Thermal effects of prolonged treadmill exercise in the heat

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Abstract
Six trained men 21–38 yrs old ran 19–29 km (1.5 to 2.5 hrs) approximately 75% of their V̇o2 max on a level treadmill in the heat (33.5/21.5°C db/wb, wind velocity 36 m/min). Every 20 min while running they (a) drank 200 ml of 10°C water, (b) drank 200 ml of water at core body temperature, or (c) sponged their faces, arms, and trunks for 2 min with a towel soaked in 10°C water. In another series of experiments each subject (d) hydrated 30 min before the run by drinking 1 liter of water, (e) hydrated and performed (a) above, and (f) hydrated and performed (b) above. Rectal temperature (Tre) and body weight loss (WL) were determined every 30 min and metabolism at 45 min of running. Terminal Tre after (a) through (f) averaged 39.1, 39.3, 39.9, 39.7, 39.1, and 39.3°C respectively. During a control run the corresponding mean Tre was 39.9°C. In a 66.7 kg man with a body weight/surface area (BW/SA) ratio of 37.5 and running at 74% of his V̇o2 max, Tre rose 0.4°C/l% WL with increasing WL's greater than 2.0% of his initial body weight regardless of the intervention imposed. In another subject weighing 70 kg with a BW/SA ratio of 38.8 and running at 83% of his V̇o2 max, Tre rose 0.49°C/l% WL over the entire range of WL's, These data indicate: (1) the rise in Tre during prolonged work in the heat is related to the %WL incurred; (2) consuming 1 liter of warm or cold water on the run is more effective in preventing a marked rise in Tre than drinking an equal volume of water 30 min before the run; and (3) sponging the upper body with a cold towel while running is ineffective in preventing a marked rise in Tre.

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