Human temperature regulation during exercise in the heat : effects of the menstrual cycle and ambient thermal profile : 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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Date
2018
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Massey University
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Abstract
Behavioural thermoregulation is the most effective means with which we regulate our body
temperature at rest and during exercise. Yet, research into behavioural thermoregulation
during exercise is still at an emergent stage, as it has not included females, or investigated
different thermal profiles. In particular, limited studies are available to describe the
behavioural and physiological differences between dry and humid heat for both sexes.
Furthermore, it remains unknown whether ambient humidity or temperature alone contribute
to the initiation of the behavioural responses during exercise in the heat. Therefore, the first
part of this thesis investigated the effects of endogenous and exogenous female ovarian
hormones on behavioural and autonomic responses, in both dry and humid heat environments
matched according to the heat stress index, WBGT (Chapter Five and Six). The results from
Chapter Five clearly show that behavioural and autonomic responses were less affected by
menstrual phase, but were affected by the environmental conditions. In particular, trained
women reduced their power output in order to nullify the autonomic strain from a humid heat
environment. Chapter Six then extended this observation to (trained) women taking
combined hormonal contraception, compared to eumenorrheic women in Chapter Five. The
results from Chapter Six indicate that greater autonomic strain was observed in women with
hormonal contraception, compared to eumenorrheic women, in both dry and humid heat,
whilst the behavioural response was similar between those two groups. Furthermore, the
behavioural response was different between dry and humid heat, with power output being
lower in the humid heat environment compared to dry heat. The second part of this thesis
investigated the effects of ambient temperature per se on the interaction of thermoregulatory,
cardiovascular and perceptual responses to exercise (Chapter Seven), as well as assessing
different exercise modalities (variable-intensity versus fixed-intensity exercise) and their
effects on thermoregulation when the duration and average power output were matched
(Chapter Eight). The results from Chapter Seven indicate that thermoregulatory and
cardiovascular responses were not affected by ambient temperature but that perception was,
when vapour pressure was matched between two different thermal profiles. The results from
Chapter Eight indicate that self-pacing (behaviour) did not modulate thermoregulatory strain,
when both self-paced and fixed-intensity were matched at the same exercise intensity and
duration. In conclusion, this thesis extends the knowledge-base on behavioural thermoregulation in trained women and also provides evidence that behavioural and
autonomic thermoregulation is influenced more by vapour pressure than ambient temperature
of the environment in men. Furthermore, the findings of this thesis confirm that behavioural
thermoregulation is effective in modulating physiological strain only when there is a
reduction in metabolic heat production.
Description
Figures 3 (p.12), 5 (p.15), 6 (p.17), 7 (p.23), 8 (p.27), 9 (p.29), 10 (p.32) & 11 (p.39) have been removed for copyright reasons, but may be accessed via their source listed in the References. Figure 11 is accessible via https://theconversation.com/blood-sweat-and-tears-the-menstrual-cycle-and-the-olympics-8099
Keywords
Body temperature, Regulation, Athletes, Physiology, Exercise, Physiological aspects, Heat, Physiological effect, Oral contraceptives, Menstrual cycle