Yes / Introduction: Investigate the effectiveness of short-term heat acclimation (STHA), over 5-days (permissive dehydration), on an intermittent sprint exercise protocol (HST) with females. Controlling for menstrual cycle phase. Materials and Methods: Ten, moderately trained, females (Mean [SD]; age 22.6 [2.7] y; stature 165.3 [6.2] cm; body mass 61.5 [8.7] kg; VO˙ 2 peak 43.9 [8.6] mL·kg−1 ·min−1 ) participated. The HST (31.0◦C; 50%RH) was 9 × 5 min (45-min) of intermittent exercise, based on exercise intensities of female soccer players, using a motorized treadmill and Wattbike. Participants completed HST1 vs. HST2 as a control (C) trial. Followed by 90 min, STHA (no fluid intake), for five consecutive days in 39.5◦C; 60%RH, using controlled-hyperthermia (∼rectal temperature [Tre] 38.5◦C). The HST3 occurred within 1 week after STHA. The HST2 vs HST3 trials were in the luteal phase, using self-reported menstrual questionnaire and plasma 17β-estradiol. Results: Pre (HST2) vs post (HST3) STHA there was a reduction at 45-min in Tre by 0.20◦C (95%CI −0.30 to −0.10◦C; d = 0.77); Tsk (−0.50; −0.90 to −0.10◦C; d = 0.80); and Tb (−0.25; −0.35 to −0.15◦C; d = 0.92). Cardiac frequency reduced at 45-min (−8; −16 to −1 b·min−1 ; d = 1.11) and %PV increased (7.0; −0.4 to 14.5%: d = 1.27). Mean power output increased across all nine maximal sprints by 56W (−26 to 139W; d = 0.69; n = 9). There was limited difference (P > 0.05) for these measures in HST1 vs HST2 C trial. Discussion: Short-term heat acclimation (5-days) using controlled-hyperthermia, leads to physiological adaptation during intermittent exercise in the heat, in moderately trained females when controlling for menstrual cycle phase.

This thesis evaluated physiological responses of young and older adults during extreme heat events and the extent to which commonly recommended heat-health guidelines (indoor temperature limits) and heat mitigation strategies (cooling centres) are effective at limiting hyperthermia and cardiovascular burden. A multidisciplinary narrative review and three experimental studies were conducted. In the review, the mechanisms by which aging impairs the regulation of body temperature and hemodynamic stability, and how they may contribute to the increased risk of heat-related mortality and morbidity in older adults, were summarized. A lack of ecologically minded study designs in previous research evaluating the physiological responses supporting homeostasis and health during heat stress (i.e., body temperature regulation and cardiovascular stability) was also identified. The three experiments were therefore designed as day-long (8-9 hour) extreme heat simulations to 1) evaluate age-related alterations in thermoregulatory and cardiovascular function during peak heat conditions; 2) assess how these responses translate to indoor environments; and 3) quantify the effectiveness of cooling centers, a widely recommended heat mitigation strategy, for limiting hyperthermia and cardiovascular burden. In the first study, healthy older adults (age: 64-78 years; n=19) stored 87 kJ [95% confidence limits: 33, 141] more heat than their younger (age: 19-31 years; n=20) counterparts (328 [71] kJ vs. 241 [SD: 87]; P<0.001) during the first three hours of a 9-hour exposure to extreme heat (40°C and 15% relative humidity). This resulted in a 0.4°C [0.2, 0.6] greater increase in body core temperature in the older adults that was maintained throughout exposure (1.0 [0.3] vs 0.6 [0.3]°C; P<0.001). These findings were extended in the second study, wherein it was demonstrated that healthy older adults (age: 66-78 years, n=8) exhibit progressive elevations in body temperatures (P<0.001) and attenuations in cardiovascular autonomic function (P<0.001) during 8 hours of rest in conditions representative of those experienced indoors during extreme heat events. These ranged from an actively cooled environment (22°C), through indoor temperature thresholds recommended by Toronto Public Health (26°C) and the World Health Organization (31°C), to poorly insulated and ventilated homes and/or dwellings without access to air conditioning (36°C; 45% relative humidity in all conditions). In the third study, it was shown that short-term exposure to a cool environment midway through (hours 5-6) a day-long (9 hour) simulated heat event reduced core temperature in a group of healthy older adults (age: 67-78 years; n=8) by 0.8°C [0.6, 1.0] compared to an age-matched group not removed from the heat (from study 1). Despite this, core temperature rose rapidly upon return to the heat and was statistically equivalent in both groups by the end of exposure (37.8 [0.3] vs 37.9 [0.3]°C; P=0.011). The findings of this thesis indicate that even healthy older adults experience sustained elevations in body temperature and cardiovascular burden during extreme heat events and that commonly recommended heat-health guidelines (indoor temperature limits) and mitigation strategies (cooling centres) may not provide adequate protection. Collectively, this work represents a considerable advance in our understanding of the physiological burden experienced by older adults during hot weather and extreme heat events.