Exercise in the heat is limited by a critical internal temperature

J Appl Physiol (1985). 2000 Aug;89(2):799-806. doi: 10.1152/jappl.2000.89.2.799.

Abstract

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 degrees C to 42.8 degrees C (T(hyp)) and 42.1 degrees C (T(rec)). The treadmill speed was 17 m/min (8 degrees grade), and the ambient temperature during exercise was 35 degrees 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 degrees C (T(hyp)) and 42.2-42.5 degrees 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.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Body Temperature / physiology*
  • Hot Temperature / adverse effects*
  • Hypothalamus / physiology
  • Male
  • Microwaves
  • Physical Exertion / physiology*
  • Rats
  • Rats, Sprague-Dawley
  • Running / physiology
  • Stereotaxic Techniques
  • Weight Loss / physiology