Heat Stress With Once Vs Thrice Daily Exposures To Heat

PURPOSE: To determine the sex differences and effect of periodic vs sustained heat exposures during heat acclimation on core body temperature.

METHODS: Males and females were divided into two exposure groups and underwent 90 minutes of heat acclimation (38 °C, 60% RH) at an absolute intensity on a treadmill (1.6 m·s-1, 5% grade) for 1 week. A sustained group completed the single 90-minute session continuously daily. A periodic group completed three 30-minute sessions 3 hours apart daily. Therefore, total heat exposure was matched. On days 1, 4, and 7 a telemetry pill was ingested at least 4 hours prior to exercise. Peak core temperature was tracked, and the area under the curve (AUC) of core temperature over time was calculated. Post-heat exposure measures for the sustained group were taken during the 6 hours following exposure. Post-heat exposure measures for the periodic group were taken in the 2 hours following each exposure. AUC was calculated using the trapezoid method.

RESULTS: Of the 180 telemetry pills ingested, only 162 were usable for peak core temperature analysis and 126 for AUC calculation due to early excretion or other technical issues related to the telemetry system. Peak core temperature during each heat acclimation session did not differ by sex regardless of exposure schedule (males, 38.4 ± 0.5 °C; females, 38.4 ± 0.4 °C; p = 0.724). However, AUC calculations showed that males accumulated less heat than females (males, 13,475 ± 89; females, 13,524 ± 79; p = 0.038). The sustained group (38.6 ± 0.5 °C) experienced a higher peak core temperature than the periodic group (38.2 ± 0.3 °C, p < 0.001) throughout acclimation. Calculated AUC showed that accumulated core temperature decreased from day 1 over 7 days of heat acclimation in the sustained group (Day 1, 13,535 ± 72; Day 4 13,485 ± 105; Day 7, 13,458 ± 85, p < 0.05), but not the periodic group (Day 1, 13,519 ± 65; Day 4, 13,519 ± 90; Day 7, 13,500 ± 75; p > 0.05).

CONCLUSIONS: Peak core temperature did not differ between sex, however, males accumulated less heat than females. The sustained group experienced higher peak core temperatures than the periodic group and exhibited greater heat acclimation than the periodic group.