Exercise in the heat is limited by a critical internal temperature

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.

Differential effects of targeted tongue exercise and treadmill running on aging tongue muscle structure and contractile properties

“…For the ET, rats were given a 1-min warm-up running period at 20 cm/s. Treadmill speed was then increased to 30 cm/s on a 20°incline, and rats ran until exhaustion, which was defined as the inability of the rat to return to and maintain the pace of the treadmill for two continuous minutes (22,40,42,85). Time to exhaustion (s) was recorded.…”

Thermoregulatory responses to RF energy absorption

“…The resulting DT co was thereby held to $1.0 8C. Walters et al [2000b] examined whether fatigue during exertional heat stress occurred at a critical T co independent of the initial T co at the start of exercise. Microwaves (2.1 GHz; 100 mW/cm 2 ) were used to rapidly (3-8 min) heat rats (n ¼ 11) to one of three levels before treadmill exercise to exhaustion.…”

The effects of heat stress on neuromuscular activity during endurance exercise

“…The body's inability to control its increase in temperature (thermoregulatory failure), cardiovascular changes and changes in metabolic rate may also affect exercise capacity in a hot environment (17,23). It has been suggested that rather than dehydration, energy production or metabolic rate changes, absolute core temperature is the critical factor limiting exercise capacity in the heat (8,25). It is likely that when core temperature reaches critically high levels, an afferent signal is sent to the CNS, which responds by sending an efferent command to alter the neuromuscular recruitment strategy, to prevent cellular damage or rigor from occurring (19).…”