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Additional file 1.

Human performance in extreme environments is characteristic of both physiological and psychological stress. Much of the research investigating the mechanisms underpinning performance in these environments, however, often focuses only on physiological mechanisms to explain performance outcomes. Yet, bodies and psyches differ when they are confronted with stress. Therefore, the stress response of an individual in extreme environments may also be due to, or exacerbated by, psychological mechanisms. Hence, this thesis aimed to investigate both psychological and physiological stress on human performance in extreme environments. In Chapter 1 of this thesis, the current literature on human performance in extreme environments is reviewed where both physiological and psychological perspectives are critically evaluated. Following this, an outline of the methodologies used throughout the experimental studies are presented in Chapter 2. The first experimental study is presented in Chapter 3 which investigated the effectiveness of the current methods being used for inducing mental fatigue in performance research. In this study, previous methods used in research investigating the impact of mental fatigue on physical performance (i.e. long duration (> 30-min), non-individualised, single-task tests) were compared against a novel method (i.e. Time load Dual-back test) for inducing mental fatigue (i.e. short duration (16-min), individualised, dual-task test). The results demonstrated that the TloadDback test was a more effective method for inducing mental fatigue by inducing measurable reductions in mental resources, independent of sleepiness, whilst maintaining physiological arousal. To further explore the impact of mental fatigue on performance, the second experimental study, outlined in Chapter 4, investigated the impact of mental fatigue, using the novel approach from Study 1 (Chapter 3), on physical and cognitive performance in normoxia and hypoxia. This experiment, and the following experiments (i.e. Chapter 4, 5, 6), adopted a combined stressors approach whereby stressors were investigated both independently and combined on performance outcomes. Therefore, Study 2 included a combination of three levels of hypoxia (i.e. 0.209 FiO2 (sea level), 0.13 FiO2 (~3500m), 0.10 FiO2 (~5800m)), and two levels of mental fatigue (i.e. mental fatigue or no mental fatigue). Physical performance was tested using a self-paced time trial, followed by an assessment of neuromuscular fatigue. Cognitive performance was measured using the Tower of Hanoi cognitive test. The results from this study indicated that hypoxia impaired physical performance but not cognitive performance. Conversely, mental fatigue had no discernable impact on physical or cognitive performance in normoxia or hypoxia. Given that mental fatigue had no impact on performance measures, despite incorporating a more effective and explicit method of inducing mental fatigue, further questions the methods and results of previous research investigating this phenomenon. At a neural processing level, mental fatigue may not be a strong enough stressor to override the impact of concurrent physical and environmental stressors. Following the results of Study 2 (Chapter 4), it was evident that methods to aid in mitigating the performance decrements observed in hypoxia warranted further investigation. Given the exacerbated subjective responses of discomfort and exertion in hypoxia (Study 2; Chapter 4), psychological strategies which lower subjective states of discomfort and exertion may help to mitigate the performance decrements. Therefore, Study 3 (Chapter 5) aimed to investigate whether self-selected motivational music would improve physical performance in normoxia and hypoxia. Using an identical physical performance testing protocol as in Study 2 (Chapter 4), the impact of self-selected motivational music on physical performance was investigated in two levels of hypoxia (i.e. 0.209, 0.13 FiO2) and two levels of music (i.e. with music and without music). Similar to Study 2, a combined stressors approach was incorporated to investigate the differential impact of music and hypoxia on performance. The results found that music improved physical performance in both normoxia and hypoxia through enhancing neural drive and diminishing detrimental mental processes. Subjective ratings of discomfort, exertion and mood were largely unchanged due to the mechanisms of dissociation, a coping mechanism, which facilitates an individual in blocking out the increased pain and discomfort in hypoxia and normoxia. In corroboration with previous research investigating human performance in extreme environments, Studies 2 and 3 observed interindividual variability in physical and cognitive performance measures. Study 4 (Chapter 6) aimed to investigate whether some of the variability observed may be due to psychological mechanisms. In particular, Study 4 investigated the impact of trait and state anxiety on physical performance in hypoxia (0.10FiO2), heat (35⁰C) and a temperate-normoxic (0.209 FiO2, 21⁰C) control. Taking a combined stressors approach, each environmental condition (i.e. heat, hypoxia, and control) consisted of two main experimental trials, one of which consisted of the environmental stress only (i.e. hypoxia, heat or control); and the other consisted of the environmental stress combined with a unique protocol which involved inducing an additional psychological stress (i.e. state anxiety). The physical performance assessment consisted of a voluntary exhaustion protocol. Results from Study 4 indicated that in all environmental conditions state anxiety reduced times to exhaustion. In the heat alone, trait anxiety accounted for 35% of the variation in physical performance. In hypoxia alone, trait anxiety was not associated with impaired performance, however trait anxiety did account for 50% of the impaired performance observed as a result of the state anxiety protocol in hypoxia. The findings from Study 4 have important implications for individuals with trait anxiety, and also individuals performing in environments characteristic of state anxiety. Taking all studies together, the results from this thesis highlight the need for a more individualised approach to human performance testing in extreme environments, and further consideration into the impact of psychological constructs on performance outcomes in these environments.