thermoregulation 60-65% of total energy expenditure lost as heat transfer of heat from body...
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Effects of exercise intensity on sweat rate
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How much sweat evaporation is necessary to maintain thermoregulation at moderate intensity
for 20 min?
Metabolic efficiency is ~23% Evaporation of 1000 mL of sweat requires
580 kcal of energy Cycling at 2 L/min (= 10 kcal/min) for 20 min
– Total energy expended = 20 min 10 kcal/min = 200 kcal
– Total heat produced = 200 kcal 0.77 = 154 kcal– Total evaporation necessary = 154 kcal / 580
kcal/L = 0.27 L of sweat(Assumes no other heat loss from other mechanisms)
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Effects of Exercise in Heat
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Physiological responses to heat accumulation
EPI release– HR upward drift glycogen use
surface blood flow is increased at expense to other tissues
blood volume cardiac efficiency ( SV, HR)
fatigue (central)
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Effect of Dehydration on HR
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Dehydration and Performance
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Fluid balance
dehydration impairs endurance performance, – minimal effect on power and speed events
blood volume will skin blood flow and heat dissipation
thirst mechanism doesn’t keep up with dehydration
need for water replacement > than electrolyte replacement
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Maintaining fluid balance
fluid intake during exercise will:– minimize dehydration– minimize rise in body temperature– reduce CV stress
[CHO] > 6-8% slows absorption from gut
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Sport Nutrition: Sport Drinks
[CHO] affects fluid absorption rate in gut– Gatorade – 6%– Exceed – 7.2%– Coca-Cola – ~11%– Sprite – 10.2%– cranberry juice – 15%– orange juice – 11.8%
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ACSM Fluid Replacement Recommendations
Before exercise, drink: – ~500 ml 2 hr before exercise
During exercise, drink:– early and at regular intervals– to replace lost fluids (if tolerated)– over-hydration can result in hyponatremia
After exercise, drink:– excess of that lost during exercise
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Acclimatization to exercise in heat
body fluids/blood volume (w/in 3-5 d) skin blood flow rate of sweating (may take up to 10 d) electrolyte loss heat tolerance
Achieving heat acclimatization normal workouts in heat for 5-10 d
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Heat acclimatization
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Suggested Readings(available at gssiweb.com under Sport Science Exchange)
Murray, R. Fluid replacement: The American College of Sports Medicine Position Stand. GSSE 9(4) SSE#53, 1996.
Maughan, RJ and SM Shirreff. Preparing athletes for competition in the heat: developing an effective acclimatization strategy. GSSE 10(2) SSE#65, 1997.
Murray, R, J Stofan, ER Eichner. Hyponatriemia in athletes. SSE 88, 16(1), 2003.
Cheuvront,SN, MN Sawka. Hydration Assessment of Athletes SSE, 18(2), 2005.
Bonci, L. Energy Drinks: Help, Harm, or Hype? SSE 84, 15(1), 2002.
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The primary mode of heat dissipation during exercise is
A. convection
B. radiation
C. conduction
D. sweat evaporation
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What likely is the primary cause of HR drift during moderate-intensity exercise in heat?
A. plasma volume leading to SV
B. EPI release
C. temperature
D. sweating
E. body weight
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Which of the following fluids would best maintain hydration during exercise in heat?
A. water
B. orange juice
C. Coke
D. Coke without carbonation
E. fruit juice
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Which of the following is an adaptation to repeated exercise in the heat?
A. Earlier onset of sweating
B. Increased sweat rate
C. Decreased sweat Na+ concentration
D. Increased plasma volume
E. All of the above are adaptations