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 degrees 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 degrees C) than in the neutral climate (0.21 +/- 0.25 degrees C), whereas resting T (co) declined similarly in the heat (0.20 +/- 0.25 degrees C) and the neutral environment (0.17 +/- 0.23 degrees 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 degrees 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 degrees C) than for males (0.21 +/- 0.23 degrees 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.