Short Duration Heat Acclimation in Australian Football Players

Abstract

This study examined if five sessions of short duration (27 min), high intensity, interval training (HIIT) in the heat over a nine day period would induce heat acclimation in Australian football (AF) players. Fourteen professional AF players were matched for VO2peak (mL·kg(-1)·min(-1)) and randomly allocated into either a heat acclimation (Acc) (n = 7) or Control (Con) group (n = 7). The Acc completed five cycle ergometer HIIT sessions within a nine day period on a cycle ergometer in the heat (38.7 ± 0.5 °C; 34.4 ± 1.3 % RH), whereas Con trained in thermo-neutral conditions (22.3 ± 0.2 °C; 35.8 ± 0. % RH). Four days prior and two days post HIIT participants undertook a 30 min constant load cycling test at 60% V̇O2peak in the heat (37.9 ± 0.1 °C; 28.5 ± 0.7 % RH) during which VO2, blood lactate concentration ([Lac(-)]), heart rate (HR), rating of perceived exertion (RPE), thermal comfort, core and skin temperatures were measured. Heat acclimation resulted in reduced RPE, thermal comfort and [Lac(-)] (all p < 0.05) during the submaximal exercise test in the heat. Heart rate was lower (p = 0.007) after HIIT, in both groups. Heat acclimation did not influence any other measured variables. In conclusion, five short duration HIIT sessions in hot dry conditions induced limited heat acclimation responses in AF players during the in-season competition phase. In practice, the heat acclimation protocol can be implemented in a professional team environment; however the physiological adaptations resulting from such a protocol were limited. Key pointsSome minor heat acclimation adaptations can be induced in professional AF players with five 27 min non-consecutive, short duration HIIT sessions in the heat.The heat acclimation protocol employed in this study was able to be implemented in a professional team sport environment during an actual competitive season.Elevating and maintaining a high core temperature sufficient for heat acclimation likely requires a longer heat training session or some pre-heating prior to exercise.

Keywords: Core temperature; adaptation; high intensity interval training; metabolism; thermoregulation.

Figures

Figure 1.

Experimental protocol and testing timeline related to a short duration high intensity interval training program (five 27 min sessions in 9 days) in the heat (temperature = 38.7 ± 0.5 °C, relative humidity = 34.4 ± 1.3 %) or at room temperature (22.3 ± 0.2 °C; 35.8 ± 0.6 %). ↑ denotes competition on the weekend.

Figure 2.

Heart rate (A), rating of perceived exertion (B) and rating of thermal comfort (C) during session 1 and session 5 of high intensity interval training (HIIT 1 and HIIT 5) in the Acc group who performed exercise in the heat (temperature = 38.7 ± 0.5 °C, relative humidity = 34.4 ± 1.3 %) and Con who exercised at room temperature (22.3 ± 0.2 °C; 35.8 ± 0.6 %). Values are mean ± SD, n = 7 per group. Difference between HIIT 1 and HIIT 5 at the corresponding time point within the Acc group, * p < 0.05. Mean rating of perceived exertion (RPE) during HIIT 1 in Acc is different from HIIT 5 in Acc, # p < 0.05. Acc: acclimation, Con:control.

Figure 3.

Heart rate (A,B), rating of perceived exertion (C,D) and thermal comfort (E,F) during 30 min of continuous submaximal cycling exercise (SMT) at approximately 60% VO2peak in hot conditions before (pre) and after (post) 5 sessions of high intensity interval training (HIIT) in the heat (Acc; temperature = 38.7 ± 0.5 °C, relative humidity = 34.4 ± 1.3 % or Con: 22.3 ± 0.2 °C; 35.8 ± 0.6 %). Values are mean ± SD, n = 7 per group except for heart rate where n = 6 for Con. Main effect for training, ^p < 0.05. Difference between pre and post training at all corresponding time points within the Acc group, * p < 0.05. Mean rating of thermal comfort during pre is different from post training within both Acc and Con groups, # P <0.05. Acc: acclimation, Con: control, HIIT denotes high intensity interval training.