Author(s): KELLY, M., SNOW, R.J., BROWN, H.A., CONDO, D., GUY, J.H., CHALMERS, S., BROATCH, J.R., ROBERTS, S.R., PÉRIARD, J.D., BOWE, S.J., CARR, A.J., Institution: DEAKIN UNIVERSITY, Country: AUSTRALIA, Abstract-ID: 2028

INTRODUCTION:
High-intensity, intermittent exercise heat acclimation (HA) is rarely utilised compared to sub-maximal continuous exercise in the heat. However, high-intensity, intermittent exercise HA is likely more specific and perhaps beneficial for team sport athletes when preparing for competition in hot environments [1]. In females, the precise time course for heat adaptation when using a high-intensity, intermittent HA protocol is unknown, including its effect on sleep outcomes as an important marker of recovery. The aim of this study was to investigate, in female participants, the effects of six sessions of high-intensity, intermittent exercise in the heat conducted over seven days on the time course of the thermophysiological (rectal temperature (Tre), skin temperature (Tsk), heart rate (HR), sweat rate), and perceptual adaptations (thermal discomfort, rating of perceived exertion), and the effect on objective sleep measures (sleep duration, wake after sleep onset).
METHODS:
Eleven non-heat acclimatised females (mean ± SEM: age 27±2 yr; body mass 69.63±2.64 kg; body surface area 1.78±0.04 m2) of recreationally active to highly trained status [2], completed six, 60-min cycling sessions over seven days in the follicular phase of the menstrual cycle. The intermittent exercise protocol alternated work periods of 30 s at ~90% V̇O2peak and 30 s active recovery at ~30% V̇O2peak (mean work rate ~60% V̇O2peak) in the heat (~40°C, ~40% relative humidity). Tre, Tsk, HR, Tre at onset of sweating, rating of perceived exertion, thermal discomfort, and objective sleep were measured. Linear mixed models were used to compare outcomes measures at the third (HA3), fourth (HA4) and sixth (HA6) compared to the first HA session (HA1). Bonferroni post-hoc procedures were used to locate differences where appropriate. Results were considered statistically significant when p<0.05.
RESULTS:
In the final HA session (HA6), resting Tre was lower (-0.18°C; p=0.001) compared to the first session (HA1: 37.2 ± 0.1°C). Mean Tre was lower on HA6 than HA1 (-0.21°C; p<0.001), HA3 (-0.16°C; p<0.001), and HA4 (-0.12°C; p=0.001). Mean Tsk was lower on HA6 than HA1 (-0.20°C; p<0.001). Tre at the onset of sweating was lower on HA6 than HA3 (-0.17°C; p=0.035). Mean HR was lowest in HA6 than HA1 (-9 beats.min-1; p<0.001), HA3 (-3 beats.min-1; p<0.001), and HA4 (-5 beats.min-1; p<0.001). TD was lower on HA6 than HA1 (-242 mm; p<0.001), and RPE lower on HA6 than HA1 (-0.9; p=0.003). There were no significant effects on measures of sleep (p≥0.208) during the HA protocol.
CONCLUSION:
In females, high-intensity, intermittent exercise presents an effective HA protocol that induces rapid thermophysiological and perceptual adaptations, without inducing disturbances in sleep.
1. Sunderland, et al. (2008) BJSM, 2. McKay et al. (2022) IJSPP.