Lowering of resting core temperature during acclimation is influenced by exercise stimulus

The decrease in resting core temperature ( T co ) and its relation to the reduced physiological strain during heat acclimation was analysed with rectal temperature data measured in three groups of eight semi-nude persons (6 males, 2 females) who were acclimated for 15 consecutive days to dry, humid and radiant heat, respectively, with equivalent WBGT (33.5°C), by performing 2-h treadmill work. A fourth group followed the same protocol for 12 days in a neutral climate. After acclimation, both resting T co , prior to heat exposure, and final T co , measured at the end of work, were significantly reduced. The reduction in final T co increased with decreasing ambient water vapour pressure and was higher for the data pooled over the heat conditions (0.46 ± 0.31°C) than in the neutral climate (0.21 ± 0.25°C), whereas resting T co declined similarly in the heat (0.20 ± 0.25°C) and the neutral environment (0.17 ± 0.23°C). The lowering of resting and final T co after heat acclimation showed a significant correlation ( r = 0.67) and regression analysis showed that 37% of the average reduction in final T co was attributable to the lowering of resting T co . The same analysis was applied after extending the database by short-term series of clothed persons (17 females, 16 males) acclimated at 29.5 and 31.5°C WBGT for 5 days. A significant correlation was found between the lowering of resting and final T co ( r = 0.57) that did not depend on climatic conditions and gender, although the reduction in resting T co was significantly smaller for females (0.06 ± 0.22°C) than for males (0.21 ± 0.23°C). It is concluded that the lowering of resting core temperature contributes to the reduced physiological strain during heat acclimation. Similar effects under neutral conditions point to the exercise stimulus as a probable explanation.