Effects of Acute Exposure to Thermal Stress on Cardiorespiratory Function, Skeletal Muscle Oxygenation, and Exercise Performance in Healthy Males

We investigated the effects of acute thermal stress (30 degrees C and 40 degrees C) and ordinary temperature (20 degrees C) on cardiorespiratory function, skeletal muscle oxygenation, and exercise performance in healthy men. Eleven healthy males (21.5 +/- 2.3 years) performed a graded exercise test (GXT) using a cycle ergometer in each environmental condition (20 degrees C, 30 degrees C, and 40 degrees C) in a random order with an interval of 1 week between each test. Before the test, they were allowed to rest for 30 min in a given environmental condition. All dependent variables (body temperature, cardiorespiratory function parameters, skeletal muscle oxygenation profiles, and exercise performance) were measured at rest and during GXT. GXT was started at 50 W and increased by 25 W every 2 min until subjects were exhausted. Body temperature increased proportionally at rest and at the end of exercise as thermal stress increased. There were no differences in the rating of perceived exertion, oxygen uptake, respiratory exchange ratio, and carbon dioxide excretion between environmental conditions. Heart rate (HR), minute ventilation (VE), and blood lactate levels were significantly higher at 30 degrees C and 40 degrees C than at 20 degrees C, and oxygen pulse was significantly lower at 40 degrees C than at 20 degrees C at various exercise loads. None of the skeletal muscle oxygenation profiles showed significant changes at rest or during exercise. Maximal oxygen uptake, peak power, and exercise time significantly decreased proportionally as thermal stress increased, and this decrease was most pronounced at 40 degrees C. Acute thermal stress induces a decrease in exercise performance via increased body temperature, HR, VE, and blood lactate levels and decreased oxygen pulse during load-homogenized exercise. This phenomenon was more prominent at 40 degrees C than at 30 degrees C and 20 degrees C.