Exercise in Hot Environments

• WEEK 8: Exercise in Hot Environments

Announcements

• Term Test 2 is scheduled for March 6th during class time (12:40-1:40 pm), lasting 60 minutes
• The test includes 50 multiple-choice questions from the test bank
• Test content covers all material from Week 5-8, including asynchronous and in-class sessions

Today's Objectives

• The study of body temperature regulation in response to exercise
• The review of physiological responses to exercise in the heat
• Understanding the adaptations from acclimation to exercise in hot conditions

Body Temperature Regulation

• Physical exertion complicates stress due to thermal conditions
• Humans are Homeothermic beings, with an internal body temperature that remains stable
• Thermoregulation involves regulating body temperature around the physiological threshold

Body Temperature Regulation: Metabolic Heat Production

• Metabolic heat production is denoted as (M)
• Less than 25% of ATP breakdown results in cellular work (W)
• More than 75% of ATP breakdown results in metabolic heat

Transfer of heat between the body and the environment

• Heat transfers from the body core to the body shell via flood transportation
• Once heat reaches the skin, it can be dissipated through conduction, convection, radiation, or evaporation

Body Temperature Regulation: Transfer of Body Heat

• Conduction (K) is the heat transfer by the direct molecular contact of two solid mats
• An example of conduction is sitting on chilly (or hot) metal bleachers

Convection (C)

• Convection involves heat transfer by gas or liquid movement across a surface
• When there is an increase of movement across the skin surface, there is an increase heat exchange
• Convection is a major thermoregulatory factor

Radiation (R)

• Radiation involves heat loss in the form of infrared rays
• The body can give off or receive radiant heat
• Radiation is also a major daily thermoregulatory factor

Transfer of Body Heat

• C + K + R = avenues of dry heat exchange

Evaporation (E)

• Evaporation involves heat loss via the phase change from liquid to gas
• During exercise, primary heat loss occurs through evaporation, accounting for ~80% of the process
• Clothing can act as a barrier, causing resistance to evaporation

Heat Balance Equation

• If M - W ± R ± C ± K - E = 0, it means that there is heat balance
• If M - W ± R ± C ± K - E < 0, the body is experiencing heat loss
• If M - W ± R ± C ± K - E > 0, the body is experiencing heat gain
• Difference in heat balance is apparent when in a resting state, compared to exercising

Body Temperature Regulation: Other Factors

• Variable of humidity is a factor regarding heat loss
• Evaporation is affected by water vapor pressure (humidity)
• An increase in humidity results in a decrease in evaporation, which reduces heat transfer and causing overheating
• Less humidity results in an increase of evaporation, which encourages heat transfer, resulting in more hydration
• Prolonged evaporation of sweat can cause dehydration

Cooling Capacity of sweat

• If the air temperature equals or is greater than the temperature of the skin, convection will not work and the body must depend on evaporation
• For every 1.5L of evaporated sweat, 400W of cooling takes place

Physiological Responses to Exercise in the Heat

• Exercise significantly rises heat load, which disturbs thermal homeostasis
• Affects cardiovascular function

Cardiovascular Function

• Skin arterioles vasodilate, resulting in increased skin blood flow and thus an increase in convective heat loss
• Increases blood flow compared to exercising in colder environments
• The pre-optic anterior hypothalamus (POAH) triggers the SNS
• This results in increased cardiac output (via heart rate and contractility)
• Vasoconstriction to inessential tissues occurs because of the increased blood flow
• A decrease in blood volume because of sweat
• Stroke Volume (SV) doesn’t, so Heart Rate (HR) increases to compensate (cardiovascular drift).

Limitations of Exercising in Heat

• Cardiovascular system overload
• Critical temperature theory; the brain shuts down exercise at ~40-41 degrees celcius

Cardiovascular system overload

• The heart cannot provide sufficient blood flow to both exercising muscle and skin
• Impairs performance, and there's an increased risk of overheating
• Particularly affects untrained/non-acclimatized athletes

Case Study: core temperature in elite cyclists

• 2016 Road World Championships in Qatar(37 degrees Celsius and 25% RH)
• Individual time trial (ITT): 30 km for women and 40 km for men (~35 - 45 minutes)
• Team time trial (TTT) ~6 riders: 40 km for women and men (~45 minutes)
• Road race (RR): 140 km for women (~3 hours) and 260 km for men (~6 hours)

Core temperature in elite cyclists: Main Takeaways

• Exercise intensity matters more for changes in body temperature
• Elite athletes can safely conduct physical activity at core temperature above 40-41°C

Precooling and Sport Performance

• Artificially lowers core temperature before exercise via methods: Cold water immersion, cold room/shower,cooling vests, icy drinks
• Can improve sports performance, particularly during endurance events
• May hurt sprint events

Exercise the in heat: Acclimation

• Acclimation is adaptation that happens in a artificial environment
• Acclimatization is adaptation that occurs in a Natural environment
• Short Term Heat Acclimation (</= 5-7 days)
• Long Term Heat Acclimation (typically 10+ days)
• At least 60 minutes a day of continuous exercise in the heat to maintain a core temperature of 38.5 degrees celcius
• Plasma volume increases due to an increase of oncotic pressure

Plasma Volume

• Is temporary and returns to normal in ten days
• Buys time for other adaptations to occur

Heart Rate

• Heart rate decreases and cardiac output (stroke volume) increases
• Supports skin blood flow
• Involves greater heat loss, decreases core temperature

Widespread Sweating

• Occurs earlier with more dilute
• Prevents loss of sodium
• Optimizes heat loss
• Can be measured with direct calorimetry

Acclimation to Exercise in the Heat: Sex Differences

• Both genders have the same capacity for exercising at the same relative intensity
• Women are not able to tolerate the heat as well during their menstrual cycle
• Women use more CHO during exercise
• Women have lower sweat rates than men
• Women take longer to heat adapt
• Women have more active sweat glands but less sweat production per gland, which is an advantage in humid climates, but disadvantage in hot, dry climates

Summary

• Humans are homeothermic, which means that they maintain a stable internal temperature
• Heat is transferred via conduction, convection, radiation, and evaporation
• Evaporation is the primary heat loss mechanism during physical activity
• Skin blood flow increases to improve heat flow
• Due to compensation from decreased stroke volume cardiovascular drift takes place
• Brain function limit exercise when core temperature is roughly 40-41 degrees celcius
• Humidity impacts evaporation
• Intensity of exercise helps influence core temperature
• Acclimation can be short term (5-7 days) and long term (10+ days)
• Acclimation benefits are increased plasma volume, earlier/more dilute sweating, lower heart rate
• In terms of differences among the sexes, women adapt more slowly but have advantages in humid environments
• Strategies like cold water immersion, cooling vests, and drinks improve endurance performance