Purpose Cognition can be impaired during exercise in the heat, potentially contributing to military casualties. To our knowledge, the independent role of elevated core temperature during exercise has not been determined. The aim of the current study was to evaluate effects of elevated core temperature on cognition during physically encumbering, heated exercise, and to determine whether the perceptual cooling effects of menthol preserves cognition. Methods Eight participants complete three trials in randomised order: one normothermic (CON) and two with elevated (38.5°C) core temperature, induced by prior immersion in neutral versus hot water The CON trial and one hot trial (HOT) used a water mouth-rinse following each cognitive task of the trial, (HOT) while the other used a menthol mouth-rinse (MENT). Participants walked in humid heat (33°C, 75% relative humidity) in military clothing, completing a cognitive battery of reaction time, perceptual processing, working memory, executive function, cognitive flexibility, vigilance, and declarative memory. Results No differences in cognitive performance were observed between any conditions. Near-infrared spectroscopy showed greater oxygenated haemoglobin tissue content in HOT and MENT compared to CON (ΔO 2 Hb-deO 2 Hb: 2.3 ± 4.5 µM, p < .024), and lower deoxygenated haemoglobin in MENT than in CON or HOT (p = .017), suggesting higher brain metabolism during the more stressful conditions. Conclusion Moderately elevated core (38.5°C) and skin temperature does not appear to impair cognitive performance during exercise despite mildly elevated cerebral metabolism. The effects of menthol remain undetermined due to the lack of heat-mediated cognitive impairment. Keywords Cognitive • Physiology • Menthol • Exercise Abbreviations CON Control group HOT Hot group MENT Menthol group WI Water immersion (figure only) W Water mouth-rinse (figure only) M Menthol mouth-rinse (figure only) TLX Task-load index

Acute exposure to heat, such as that experienced by people arriving into a hotter or more humid environment, can compromise physical and cognitive performance as well as health. In military contexts heat stress is exacerbated by the combination of protective clothing, carried loads, and unique activity profiles, making them susceptible to heat illnesses. As the operational environment is dynamic and unpredictable, strategies to minimize the effects of heat should be planned and conducted prior to deployment. This review explores how heat acclimation (HA) prior to deployment may attenuate the effects of heat by initiating physiological and behavioural adaptations to more efficiently and effectively protect thermal homeostasis, thereby improving performance and reducing heat illness risk. HA usually requires access to heat chamber facilities and takes weeks to conduct, which can often make it impractical and infeasible, especially if there are other training requirements and expectations. Recent research in athletic populations has produced protocols that are more feasible and accessible by reducing the time taken to induce adaptations, as well as exploring new methods such as passive HA. These protocols use shorter HA periods or minimise additional training requirements respectively, while still invoking key physiological adaptations, such as lowered core temperature, reduced heart rate and increased sweat rate at a given intensity. For deployments of special units at short notice (< 1 day) it might be optimal to use heat re-acclimation to maintain an elevated baseline of heat tolerance for long periods in anticipation of such an event. Methods practical for military groups are yet to be fully understood, therefore further investigation into the effectiveness of HA methods is required to establish the most effective and feasible approach to implement them within military groups.

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BackgroundMilitary personnel often deploy into hot environments that impose substantial strain on physical and cognitive performance. Hot environments can present as arid or humid and occur in different terrains, requiring different operational approaches. The aim of this study was to characterise the physiological, cognitive and perceptual strain experienced by military personnel during typical operations in arid and humid environments. MethodsNine pack-fit military personnel participated in two heat-stress tests to exhaustion, one in an arid environment (44°C, 21% humidity) and the other in a humid environment (33°C, 78% humidity). Participants walked at 5 km.h-1 while physiological, cognitive and perceptual measures were recorded. Tests were terminated volitionally, or by excessive core temperature or heart rate. Results The operational environments induced similar physiological stress, resulting in no difference in time to exhaustion (p = .155). The humid environment saw a greater elevation in core temperature (+0.3°C, p < .001) and heart rate (+5 b.min-1, p < .001). Skin temperature was greater in the arid environment (+0.4, p < .001) as was sweat evaporation (+0.3 L.h1, p = .045). Baseline performance predictors only provided moderate predictions of performance, whereas changes in perceptual measures provided the best performance predictors during the exercise, specifically perceptions relating to thermal sensation (β = -.65 - -.80) and sleepiness (β = -.79 - -.87). While no differences in cognitive performance were observed, greater cognitive stress was reported by participants over time, regardless of environment (all p < .011). ConclusionsThe humid operational environment elicited a greater thermal strain that may threaten safety, and impair performance, to a greater degree than the arid environment. Perceptual measures of thermal sensation and sleepiness were the best predictors of test termination and could likely be used to monitor thermal tolerance in field settings.

Decompression sickness occurs from tissue supersaturation with nitrogen when pressure is reduced. It limits operational capacity in diving, aviation and extravehicular activity in space. Little is known about organ variability, and why differences exist between individuals. Procedures are based on empirical testing and mathematical modelling, with limited data from actual nitrogen load or diffusivity. Nitrogen has a positron-emitting isotope that can be made in gaseous form (13N2) which can then be monitored spatially and temporally. The feasibility to study nitrogen kinetics was assessed using hyperbaric pressure to increase the partial pressure or pre-breathing oxygen to increase tissue uptake capacity, both of which would expect to cause increased nitrogen uptake. Fifteen female Sprague Dawley rats were divided into air-breathing normobaric (CON), oxygen-breathing, including 1 h pre-breathing (OXY), and air-breathing hyperbaric, compressed to 700 kPa (HB). Breathing gases were supplemented with trace amounts of 13N2 for 30 min, with CON & OXY euthanized at 30 min. HB were decompressed and breathed air/isoflurane without 13N2 for 10-15 minutes before euthanized. Blood, liver, brain, femur (bone) and thigh muscle were analyzed in a gamma counter with the 13N2 counts normalized for organ mass. In liver (p=.026) and brain (p=.018) 13N2 was greater in OXY than both CON and HB. In the femur HB was higher than CON (p=.048). Results suggest that pre‑breathing O2 in OXY removed tissue nitrogen creating a larger gradient for 13N2 uptake. We interpret the limited increase in 13N2 in HB was due to off-gassing in high-perfused tissues during decompression, with the high femur value being due to uptake in HB but slower off-gassing during decompression. Our data suggest that 13N2 can be used to track gas kinetics in a live rodent during physiological conditions endured by humans. This study was supported by a grant from the Office of Naval Research. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.