Exercise in the heat is limited by a critical internal temperature

• Published in: Journal of applied physiology (1985) 2000-08, Vol.89 (2), p.799-806
• Main Authors: Walters, T. J, Ryan, K. L, Tate, L. M, Mason, P. A
• Format: Article
• Language: English
• Subjects: Animals; Biological and medical sciences; Body Temperature - physiology; Exercise; Fatigue; Fundamental and applied biological sciences. Psychology; Hot Temperature - adverse effects; Hypothalamus - physiology; Male; Microwaves; Physical Exertion - physiology; Rats; Rats, Sprague-Dawley; Running - physiology; Stereotaxic Techniques; Temperature; Thermoregulation. Hibernation. Estivation. Ecophysiology and environmental effects; Vertebrates: anatomy and physiology, studies on body, several organs or systems; Weight Loss - physiology
• ISSN: 8750-7587; 1522-1601
• DOI: 10.1152/jappl.2000.89.2.799

1  Air Force Research Laboratory, Directed Energy Bioeffects Division; 2  Veridian Engineering at Air Force Research Laboratory, Directed Energy Bioeffects Division, Brooks Air Force Base, 78235; and 3  Department of Biology, Trinity University, San Antonio, Texas 78212 We examined whether fatigue during exertional heat stress occurred at a critical internal temperature independent of the initial temperature at the start of exercise. Microwaves (2.1 GHz; 100   mW/cm 2 ) were used to rapidly (3-8 min) heat rats before treadmill exercise to exhaustion. In a repeated-measures design, food-restricted male Sprague-Dawley rats ( n  = 11) were preheated to three levels (low, medium, and high). In addition, two sham exposures, Sham 1 and Sham 2, were administered at the beginning and end of the study, respectively. At the initiation of exercise, hypothalamic (T hyp ) and rectal (T rec ) temperatures ranged from 39.0°C to 42.8°C (T hyp ) and 42.1°C (T rec ). The treadmill speed was 17 m/min (8° grade), and the ambient temperature during exercise was 35°C. Each treatment was separated by 3 wk. Run time to exhaustion was significantly reduced after preheating. There was a significant negative correlation between run time and initial T hyp and T rec ( r  = 0.73 and 0.74, respectively). The temperatures at exhaustion were not significantly different across treatments, with a range of 41.9-42.2°C (T hyp ) and 42.2-42.5°C (T rec ). There were no significant differences in run time in the sham runs administered at the start and end of the investigation. No rats died as a result of exposure to any of the treatments, and body weight the day after each treatment was unaffected. These results support the concept that a critical temperature exists that limits exercise in the heat. brain; electromagnetic fields; hypothalamus; microwaves; thermoregulation; treadmill