The Development and Evaluation of a Novel Two-day 'Condensed' Heat Acclimation Intervention

Abstract

Short duration heat acclimation (HA) interventions (≤5 days) are more desirable to time- poor individuals but may result in incomplete heat adaptation compared to longer HA (8- 10 days) possibly due to the lower ‘thermal dose’ accumulated in the shorter time scale. Accordingly, the studies presented in this thesis investigated the physiological responses to a novel 2-day Condensed HA program compared a Traditional HA program (8 consecutive daily exposures). This thesis comprises four empirical investigations, each with distinct aims, based upon the data collected from largest HA study of its kind.
The first study within this thesis used a between-groups design, with participants completing either 8 heat exposures delivered over 2 days (4 sessions.day-1; Condensed; n = 17) or once-daily (Traditional; n = 15) to examine the induction of the heat adaptations. The results of this investigation demonstrated that the Condensed HA induced an array of hallmark adaptations to heat during a standardised exercise-heat exposure (40 °C, 50% relative humidity) which were not different in magnitude from the Traditional HA. However, improvements in evaporative capacity increased to greater extent in the Traditional HA compared to Condensed HA. The second study examined the effects of the two HA programs on temperate exercise performance and hypoxic tolerance. The only improvement in parameters of temperate exercise performance was an increased power at the lactate threshold in Traditional HA, but this did not translate into an ergogenic effect. Additionally, small improvements in oxygen pulse and peripheral oxygen saturation were observed during exercise in hypoxia after HA but these were not different between groups. The next experimental study was an exploration of the inflammatory, gastrointestinal, renal and perceptual stress responses to the HA interventions. Generally, the Condensed HA was well-tolerated, except a reduction in estimated glomerular filtration rate which did not translate to increased markers of acute kidney injury. Markers of inflammation were elevated in Condensed HA at rest after HA, however, these levels were below clinical levels. The final study addressed the decay of the heat adaptations induced by the two HA programs. We demonstrated that adaptations induced in 2-days are well maintained after a period of ~3 weeks without heat exposures, with the exception of a reduction sweat rate and contraction of plasma volume. However, sweat rates remained elevated above baseline levels in both groups.
In summary, the findings reported in this thesis demonstrate for the first time that heat adaptations can be induced in 2 days where thermal exposure is matched to an 8-day HA program. Moreover, the large thermal dose accumulated in Condensed HA is well- tolerated and does not induce excessive physiological stress. Despite the short time frame of induction, the heat adaptations induced are well-maintained after 3 weeks of no heat exposure. These extensive findings provide an option for HA for time-poor individuals with minimal disruption to preparation schedules, particularly in the case of rapid deployment.

Date of Award	14 Feb 2024
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Jo Corbett (Supervisor), Joe Costello (Supervisor) & Mike Tipton (Supervisor)