S2, L9: Exercise in the heat

Which athletes are more susceptible to heat related illnesses?

Those that have previously experienced heat related illnesses

What is hypohydration?

Dehydration due to lots of sweating, loss of blood volume, and less blood volume to dissipate heat

How does hypohydration lead to a shortage of fuel?

The reduced blood volume is shifted to the periphery for heat loss, so less blood reaches the gut and intestines, meaning that there is less absorption of glucose, so glycogen stores are used more as less exogenous glucose is taken up

What does homeothermic mean?

Body temperature is maintained within a narrow range

Describe how heat is lost (4)

• Radiation - any object that emits heat, the largest being the sun

• Conduction - heat transfer between objects

• Convection - heat transfer from air or liquid, huge heat loss in cycling due to large air flow

• Evaporation - 80% of total heat loss during exercise is through sweat evaporation and it makes humans the best mammal at dealing with heat

Describe how heat is gained (2)

• Metabolic heat - humans are 25% efficient (25% of energy is used and the rest is heat)

• Some heat gain from radiation, convection and conduction

What methods of measuring core temperature are there? (5)

• Rectal probe

• Pills

• Oral temperature

• Core temperature monitor

• Probe in the armpit, under the tongue, or on the forehead

What structure regulates temperature?

The preoptic-anterior hypothalamus

How is temperature regulated?

Thermoreceptors detect changes in temperature and activate a negative feedback system to return temperature to normal via the sympathetic nervous system to reduce core body temperature

What are the physiological responses to exercise in the heat? (6)

• Skin arterioles vasodilate to increase skin blood flow

• Increased cardiac output to increase blood flow to skin and muscles

• Reduced stroke volume due to a reduced blood volume

• Heart rate increases to a certain limit (cardiovascular drift)

• Performance reduces due to insufficient blood flow to the muscles

• Increased sweating, causing reduced blood volume

Describe the relationships of heart rate, stroke volume, and cardiac output with exercise intensity at 26°C

• Heart rate has a positive linear relationship with exercise intensity

• Stroke volume increases after a VO₂ of 2 L/min

• Cardiac output has a positive linear relationship with exercise intensity

Describe the relationships of heart rate, stroke volume, and cardiac output with exercise intensity at 40°C

• Heart rate has a positive linear relationship with exercise intensity, but has a steeper gradient than at 26°C

• Stroke volume has a negative linear relationship with exercise intensity

• Cardiac output is unaffected until a VO₂ above 2 L/min, when it drops

Describe the relationship between heat and performance

Cycling to exhaustion at 70% of VO_{2 max} gave an inverted-U relationship between temperature and performance, with 11°C being optimal

Explain why the relationship between heat and performance is an inverted-U

• Heat causes substrate-independent fatigue at 31°C

• There is a reduction in mechanical efficiency at 4°C

How does humidity affect time to exhaustion?

There was a significantly faster time to exhaustion at 60% and 80% humidity

How does solar radiation affect time to exhaustion?

There was a significantly faster time to exhaustion at solar radiations of 250, 500, and 800 W/m² compared to 0 W/m²

What temperature measure accounts for humidity, water vapour, solar radiation and air temperature?

Wet-bulb globe temperature (WBGT)

How does the heat impact different distances of running events?

• Performance improvement in sprints

• No difference in middle distance events

• Performance decrease in long distance events

How does fluid intake affect core body temperature during exercise in the heat?

• Lots of fluid intake attenuates hyperthermia by maintaining or reducing the decrease in blood plasma volume, therefore allowing for increased skin blood flow and therefore heat loss

• More fluid, up to about 2.5 L can reduce core temperature by about 0.8°C

What is acclimation?

Physiological changes that occur over a short period of time

What is acclimatisation?

Physiological changes as a result of living in a certain environment for months/years

How is heat acclimation beneficial?

It can develop beneficial physiological changes that may help to maintain or prevent reductions in performance

Describe the benefits of heat acclimation? (6)

• Plasma volume peaks at 115% after 6 days, then gradually decreases to 105% at 14 days

• Heart rate drops to 80% after 6 days, then plateaus

• Core temperature drops to 95% after 4 days, then plateaus

• Skin temperature drops to 97% after 3 days, then plateaus

• Sweating rate only increases after 4 days up to 120% at 11 days before plateauing

• Exercise capacity increases linearly to 115% at day 13 before plateauing

Describe the methods of heat acclimation (4)

• Baseline and repeat testing - heat stress tests for 30 minutes monitoring core body temperature

• Fixed duration - 90 mins in heat at 50% VO_{2 max}

• Isoprogressive - maintaining core temperature at 38.5°C

• Exercising until a specified core temperature is achieved - 38.5°C

What are the key adaptations to heat acclimation? (4)

• Increased capacity and performance

• Reduced resting heart rate

• Reduced resting core temperature

• Increased sweat rate

Describe the effectiveness of hot baths for reducing core temperature and performance improvements

• Hot water immersion (40°C) significantly reduced resting and final exercise core temperature in conditions of both 18°C and 33°C

• Hot water immersion significantly reduced 5k time in both 18°C and 33°C

What are the symptoms of heat cramps? (4)

• Painful muscle cramps

• Profuse sweating

• Excessive thirst

• Fatigue

What are the symptoms of heat exhaustion? (3)

• Nausea

• Chills/goosebumps

• Headaches

What are the symptoms of heatstroke? (3)

• Cessation of sweating

• Confusion

• Loss of consciousness

What is the treatment for exertional heatstroke?

Cold water immersion as quickly as possible

What are the practical considerations for exercise in the heat? (7)

• Consider cancellation if WBGT > 28°C

• Ensure adequate fluid intake

• Practice early morning or late evening

• Factor in breaks in play

• Minimise the amount of clothing worn

• Ensure athletes and coaches are aware of exertional heatstroke

• Period of heat acclimation