Transcript AIS Chapter 9 Heat

AIS Chapter 9
Heat
Heat Exchange During Exercise
The Hypothalamus 下視丘 —The
Body’s Thermostat
• Increased core temperature • Cold exposure
– Anterior hypothalamus
– Commencement of sweating
– Increased skin blood flow
– 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), 1.0101.020 (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/