Boynton, JR, Peiffer, JJ, and Abbiss, CR. Effects of HIIT in cool and hot on temperate performance and physiological response in trained cyclists. J Strength Cond Res 39(3): e485-e495, 2025-This study investigated cardiopulmonary responses in hot and cool high-intensity interval training (HIIT) and the subsequent effects on time-trial (TT) performance and physiological responses in temperate conditions. Twenty trained cyclists were separated into 2 groups and completed a 4-week HIIT intervention (8 sessions, 5 × 4 minute) at an environmental temperature (TA) of either 13° C (HIIT13) or 35° C (HIIT35). Cardiorespiratory data (e.g., heart rate [HR] and time above 90% V̇o2max [T >
90% V̇o2max]) were assessed for the first and last interval sessions. Subjects completed 20 km TTs in temperate conditions (22° C) before (TT1) and after (TT2) HIIT training, during which power output, HR, and thermoregulatory measures were recorded. T >
90% V̇o2max was greater in HIIT13 (875 ± 295 seconds
p = 0.007), compared with HIIT35 (420 ± 395 seconds). Average power output during the TT increased from TT1 to TT2 in both groups (HIIT13p = 0.023, 3.3 ± 3.4%
HIIT35p = 0.003, 7.3 ± 6.3%) but with no significant interactions or differences between groups (p = 0.115
p = 0.421, respectively). Within-subject increases for HR and core temperature were observed during TT2 for HIIT13 (2.7 ± 2.1%, 0.5 ± 0.6%) but not HIIT35 (1.5 ± 4.9%, 0.0 ± 0.7%). After 4 weeks of HIIT, the improvement in temperate 20-km TT performance was not greater for HIIT13 than HIIT35, despite greater T >
90% V̇o2max during 13° C HIIT vs. 35° C HIIT. Physiological responses (e.g., HR and thermoregulation) during the TT differed between HIIT13 and HIIT35, indicating varying adaptive responses.