Thermal tolerance can be maintained and enhanced by passive, post-exercise intermittent heat exposure following heat acclimation in a military context

Military personnel are required to operate in hot environments upon short notice. To preserve performance an elevated thermal tolerance could be maintained prior to deployment using heat acclimation (HA) followed by intermittent heat exposure (IHE). In a randomised manner, 19 participants completed 5 d of passive, post-exercise HA in either sauna or hot-water immersion (HWI) followed by 19 d of decay, or IHE every 2-3 days. A heat-stress test involving walking for 1 h in 33°C, 75% humidity in military dress was conducted before HA, after HA, and following the decay or IHE period. Following HA, performance was unaffected, while mean rectal temperature (↓0.3°C), and mean heart rate reduced (↓8 bpm). Following the decay period, performance reduced in the decay group (↓2.6 min) while no change was seen with IHE. IHE saw mean rectal temperature continued to decrease (↓0.1°C) while sweat rate increased (↑0.1 L.h-1), changes not seen in the decay group. These findings showed HA to induce beneficial thermoregulatory adaptations that could be enhanced by IHE in a manner that can be practicably implemented within groups that need to deploy into hot environments at short notice.