In order to verify the effects of heat and exercise acclimation (HA) on resting and exercise-induced expression of plasma and leukocyte heat shock protein 72 (Hsp72) in humans, nine healthy young male volunteers (25.0± 0.7 years; 80.5±2.0 kg; 180±2 cm, mean ± SE) exercised for 60 min in a hot, dry environment (40±0°C and 45±0% relative humidity) for 11 days. The protocol consisted of running on a treadmill using a controlled hyperthermia technique in which the work rate was adjusted to elevate the rectal temperature by 1°C in 30 min and maintain it elevated for another 30 min. Before and after the HA, the volunteers performed a heat stress test (HST) at 50% of their individual maximal power output for 90 min in the same environment. Blood was drawn before (REST), immediately after (POST) and 1 h after (1 h POST) HST, and plasma and leukocytes were separated and stored.
There is evidence that brain temperature (Tbrain) provides a more
sensitive index than other core body temperatures in determining physical
performance. However, no study has addressed whether the association between
performance and increases in Tbrain in a temperate environment is
dependent upon exercise intensity, and this was the primary aim of the present study.
Adult male Wistar rats were subjected to constant exercise at three different speeds
(18, 21, and 24 m/min) until the onset of volitional fatigue. Tbrain was
continuously measured by a thermistor inserted through a brain guide cannula.
Exercise induced a speed-dependent increase in Tbrain, with the fastest
speed associated with a higher rate of Tbrain increase. Rats subjected to
constant exercise had similar Tbrain values at the time of fatigue,
although a pronounced individual variability was observed (38.7-41.7°C). There were
negative correlations between the rate of Tbrain increase and performance
for all speeds that were studied. These results indicate that performance during
constant exercise is negatively associated with the increase in Tbrain,
particularly with its rate of increase. We then investigated how an incremental-speed
protocol affected the association between the increase in Tbrain and
performance. At volitional fatigue, Tbrain was lower during incremental
exercise compared with the Tbrain resulting from constant exercise
(39.3±0.3 vs 40.3±0.1°C; P<0.05), and no association between the
rate of Tbrain increase and performance was observed. These findings
suggest that the influence of Tbrain on performance under temperate
conditions is dependent on exercise protocol.
Thermal environmental stress can anticipate acute fatigue during exercise at a fixed intensity (%VO 2max ). Controversy exists about whether this anticipation is caused by the absolute internal temperature (T int , ºC), by the heat storage rate (HSR, cal/min) or by both mechanisms. The aim of the present study was to study acute fatigue (total exercise time, TET) during thermal stress by determining T int and HSR from abdominal temperature. Thermal environmental stress was controlled in an environmental chamber and determined as wet bulb globe temperature (ºC), with three environmental temperatures being studied: cold (18ºC), thermoneutral (23.1ºC) or hot (29.4ºC). Six untrained male Wistar rats weighing 260-360 g were used. The animals were submitted to exercise at the same time of day in the three environments and at two treadmill velocities (21 and 24 m/min) until exhaustion. After implantation of a temperature sensor and treadmill adaptation, the animals were submitted to a Latin square experimental design using a 2 x 3 factorial scheme (velocity and environment), with the level of significance set at P<0.05. The results showed that the higher the velocity and the ambient temperature, the lower was the TET, with these two factors being independent. This result indicated that fatigue was independently affected by both the increase in exercise intensity and the thermal environmental stress. Fatigue developed at different T int and HSR showed the best inverse relationship with TET. We conclude that HSR was the main anticipating factor of fatigue.
Key wordsIt has been well documented that the total exercise time (TET) is reduced in warmer environments and internal body temperature (T int ) has been considered to be a limiting factor during prolonged physical exercise (1-5). This anticipation of fatigue seems to occur in order to prevent an increased T int from compromising homeostasis. The mechanisms responsible for exercise interruption due to an increase in T int are still not well understood; controversy exists about whether there is a critical absolute T int (in ºC) or whether the heat storage rate (HSR, in cal/min) is as important or even more important than T int in the origin of fatigue. Fuller et al. (6), studying the effect of warm environments on T int in rats during treadmill exercise, reported a shorter TET in animals exercising in warmer environments and similar abdominal and hypothalamic temperatures at the end of exercise at the time of fatigue, and suggested the existence of a critical absolute T int which may lead to the interruption of activity. Walters et al. (7),
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