Effects of exercise intensity on sweat rate

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)

Effects of Exercise in Heat

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)

Effect of Dehydration on HR

Dehydration and Performance

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

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

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%

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

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

Heat acclimatization

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.

The primary mode of heat dissipation during exercise is

A. convection

B. radiation

C. conduction

D. sweat evaporation

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

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

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