Author(s): COLE, E., JACKSON, K., DONNAN, K.J., SIMPSON, A.J., GARRETT, A.T., Institution: UNIVERSITY OF HULL, Country: UNITED KINGDOM, Abstract-ID: 1627

INTRODUCTION:
Hot water immersion (HWI) is being explored as a practicable and accessible method of heat acclimation for the general population. However, there are questions as to the applicability of these protocols in relation to people’s tolerance, affective responses and safety. This research aimed to identify practical insights into more accessible heat acclimation methods for an ageing population (45-75 years) by comparing the efficacy a potentially more tolerable Arms Out method (thereby reducing the submerged surface area), against an “Arms In” submersion protocol.
METHODS:
Employing a randomised experimental cross-over design, nine healthy highly trained adults (7 males, 2 females; aged 52±6 years) participated. Participants first engaged in a 6-min walk test to establish fitness levels, followed by two 45-min controlled HWI trials (‘Arms In’ and ‘Arms Out’) in a hot tub (40°C) located in an environmental chamber (21°C, 45% relative humidity). The ‘Arms In’ condition required participants to be submerged up to the base of the neck, whereas ‘Arms Out’ required participants to be submerged up to mid sternum, with their arms exposed on the side of the hot tub. Physiological measurements included heart rate, mean skin temperature, and mean rectal temperature. Perceptual measures included thermal sensation, thermal comfort, affective valence, and the 32-item Brunel Mood Scale. Each measurement was recorded at baseline then every 5-mins, except the Brunel Mood Scale which was taken at baseline and at 15-min intervals.
RESULTS:
All participants completed both trials. The ‘Arms In’ immersion protocol induced a significant rise in core temperature from 20-mins (p<0.05), with all participants experiencing a ≥1°C increase by 35-mins. ‘Arms Out’ also showed a significant rise in core temperature across the protocol, however it was significantly lower than ‘Arms In’. Mean temperature change from baseline to end point for ‘Arms In’ was 1.64±0.55°C (p>0.05) vs 0.54±0.33°C (p>0.05) for ‘Arms Out’. Participants, however, reported significantly higher discomfort with ‘Arms In’. This was shown in increased mean values for thermal comfort (p<0.05), affective valence (p<0.05) and elevated values for tension on the Brunel Mood Scale (p<0.05).
CONCLUSION:
These findings highlight the need to consider perceptual tolerance alongside efficacy of HWI protocols to develop practical heat acclimation methods for the general population that will both be utilised (i.e., are less unpleasant) and induce adequate physiological adaptations. The ‘Arms In’ method utilised shows the potential effectiveness of HWI to induce sufficient physiological strain for heat acclimation method, however the perceptual tolerance of such protocols is important to consider. A protocol that encompasses the subjective discomfort felt against the physiological stress needed to achieve adaptations should be the avenue for further study.