Factors influencing the exertional heat stress response in athletic females : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy, School of Sport, Exercise and Nutrition, Massey University, Palmerston North, New Zealand

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Climate change continues to expose an increasing proportion of the global population to more frequent extreme heat events. Concurrently, our society has seen an increase in the number of women that participate in physically demanding leisure time (exercise and sport) and occupational activity. Yet, limited research has been conducted to understand the female physiological responses to exercise-heat stress, especially when considering their various ovarian hormone profiles. This thesis expands our understanding of the female physiological responses to heat stress from different perspectives. Firstly, previous evidence has demonstrated that ambient heat stress amplifies the increase in the cytokine interleukin-6 following exercise, an up-regulator of hepcidin - the hormone that downregulates iron metabolism. In Chapter Five iron sufficient females’ serum iron parameters and hepcidin levels following a self-paced cycling work trial were compared in temperate and hot conditions, and different menstrual phases. IL-6 and hepcidin both increased post-exercise (198% and 38%, respectively), interestingly, neither were affected by ambient temperature or menstrual phase (all p>0.15). Chapter Six determined the measurement error of a 30-min self-paced cycling protocol in moderate, warm-dry and warm-humid environments using thirty-three athletic women distinguished by their ovulatory status and ovarian hormone concentrations. With an ICC=0.90, p<0.01, and a mean CV of 4.7%, SEM of 3.8 kJ (2.1 W) and reliable bias of -2.1 kJ (-1.2 W), it was confirmed that this protocol has high test-retest reproducibility that is not influenced by ambient environment or a female’s hormonal/ovulatory status. Finally, in order to better predict females’ risk for exertional heat stress, Chapter Seven attempted to clarify the role of the ovarian hormones when explaining the variance of the core temperature response using the 30-min self-paced cycling protocol from Chapter Six in thirty-six trained women. It was found that estrogen contributes minimally, whilst baseline core temperature and power output contribute the most to peak core temperature during exercise. Taken together, this thesis deepens our understanding of females’ physiological responses and testing norms for current sport science practices. Specifically, it details iron metabolism responses when exposed to exercise-heat stress, elucidates the role of ovarian hormones regarding exertional heat strain and adds test-retest norms specific to athletic women to the available literature.
Women athletes, Health and hygiene, Physiology, Exercise, Physiological aspects, Heat, Physiological effect