Impact of acute heat exposure on the Critical Power Concept and time to exhaustion during a high-intensity interval test in men and women

Introduction: The effect of heat on interval exercise is, up to now, not well documented, in contrast to sprint and endurance exercise. The aim of this study was to investigate the impact of acute heat exposure on the determination of the Critical Power Concept parameters and the recovery kinetics during a high-intensity interval test (HIIT) in a physically active male and female population. Methods: Twelve men (V̇O2peak: 58.2 ± 5.2 ml.min-1.kg-1, age: 25.4 ± 3.3 years, height: 1.82 ± 0.05 m, body mass 74.3 ± 9.1 kg) and twelve women (V̇O2peak: 41.2 ± 5.3 ml.min-1.kg-1, age: 26.4 ± 3.4 years, height: 1.69 ± 0.05 m, body mass 61.9 ± 5.5 kg) performed three constant load tests between 2 and 15 min to establish critical power (CP) and the curvature constant (W’), both in 18°C (TEMP) and 36°C (HOT) with 40% relative humidity. Afterwards, three HIIT were executed., one in TEMP based on CP and W’ determined in TEMP (HIIT-1) and two in HOT, of which one based on CP and W’ determined in TEMP (HIIT-2) and one in HOT (HIIT-3), respectively. HIIT consists of three work bouts (WB) to exhaustion, separated by 5 min active recovery, respectively at the intensity equal to the individual theoretical time to exhaustion (TTE) in 4 min based on CP concept (P4) and 90 % of individual gas exchange threshold (90%GET). Results: CP was lower in HOT compared to TEMP in men (267 ± 29 vs. 249 ± 30 W; p < 0.001) and women (187 ± 30 vs. 176 ± 29 W; p < 0.001), while W’ was higher in men (20.0 ± 3.1 vs. 22.7 ± 4.5 kJ; p = 0.026) and women (12.8 ± 1.6 vs. 14.3 ± 2.6 kJ; p = 0.032). P4 was slightly lower in men (350 ± 32 vs. 344 ± 32 W; p = 0.055) and women (240 ± 32 vs. 235 ± 35 W; p = 0.044). Predicted TTE at WB1 was only correctly estimated (i.e., 240 s) in HIIT-1 in men (235 ± 38 s; p = 0.651 ) and women (226 ± 32 s; p = 0.165). TTE was different in HIIT and WB, regardless of sex of the participants (p = 0.008). Conclusion: This study showed that acute heat exposure affected the determination of CP and W’, using multiple constant load tests, in an opposite direction. The hyperbolic relationship was skewed with acute heat exposure, as short trials (2-5 min) were less impacted, in contrast to long trials (12-15 min). It has been shown that the use CP concept during interval exercise in heat needs special attention. Even though CP and W’ are determined in HOT, TTE in HIIT was still diminished compared to TEMP in both men and women