The effects of acute operational stress and passive heat stress on physiological and subjective stress responses in military personnel

Military personnel often encounter situations that can trigger acute stress, which may affect operational performance. Therefore, it is important to examine stress responses in controlled environments to obtain more insights in performance-influencing effects of acute stress. This study investigated the impact of passive heat exposure combined with virtual combat scenarios on cardiovascular and psychophysiological parameters in a controlled setting. Sixty-eight healthy servicemembers were randomized into a low-stress or high-stress condition. Both groups engaged in two virtual combat scenarios. The low-stress group, however, underwent testing under thermoneutral conditions (M_Temp = 22.4°C, M_{RelativeHumidity} = 41.4%) whereas the high-stress group was exposed to passive heat exposure (M_Temp = 35.9°C, M_{RelativeHumidity} = 66.4%) using a portable environmental facility. While virtual combat scenarios alone led to a decrease in heart rate variability (HRV) without affecting heart rate (HR), the addition of passive heat exposure elicited a more pronounced physiological stress response, characterized by significantly higher HR and lower HRV in the high-stress condition. However, no significant changes were observed in respiratory rate, salivary cortisol, or alpha-amylase levels across the conditions, suggesting that there was no activation of either the hypothalamic-pituitary-adrenal or the sympathetic-adrenal-medullary axis. Furthermore, subjective stress and anxiety scores did not differ between conditions, underscoring the physiological nature of the observed changes. Resultantly, the physiological response was likely a thermal reaction rather than an acute stress response. These findings highlight the importance of incorporating environmental stressors into military training protocols to enhance realism and prepare personnel for operational stressors. However, the mild response observed suggests that higher ambient temperatures and longer exposure times may be necessary to evoke a more robust stress response for effective stress inoculation training.