AIS Chapter 9Heat

Heat Exchange During Exercise

The Hypothalamus 下視丘 — The Body’s Thermostat

• Increased core temperature– Anterior hypothalamus– Commencement of

sweating– Increased skin blood

flow

• Cold exposure– Posterior

hypothalamus– Increase heat

production• Shivering

– Decrease heat loss• Decreased skin blood

flow

Heat Exchange During Exercise

• Evaporative heat loss: Most important • Convective heat loss: Small contribution• Radiative heat loss: Small contribution

Heat Exchange During Exercise

Body Temperature Increase During Exercise

• Increase in body temperature with work rate– Linear across wide range of temperatures

Heat Exchange During Exercise: Effect of Ambient Temperature

• As ambient temperature increases:– Heat production remains constant– Lower convective and radiant heat loss– Higher evaporative heat loss

Heat Exchange During Exercise: Effect of Exercise Intensity

• ↑exercise intensity, ↑ heat production– Lower convective and radiant heat loss– Higher evaporative heat loss

Exercise in Hot/Humid Environments

• Cardiovascular responses– ↑skin temperature, ↑cutaneous blood flow– ↑heart rate to maintain cardiac output– ↑sweat rate, ↓plasma volume, ↑plasma osmolality

• Thermal responses– ↑ Peripheral and core temperature

• Hyperthermia: esophageal temperature >~40 C in well-trained subjects– An independent cause of fatigue– ↓ Self-selected exercise pace

Exercise in Hot/Humid Environments

• Metabolic responses– ↑anaerobic metabolism, ↑blood lactate– ↓ADP rephosphorylation (ADP ATP), ↓muscle

blood flow, ↑respiratory exchange ratio– ↑glycogen usage, ↑glycolysis

Core Temperature and Sweat Rate During Exercise in Heat/Humidity

Performance in different temperature

Thermal assessment in athletes

• Heat generation and dissipation during exercise highly individual

• Ingestible thermometer• Rectal thermometer• Esophageal thermometer• Data analyzed in conjunction with heart rate,

sweat rate, rate of perceived exertion, thermal sensation, performance data

Measurement of core temp during ex

Acclimatization of heat

• Passively: sustained passive heat exposure– Not recommended

• Actively: physical training in warm to hot conditions– Limit heat dissipation, promote heat storage

• Initial adaptations: ↑plasma volume, ↑cutaneous blood flow, ↓heart rate, ↓perception of effort

• Subsequent adaptations: ↓resting core temp, ↓exercise core temp, earlier onset of sweating, ↓sweat secretion

Acclimatization of heat

• Duration: ~ 2 weeks, large individual difference– Elite endurance runners already partially heat acclimatized

• Athletes predominantly train and compete in worm to hot environments already heat acclimatized

• Focus: practice pacing and heat management strategies– May require 2 weeks

• Require planning to ensure that tapering not limit adaptation– Similar efforts to those of competition

Heat management strategies

• Acclimation: what to do in cold environment• AIS prepare for Olympics in Asia– 7-8 weeks, 60-90 min exercise in heat tent, once per week

prior to departure– Further 10 days in competition venue– Gradually increase exercise intensity in heat– Exposed to heat, but allow adequate recovery between

sessions• Training in spring to prepare for summer competitions– 1-2 sessions/week train outdoors at noon, or in hot

rooms/gyms

Heat management strategies

• Hydration: <2% body weight loss during exercise– Calculate dehydration (%): (preBW – postBW) x 100/

preBW– Calculate sweat rate (L/hr): (PreBW - postBW + fluid

consumption – urine output) / Time– Urine specific gravity: <1.010 (well hydrated), 1.010-

1.020 (minimal dehydration)• Fluid consumption schedule according to

dehydration and sweat rate• Ad libitum drinking 50-70% sweat loss during ex

Heat management strategies: Cooling

• usually ↓core temp ~0.5 C• Ice jackets• Water immersion, can be used during breaks– < 10 min at < 15 C– 20-30 min at 25-28 C: more popular

• Crushed ice ingestion 冰沙– 7.5 ml/kg ice ↓core temp 0.7 C, improve performance

• Ice towels• forearm/ and hand immersion– 20 min at 10 C ↓core temp 0.6 C

Effect of immersion on core temp

Commercialized ice sports bar

http://www.powerice.com/product-info/