling

• Pre-cooling is not as effective in sprint type workouts
• Per-cooling is during exercise, but is limited compared to pre and post, is often used in tennis matches
• Post-cooling happens after exercise

Cooling Rate

• A higher cooling rate helps decrease body temperature quicker
• Cold water immersion covers most surface area
• All modalities with a rate below 0.05 is basically ineffective
• Mixed method cooling is the most common and effective
• No matter what method is used, you can expect a 5.7% increase
• Hydration has a bigger impact than cooling
• Feasibility does not guarantee effectiveness