Repeated-bout exercise in the heat in young athletes: physiological strain and perceptual responses

A short recovery period between same-day competitions is common practice in organized youth sports. We hypothesized that young athletes will experience an increase in physiological strain and perceptual discomfort during a second identical exercise bout in the heat, with 1 h (21 degrees C) between bouts, even with ample hydration. Twenty-four athletes (6 boys and 6 girls: 12-13 yr old, 47.7 +/- 8.3 kg; 6 boys and 6 girls: 16-17 yr old, 61.0 +/- 8.6 kg) completed two 80-min intermittent exercise bouts (treadmill 60%, cycle 40% peak oxygen uptake) in the heat (33 degrees C, 48.9 +/- 6.1% relative humidity). Sweat loss during each bout was similar within each age group (12-13 yr old: bout 1, 943.6 +/- 237.1 ml; bout 2, 955.5 +/- 250.3 ml; 16-17 yr old: bout 1, 1,382.2 +/- 480.7 ml; bout 2, 1,373.1 +/- 472.2 ml). Area under the curve (AUC) was not statistically different (P > 0.05) between bouts for core body temperature (12-13 yr old: bout 1 peak, 38.6 +/- 0.4 degrees C; bout 2, 38.4 +/- 0.2 degrees C; 16-17 yr old: bout 1 peak, 38.8 +/- 0.7 degrees C; bout 2, 38.7 +/- 0.6 degrees C), physiological strain index (12-13 yr old: bout 1 peak, 7.9 +/- 0.9; bout 2, 7.5 +/- 0.7; 16-17 yr old: bout 1 peak, 8.1 +/- 1.5; bout 2, 7.9 +/- 1.4), or thermal sensation for any age/sex subgroup or for all subjects combined. However, rating of perceived exertion AUC and peak were higher (P = 0.0090 and 0.0004, respectively) during bout 2 in the older age group. Notably, four subjects experienced consistently higher responses throughout bout 2. With these healthy, fit, young athletes, 1 h of complete rest, cool down, and rehydration following 80 min of strenuous exercise in the heat was generally effective in eliminating any apparent carryover effects that would have resulted in greater thermal and cardiovascular strain during a subsequent identical exercise bout.