On the edge of thermoregulation
: a matter of physiology and physics

  • Ella Walker

Student thesis: Doctoral Thesis


This thesis describes experiments designed to investigate the point of uncompensable heat stress in healthy exercising males by manipulating variables in the heat balance equation.

The addition of minimal clothing did not have a clear influence on the mean skin temperature (T̅sk) and thermoeffector responses (local sweat rate and skin blood flow) during incremental exercise at 40 °C (it did at 30 °C). A comparison between the two different ambient temperatures however, showed a significant effect on T̅sk, which was on average 1.55 (0.29) °C higher in 40 °C compared to 30 °C. Subsequently, a protocol was developed where ambient temperature was incrementally increased. This was shown to be a reliable and valid method to evoke uncompensable heat stress, and allowed the comparison of variables (humidity, work rate) and groups (fitness). Further experiments in the thesis show that uncompensable heat stress occurs at a similar deep body temperature (Tc) for males with a range of aerobic fitness in high and low humidity environments. It was also shown that high aerobic fitness (compared to low aerobic fitness): 1) does not offer any benefit in terms of delaying the transition to uncompensable heat stress when exercising at a matched absolute work rate (60 Watts) in a low and high humidity environment, or at a matched relative work (40 % of VO2max) in a low humidity environment, 2) may cause uncompensable heat stress to occur at a lower ambient temperature when working at matched relative work rate in a humid environment.

Importantly, in these experiments, local sweat rate continued to rise beyond the point of uncompensable heat stress – or upper limit of the “thermoregulatory zone”, regardless of the work rate, humidity or aerobic fitness status of the individual, indicating a thermoeffector reserve remained. It is therefore concluded, that the transition into uncompensable heat stress is not synonymous with maximal thermoeffector output and the sweating and skin blood flow responses of an individual are determined by the thermal profile. An elevated and increasing Tc may be necessary to facilitate increased levels of thermoeffector output. This is an important and novel contribution to the understanding of the thermoregulatory response to heat stress.
Date of AwardSept 2015
Original languageEnglish
SupervisorJo Corbett (Supervisor), James R. House (Supervisor) & Mike Tipton (Supervisor)

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