Physiological Strain and Perceptual Differences during Repeated-bout Exercise in the Heat

Abstract

One of the biggest challenges facing numerous young athletes is attempting to train or perform safely and effectively in the heat. An even greater performance challenge and clinical risk for incurring heat injury is encountered when a young athlete has to train or compete again soon on the same day, following a prior heat exposure and strenuous exercise session or competition. PURPOSE: To test the hypothesis that young athletes will experience an increase in physiological strain and related ratings of perceptual discomfort during a second identical exercise bout (compared to the first) in the heat, with a one-hour recovery period (21°C) in between bouts, even when maintaining adequate hydration (6% carbohydrate-electrolyte drink; [Na+]: 21.1 mmoH-1). METHODS: 24 healthy, young soccer players (6 boys, 6 girls 12–13 y: 12.6 ± 0.5 y, 161.8 ± 7.5cm, 49.9 ± 7.9 kg, BMI: 18.6 ± 2.2; 6 boys, 6 girls 16–17 y: 16.5 ± 0.5y, 166.0 ± 8.0 cm, 61.0 ± 8.6 kg, BMI: 21.8 ± 2.0) each completed two 80-min intermittent exercise (treadmill: 60% VO2max; cycle ergometer: 40% VO2max) sessions in the heat (33°C; relative humidity: 48.9 ± 6.0%) on the same day. RESULTS: Sweat loss incurred during each bout (B) of exercise was similar within each age group (12–13 yB1: 943.6 ± 237.1 ml, B2: 955.5 ± 250.3 ml; 16–17yB1: 1382.2 ± 480.7 ml, B2: 1373.1 ± 472.2 ml). Area under the curve (AUC) was not statistically different (p>0.05) between exercise bouts for core body temperature (12–13 y B1 peak: 38.6 ± 0.4°C, B2: 38.4 ± 0.2°C; 16–17y B1 peak: 38.8 ± 0.7°C, B2: 38.7 ± 0.6°C), physiological strain index (PSI) (12–13 yB1 peak: 7.9 ± 0.9, B2: 7.5 ± 0.7; 16–17 yB1 peak: 8.1 ± 1.5,B2: 7.9 ± 1.4), or thermal sensation for any age/gender sub-group or for all subjects combined. However, rating of perceived exertion AUC for all subjects combined was higher (p=0.0002) during B2 (B1 peak: 15.3 ± 2.0; B2: 16.2 ± 2.2), with the biggest difference shown in the older boys. Notably, 4 subjects experienced a consistently higher core temperature and PSI throughout exercise B2 compared to B1. CONCLUSION: With fit, young athletes, one hour of cool-down and rehydration, following 80 min of exercise in the heat, is insufficient time to avert greater perception of effort, and for some, greater physiological strain, during a subsequent identical exercise bout. Supported by Gatorade Sports Science Institute