Active and passive heat acclimation for exercise performance in female and male athletes

Kirby, Nathalie Victoria ORCID: 0000-0002-2899-2115 (2021). Active and passive heat acclimation for exercise performance in female and male athletes. University of Birmingham. Ph.D.

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Abstract

Heat acclimation is an effective strategy to drive favourable physiological adaptations that attenuate exercise performance impairments in hot environments. However, there is limited data investigating heat acclimation in female participants. This thesis investigated novel aspects of active (heat stress during exercise) and passive (heat stress independent of exercise) heat acclimation; namely, short-term heat acclimation, post-exercise sauna bathing, heat acclimation decay and heat acclimation as an ergogenic aid for performance in cool conditions. This thesis focused particularly on the female athlete’s response to these active and passive heat acclimation protocols.

Short-term heat acclimation (<7 days) is more cost- and time-efficient than traditional heat acclimation (~10 days) and has been repeatedly demonstrated to be effective in males. The first two experimental chapters of this thesis (\(\textbf{Chapters 4 & 5}\)) showed that short-term heat acclimation (4-5 days) did not improve exercise performance in hot or cool conditions in a female cohort. Conversely, 9-10 days heat acclimation did improve performance in both conditions in this female cohort, despite participants’ reports of fatigue during time trials. Notably, performance improvements in cool conditions were not as meaningful for participants with better baseline performances. Overall, short-term heat acclimation appeared to be an ineffective strategy for females, whilst 9-10 days heat acclimation was fatiguing.

Though popularised as a more practical heat acclimation strategy for athletes, post-exercise sauna bathing had not previously been shown to improve exercise-heat tolerance. \(\textbf{Chapters 6-8}\) demonstrated that three weeks of intermittent post-exercise sauna bathing was an effective heat acclimation strategy, as it induced thermal, cardiovascular and sudomotor adaptations during exercise-heat stress without disrupting athletes’ normal training. Body temperature and heart rate adaptations were similar, if not superior, in females as compared to males. Additionally, these investigations revealed a novel sex difference in the adaptation of physiological heat loss mechanisms, whereby females showed sudomotor adaptations whilst males showed improved forearm blood flow. Adaptations observed during exercise-heat stress were partially retained for two weeks post-intervention without exogenous heat stress. Specifically, body temperature and heart rate adaptations decayed by 25-33%, similar to rates of decay observed following consecutive-day, active heat acclimation. Extending the intervention from three to seven weeks induced only marginal additional physiological improvements during exercise-heat stress. Lastly, females and males similarly improved markers of exercise performance in temperate conditions following three weeks of intermittent post-exercise sauna bathing, more than endurance training alone. These performance markers included: maximal aerobic capacity, running speed at 4 mmol·L\(^{-1}\) blood lactate concentration and time-to-exhaustion. Thus, post-exercise sauna bathing is an effective heat acclimation strategy, as well as an effective ergogenic aid, for both male and female athletes.

Overall, the findings detailed in this thesis provide useful information for female and male athletes undertaking heat acclimation to improve exercise performance in both hot and cooler conditions.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):