Thermoregulatory Responses to Constant versus Variable-Intensity Exercise in the Heat

Abstract

To compare the thermoregulatory responses between constant (CON) and variable-intensity exercise (VAR) in a dry-hot environment (36 degrees C, 29% relative humidity, and 2.5 m x s(-1) airflow). In a random order, seven endurance-trained, heat-acclimated subjects cycled either at 60% VO2max (CON) or alternating 1.5 min at 90% VO2max with 4.5 min at 50% VO2max (VAR). Total work output (915 +/- 100 kJ) and exercise duration (90 min) were identical in both trials. Net metabolic heat production was not different between trials (394 +/- 12 vs 408+/- 11 W x m(-2) for VAR vs CON). However, heat storage (60 +/- 3 vs 48 +/- 4 W x m(-2)), the increase in rectal temperature (1.6 +/- 0.1 vs 1.3 +/- 0.1 degrees C), and final heart rate (HR; 147 +/- 5 vs 141 +/- 4 beats x min(-1)) were all higher for VAR than for CON (P < 0.05). During VAR, averaged forearm skin blood flow (S(K)BF) was lower, whereas whole-body sweat rate (1.23 +/- 0.1 vs 1.11 +/- 0.1 L x h(-1)) and dehydration (2.8 +/- 0.1% vs 2.5 +/- 0.2%) were higher than during CON (P < 0.05). Final blood lactate during VAR was higher than during CON (3.5+/- 0.4 vs 2.1 +/- 0.3 mmol x L(-1); P < 0.05). Ninety minutes of variable-intensity exercise in a hot environment increases heat storage and fluid deficit in comparison to the same amount of work performed in a constant-load mode. VAR increases not only thermal (i.e., heat storage) but also cardiovascular (i.e., heart rate) and metabolic (i.e., blood lactate) stresses, which makes it less advisable than CON when the goal is to minimize physiological stress.