No significant effects of supplementing high-intensity interval training with post-exercise hot water immersion on either temperate endurance performance or mitochondrial adaptations

Objective: The primary aim of this study was to examine the effects of 30-min hot water immersion (HWI) after nine high-intensity interval training (HIIT) sessions on endurance exercise performance in a temperate environment. A secondary aim was to determine the effects of this strategy on mitochondrial adaptations. Methods: Fourteen male cyclists were assigned to either an HWI (n = 7) group or a control (CON, n = 7) group and completed nine HIIT sessions over 3 weeks. Immediately after each HIIT session, participants from the HWI group were immersed in 42 degrees C water, whereas those from the CON group were immersed in 34 degrees C water. Before and after the interventions, the cyclists performed an incremental test to exhaustion and a 20-km time trial test at 18-20 degrees C. Resting muscle biopsies were obtained from the vastus lateralis pre- and post-intervention to assess changes in mitochondrial respiratory proteins (Complex I to V), maximal citrate synthase activity, and mitochondrial respiration. Results were interpreted based on the p-value and effect size (ES). Results: The time to complete the 20-km time trial test was reduced to a similar extent (F(1, 12) = 0.00, p = 0.99) in the HWI group (1.5%, p = 0.04) and the CON group (1.7%, p = 0.03). A trivial between-group ES (0.0) was observed for the time trial performance, confirming that supplementing HIIT with post-exercise HWI provided no greater ergogenic effect than the CON group. Neither the determinants of endurance performance (i.e., lactate threshold, peak power output, and maximal oxygen consumption) nor markers of mitochondrial content and respiration showed significant changes following the 3-week intervention period. Conclusion: Nine HIIT sessions led to a significantly improved 20-km time trial performance in temperate conditions, but adding HWI after the HIIT sessions did not provide additional performance benefits.