Thirteen healthy subjects (7 males and 6 females) were passively acclimated by means of 7 sessions with passive lower body heating repeated every other day for two weeks. On every successive occasion, the subjects were sitting for 45 min immersed up to the waistline in a water bath at ~44 °C (air temperature 23 °C, rh 40%). Rectal (T_re), skin (T_sk) temperatures at three sites (back, thigh and forearm) and body mass were measured before and after passive heating. Heart rate (HR), ratings of thermal sensation (RTS) and comfort (RTC) were collected every 5 min during passive heating. During the first and the last HA session, participants performed a 2-min muscular voluntary contraction (MVC) of knee extensors. After acclimation, both the resting T_re prior to heat exposure and the final temperature measured at the end of passive heating were significantly reduced, but had no effect on T_sk. Heat acclimation had a significant effect on increased sweating and decreased HR, RTS, RTC and the physiological stress index. Lower body heating resulted in a greater torque fatigue compared to the thermoneutral condition during 2-min MVC. Following HA, MVC torque was uninfluenced. We have concluded that passively induced heat acclimation is reliable for lowering T_re at rest. Although passively induced heat acclimation for two weeks improved the symptoms of acclimation, it did not change MVC torque fatigue. Changes in T_re, HR and RTC during passive heating are closely related to the shift of T_re at rest, following heat acclimation.

Keywords: rectal temperature, heat stress, symptoms of acclimation, thermoregulation