Effects on heat tolerance of physical training in water and on land

A 4-wk training program was undertaken by 15 untrained non-heat-acclimated males who were divided into three groups matched on maximal aerobic capacity (VO2max) and trained either in water or on land to determine how physical training (PT) in these different media affects heat tolerance. Subjects trained on a cycle ergometer for 1 h/day, 5 days/wk at 75% VO2max, with the exercise intensity progressively increased to maintain a constant training stimulus. Group I exercised on land, whereas groups II and III exercised while immersed to the neck in water of either 32 degrees C (II) or 20 degrees C (III). Daily exercise increased core temperature (Tc) in groups I and II but not in group III. Training elicited similar increases (approximately 15%) in VO2max in the three groups. Before and after PT, all subjects exercised at approximately 30% VO2max for 3 h at 49 degrees C, 20% rh. Compared with before training, groups I and II showed a decrease in final Tc and heart rate (HR) in the posttraining heat exposure. Sweat rate increased 25% in group II but remained the same in group I. Group III demonstrated a decrease in final HR, but final Tc was higher than before training. Sweat rate did not increase in group III and was lower than the other groups. It was concluded that PT can improve the cardiovascular response to dry heat without affecting thermoregulatory capacity. PT appears to enhance heat tolerance only if Tc is permitted to rise during exercise, thus stimulating the temperature-regulating center for heat dissipation.