Journal of Applied Physiology, Vol 76, Issue 2 589-597, Copyright © 1994 by American Physiological Society

ARTICLES

Muscle metabolism during exercise and heat stress in trained men: effect of acclimation

M. A. Febbraio, R. J. Snow, M. Hargreaves, C. G. Stathis, I. K. Martin and M. F. Carey
Exercise Metabolism Unit, Victoria University of Technology, Footscray, Australia.

Exercise metabolism was examined in 13 endurance athletes who exercised on three occasions for 40 min at 70% of maximal O2 uptake in an environmental chamber at either 20 degrees C and 20% relative humidity (RTT) or 40 degrees C and 20% relative humidity before (PRE ACC) or after (POST ACC) 7 days of acclimation. Exercise in the heat resulted in a lower (P < 0.05) mean O2 uptake (0.13 l/min) and higher (P < 0.01) heart rate and respiratory exchange ratio. Acclimation resulted in a lower (P < 0.01) mean heart rate and respiratory exchange ratio. Postexercise rectal temperature, muscle temperature, muscle and blood lactate, and blood glucose were higher (P < 0.01) in the PRE ACC than in the RTT trial, but all were reduced (P < 0.01) in the POST ACC compared with the PRE ACC trial. Muscle glycogenolysis and percentage of type I muscle fibers showing glycogen depletion were greater (P < 0.05) in the PRE ACC than in the RTT trial. Muscle glycogenolysis was unaffected by acclimation during exercise in the heat, although the percentage of depleted type I fibers was higher (P < 0.05) in the unacclimated state. Plasma epinephrine was higher (P < 0.01) during exercise in the heat in the unacclimated individual relative to RTT but was lower (P < 0.01) in the POST ACC than in the PRE ACC trial. The greater reliance on carbohydrate as a fuel source during exercise in the heat appears to be partially reduced after acclimation. These alterations are consistent with the observed changes in plasma epinephrine concentrations.

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