The first aim of this study was to compare the physiology of each environment, results showed the humid environment to be marginally more stressful, with higher core temperature, higher heart rate and a greater oxygen requirement. The understanding of these differences allows for specific preparation ahead of deployment, including heat acclimation, equipment design and mission planning. The second aim was to assess the strength of physiological variables at predicting performance in hot environments. To this end several factors were found in each environment that predicted performance, including factors unique to each environment.
Performance
Despite differences in environmental conditions, gear loadouts and physiological responses, there was no difference seen in performance between arid and humid environments (Fig. 3), with minimal differences in the reasons for test termination. Together these similarities indicate the overall thermal strain in both environments was similar, likely due to the comparable WBGT, originally developed to quantify heat stress (Yaglou and Minard 1957). However, the WBGT does not account for the difference in clothing and protective equipment worn by soldiers. It was expected that the larger and heavier pack carried in the humid environment would exacerbate endogenous heat production while also impairing evaporative heat loss and thereby cause earlier test termination (Dorman and Havenith 2009). However, it is possible that the combination of body armour and backpack in the arid environment may have comparatively restricted heat loss from the chest (Johnson, Knapik et al. 1995), helping to nullify the effects of a heavier pack in the humid condition.
Body Temperatures
Higher rectal temperatures were observed in the humid environment than the arid environment (Table 1, Fig. 4). While this is likely partially accounted for by the additional metabolic heat production caused by the heavier carried load (Dorman and Havenith 2009), there was also a reduced evaporated sweat rate, with the same absolute sweat rate, suggesting reduced evaporative heat loss. Despite no statistical difference in the rate of rise in rectal temperature, extrapolation of the data revealed that rectal temperature would reach 40°C 25 min faster in the humid environment (Fig. 4). While military personal can likely still perform beyond this threshold safely (Ely, Ely et al. 2009, Lee, Nio et al. 2010, Veltmeijer, Eijsvogels et al. 2015), it represents a limit at which heat stroke is known to occur, and therefore safety guidelines suggest that exercise should be restrained beyond this threshold (Goforth and Kazman 2015, Smith, Jones et al. 2016). Indeed, reducing core temperature below 40°C rapidly after exercise drastically reduces the mortality risk (Casa, Armstrong et al. 2012). However, in real-world military contexts, heat stress in often not alleviated and rectal temperature continues to rise even after the cessation of exercise, placing soldiers in danger, even if exercise is stopped (Giesbrecht, Jamieson et al. 2007, Smith, Withnall et al. 2017). In the humid environment a 40°C rectal temperature would have been seen only 20 min after the average termination time, highlighting the imminent danger of exercise in humid environments. However, it should be noted that the 95 min mark where rectal temperature is calculated to reach 40°C in the humid condition falls outside of the 95% confidence interval (Table 1), suggesting internal cues can help reduce risk by terminating exercise in both environmental conditions. Therefore, when operating in these environments, particularly humid environments, continuous physiological monitoring of soldiers may be valuable to ensure activities are conducted safely (Tharion, Buller et al. 2013, Buller, Welles et al. 2017, Parsons, Stacey et al. 2019), as is understanding methods for rapidly cooling individuals (Carter, Rayson et al. 2007, Casa, Armstrong et al. 2012, Epstein, Druyan et al. 2012).
The strength of the relationships between rectal temperature and performance is strengthened by the ethical termination of trials when core temperature exceeded 39.3°C, although, as mentioned above, internal cues such as central fatigue likely also lead to test termination (Nybo and Nielsen 2001, Tucker, Rauch et al. 2004, Hargreaves 2008). The stronger relationship between rectal temperature slope and performance in the humid environment may also be explained by this, where 56% of heat-stress tests were terminated due to high core temperature, compared to only 44% in the arid environment. Nonetheless, this ethical limit was put in place as it was deemed unsafe for core temperature to rise any further, and is the point where physical activity should be restrained in the field, if possible (Taylor, Patterson et al. 1997, Goforth and Kazman 2015). High rates of rise in core temperature have previously been identified to increase hyperthermia risk and heat-illness symptoms (Armstrong, Johnson et al. 2010, Maughan, Otani et al. 2012), highlighting the desire for a reduced rate of rise in core temperature (Hunt, Billing et al. 2016). Therefore, to prioritise safety, core temperature should be monitored. Although less practical, the ability to monitor core temperature during exercise, either using a heat tolerance test prior to departure or real-time monitoring of soldiers in the field (Buller, Welles et al. 2017, Epstein, Shapiro et al. 2017), provides a much stronger predictor of performance (Table 2).
The arid environment induced a higher skin temperature (Table 1), likely through heat gain from the environment that occurs when temperatures exceed 35°C (Nadel 1979). Elevated skin temperature impairs heat loss as it minimises the core-to-skin temperature gradient (Chou, Allen et al. 2018). However, as the humidity gradient is ~ 50% lower in the arid environment, evaporative heat loss is facilitated (Akerman, Tipton et al. 2016), explaining the lower rectal temperature despite a higher skin temperature. The higher skin temperature in the arid environment likely explains the stronger relationship with performance, which was of moderate strength, compared to only a weak relationship in the humid environment. Furthermore, skin temperature may directly influence the perceptual relationships with performance, which were among the strongest predictors of performance in both environments (Table 2), consistent with previous findings (Schlader, Perry et al. 2013, Flouris and Schlader 2015). Whether the higher skin temperature in the arid environment partially explains the stronger relationships between perceptual changes and performance in the arid environment, however, is uncertain as a lack of perceptual differences existed between environments (Table 1). Perceptions are produced by the brain integrating numerous physiological signals to generate behavioural responses to help cope with environmental stress (Morante and Brotherhood 2008, Schlader, Simmons et al. 2011, Fleming and James 2014, Periard, Racinais et al. 2014). Therefore, as a response to exercise becoming uncompensable, thermoregulatory behaviour leads to the termination of the test (Pimental, Cosimini et al. 1987, Cheung and McLellan 1998, Gonzalez-Alonso, Teller et al. 1999). Thereby having a lower skin temperature could delay the rate at which perceptual feelings worsen, allowing prolonged performance before voluntary termination, although there were only marginally more voluntary terminations in the arid environment. The absence of relationship between rating of perceived exertion and performance may highlight military mental toughness, hypothesised to place individuals in danger as they disregard internal cues to cease exercise (Howe and Boden 2007, Epstein, Druyan et al. 2012, Buller, Welles et al. 2017, Parsons, Stacey et al. 2019). If valid, overcoming these internal cues exacerbates the danger of these environments as continuing to exercise further elevates core temperature which can ultimately be fatal (Parsons, Stacey et al. 2019). Understanding that in these environments the accumulated heat gain from both endogenous and exogenous sources, and not simply exercise intensity alone, is the primarily cause of fatigue and casualties, may help develop monitoring strategies (Macpherson 1962). The data in the current study found that directly addressing heat in perceptual monitoring, by enquiring of how hot or sleepy individuals are feeling, provides an indication of how much longer an individual can safely exercise for.
Cardiovascular
Cardiovascular differences were apparent between the two environments, with a higher heart rate during the heat-stress test in the humid environment (Table 1). Furthermore, cardiovascular variables were better predictors of performance in the humid environment, whereas aerobic fitness, which is often cardiovascular dependent, was a better predictor in the arid environment (Table 2). While aerobic fitness and heart rate were expected to have a similar relationship with performance, it is possible that in the humid environment the heavier carried load, and subsequent increase in cardiovascular demand, accounts for part of this discrepancy. Furthermore, fitter individuals are known to be able to tolerate a higher core temperature (Cheung and McLellan 1998), therefore the withdrawal of participants due to having a high core temperature, which occurred more frequently in the humid environment, limits aerobic fitness influencing the time to exhaustion.
During exercise, elevations in cardiac output, facilitated by an increase in heart rate, are required to ensure both cutaneous and skeletal muscle circulations receive adequate blood supply (Gonzalez-Alonso and Calbet 2003, Cramer and Jay 2016). A larger underlying blood volume facilitates higher stroke volume and a more widespread distribution of blood, allowing heat loss while maintaining performance (Gonzalez-Alonso, Calbet et al. 1998, Taylor 2000). A greater blood volume may be more important in humid environments as sweat evaporation is restricted by high humidity (Maughan, Otani et al. 2012), thereby causing insensible fluid loss, where dehydration occurs without beneficial heat loss (Eichna 1943, King, Clanton et al. 2016, Taylor 2017). As central blood volume declines a greater stress is placed on the cardiovascular system (Gonzalez-Alonso, Calbet et al. 1998, Charkoudian 2016), limiting peripheral blood flow. As the perfusion of cutaneous circulations is reduced, heat transfer becomes limited, thereby causing increases in core temperature (Nadel, Fortney et al. 1980, Gonzalez-Alonso, Calbet et al. 1998, Kenefick, Cheuvront et al. 2010, Casa, Armstrong et al. 2012). Alternatively, a greater reliance may be placed on convective heat loss mechanisms, thereby requiring an increased cardiac output to elevate cutaneous blood flow (Kenney, Stanhewicz et al. 2014, Chou, Allen et al. 2018, Tebeck, Buckley et al. 2019), shown by an elevated heart rate in the humid environment (Table 1). The importance of limiting the cardiovascular demand is further illustrated by the strong ability of the change in heart rate to predict performance (Fig. 6C). When the cardiovascular system can no longer increase cardiac output to support perfusion of both skeletal muscle and cutaneous circulations blood flow is reduced, first to cutaneous, and then to skeletal muscle circulations (Gonzalez-Alonso and Calbet 2003, Gonzalez-Alonso, Crandall et al. 2008, Kenney, Stanhewicz et al. 2014). Without the muscular blood flow to sustain oxygen requirements for exercising muscle intensity is reduced (Tucker, Rauch et al. 2004), which in this experiment meant test termination. Heart rate monitoring is one of the simplest real-time monitoring methods available (Eggenberger, MacRae et al. 2018), and by assessing the rate of rise in heart rate it allows evasive steps to be taken to prevent exhaustive limits being reached by the individual.
Sweat Rate
No differences in sweat rate were seen between the environments, although evaporated sweat was significantly reduced in the humid environment (Table 1), likely due to the vapour pressure gradient being reduced by the humidity, preventing sweat evaporation (Maughan, Otani et al. 2012). As sweat is unable to evaporate, core temperature rises (Sawka, Wenger et al. 1993, McLellan and Aoyagi 1996), underlying the elevated rectal temperature in the humid condition (Fig. 4), whereas the evaporation of sweat in the arid environment would have helped maintain a lower rectal temperature (Fig. 4) (McLellan, Meunier et al. 1992). Sweat rate changes were closely linked to performance in both environments (Table 2). In the humid environment sweat rate strongly predicted performance (Fig. 6A), while evaporated sweat rate had a weak negative relationship. This suggests sweating facilitates performance, but only if the sweat evaporates. If sweat does not evaporate then heat is not lost from the body, and water loss merely adds to dehydration (Cheung and McLellan 1998, Taylor 2017). Conversely, despite conditions favouring the evaporation of sweat, the arid environment had only a moderately strong relationship between sweat rate and performance, (Table 2). As sweat could more readily evaporate, it is likely that this was not a limiting factor, and therefore other variables were more directly linked to performance.
Metabolic
The metabolic strain during the heat stress test was greater in the humid environment, illustrated by larger oxygen uptake and carbon dioxide production. The greater pack weight in the humid environment likely accounts for some of this difference as more muscular work is required to carry the pack (Knapik 1997). Indeed, the relatively greater V̇O2 in the humid environments occurred close to the expected relative value based on load carrying energy expenditure predictions (Pandolf, Givoni et al. 1977). Furthermore, lighter individuals are known to have a greater relative metabolic demand when carrying heavy absolute loads (Bilzon, Allsopp et al. 2001). Therefore, the increased oxygen requirement from the additional relative workload likely creates a strong relationship between body mass and performance, as this would also add to metabolic heat production (Table 2).
No relationship was found between HSP70 and performance (Table 2), although most participants showed minimal plasma concentrations that were often below the detection limit of the ELISA assay. Low serum concentrations of HSP70 are not uncommon, especially at rest (Njemini et al. 2011, Walsh et al. 2001), with some studies reporting wide variations in concentrations between individuals (Lee et al. 2015). It is acknowledged that the practicality of measuring HSP70 in a real-world military context is limited due to the cost, invasiveness and laboratory expertise required to obtain results. Therefore, based on these constraints, the large variability, and our non-significant findings, HSP70 does not appear to be a worthwhile or accessible measure for predicting soldier performance in the heat.
Cognitive
Minimal changes in cognitive performance existed both within and between environments. Many of the tasks used in the cognitive assessments were relatively simple, which have been shown to be largely unaffected by heat (Hancock and Vasmatzidis 2003, Mazloumi, Golbabaei et al. 2014). However, research has shown load carriage (Caldwell, Engelen et al. 2011, Eddy, Hasselquist et al. 2015) and physical fatigue (Vrijkotte, Roelands et al. 2016) to impair simple cognitive processes. Despite the self-reported mental demand of tasks increasing across the trial (Fig. 5), no differences in cognitive performance existed. It is possible participants felt more strained when doing the tasks, but could still allocate sufficient cognitive resources to the task to complete them accurately (Lambourne and Tomporowski 2010). In real-world military contexts, soldiers may experience greater thermal stress, physical fatigue and more complex tasks, which could impair cognition.