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Background: While thermoregulatory behavior is critical for maintaining homeostasis, our knowledge of behavioral thermoeffectors in humid heat is limited compared to the control of autonomic thermoeffectors. The predictions that the frequency and duration of intensified humid heat events are expected to increase in the coming years underline this shortcoming. Therefore, this study aims to elucidate the activation of autonomic thermoregulatory responses and subjective thermal perceptions upon deciding to initiate thermal behavior in a hot and humid environment. Methods: Ten young male adults participated in an experimental trial where local cooling was permitted at any time during the behavioral assessment during passive exposure to humid heat. The air temperature and relative humidity were kept at 33 ∘ C and 80 % , respectively. Skin temperatures, core body temperature (T core ), and skin blood flow (forearm, upper arm, and upper back) were obtained 120 s preceding thermal behavior. Local sweat rate (forearm and upper arm) and subjective thermal perceptions (neck and whole-body) upon thermal behavior initiation were also recorded. Results: Mean skin temperature ( T ¯ sk ) and T core increased prior to thermal behavior initiation (p = 0.002; p = 0.001). An increase in mean body temperature ( T ¯ body ) was also observed (p < 0.001). However, the initiation of thermal behavior is not preceded by an increase in skin blood flow (p ≥ 0.154) and local sweat rate (p ≥ 0.169). An increase in thermal discomfort and skin wetness perception was observed (p ≤ 0.048; p ≤ 0.048), while thermal sensation did not differ from the baseline (p ≥ 0.357). Conclusion: These findings suggest that when given the opportunity to behaviorally thermoregulate in a hot and humid environment, changes in skin blood flow and sweat rate are not required for thermal behavior to be initiated in resting humans. Moreover, an increase in T ¯ sk and T core , which appears to cause an increase in thermal discomfort, precedes thermal behavior. In addition, an increase in T ¯ body leading up to thermal behavior initiation was observed, suggesting that changes in T ¯ body rather than T ¯ sk and T core alone mediate thermal behavior in humid heat. Collectively, the results of this study appear to support the hypothesis that the temporal recruitment of autonomic thermoeffectors follows an orderly manner based on their physiological cost. [ABSTRACT FROM AUTHOR]

BACKGROUND: Susceptibility of healthy astronauts to orthostatic hypotension and presyncope is exacerbated upon return from spaceflight. Up to 64% of astronauts experience postflight orthostatic intolerance. A promising countermeasure for postflight orthostatic intolerance is fluid loading by giving crew salt tablets and water prior to re-entry. The primary goal of the current study was to determine the optimal time window prior to re-entry when individual crewpersons would initiate fluid loading. METHODS: There were 16 subjects who were given two 6-h exposures, with and without fluid loading (conditions), to head-down tilt (HDT) to simulate the effects of microgravity. Pre- and post-HDT stand tests of orthostatic tolerance were given. Physiological measurements recorded included heart rate, blood pressure, peripheral blood volume, total peripheral resistance, and impedance cardiography. Echocardiography measures of stroke volume and cardiac output were also recorded. RESULTS: Data were analyzed with three-way repeated measures ANOVA (gender x condition X time). Only the condition X time interaction was significant for mean arterial pressure. Post hoc multiple comparison tests revealed significant increases in mean arterial pressure occurred between hours 1 and 3 of HDT after fluid loading (10 mmHg higher than no fluid). DISCUSSION: These findings indicate that the optimal time for crew to begin fluid loading is within 1 to 3 h prior to re-entry. Nonsignificant trends of multiple cardiovascular responses showed similar time profiles. The large amount of individual variability suggests that fluid loading alone may be an inadequate countermeasure for all crewmembers. Further research is needed on possible adjunct methods of tailoring countermeasures for individuals. [ABSTRACT FROM AUTHOR]