High-intensity Interval Training In The Heat: A "hotter" Alternative For Promoting Cardiovascular Health And Performance?

Abstract

Chronic heat exposure during exercise, exercise heat acclimation (EHA), and high-intensity interval training (HIIT) both promote positive physiological and performance adaptations; however, it remains unknown if HIIT acts synergistically with EHA compounding the performance benefit of HIIT. PURPOSE: To investigate the effects of HIIT and EHA (HIIT+Heat) on cardiovascular function (CV), endurance running performance, and muscle function vs. HIIT alone. METHODS: Using a randomized controlled parallel-design, 10 young healthy males and females completed 6 sessions of HIIT (8 x 30s on, 90s off) performed either in the heat (HIIT+Heat, 30°C, 50% RH) or thermoneutral (HIIT, ~20°C, 15% RH) condition. Measures of cardiovascular and muscle function, running performance and economy were measured prior to, and >72hr post, training in a thermoneutral environment. A 5 km time-trial and graded treadmill protocol were conducted to assess for running performance and economy, while power and velocity during maximal vertical jump were used to assess muscle function. CV function was assessed via, heart rate (HR), HR variability (HRV; root mean square of successive differences; RMSSD, and standard deviation of N-N intervals; SDNN), central (cBP), peripheral blood pressure (pBP), and pulse wave velocity (PWV). RESULTS: No baseline differences were observed between groups. No differences in performance improvement (-4 ± 2 vs. -4 ± 4%∆ in 5km time) or running economy were observed between groups (p>0.05). Resting HR (-2 ± 3 vs. -2 ± 2 %∆) and HRV (SDNN 5 ±2 vs. 11 ±4%) were improved in both groups but were not different between conditions (HIIT vs. HIIT+Heat, p>0.05). However, cSBP (1 ± 2 vs. -7 ± 3 %∆), pSBP (-2 ± 2 vs. -8 ± 2%∆), and PWV (0 ± 1 vs. -6 ±3%∆) only decreased in the HIIT+Heat (HIIT vs. HIIT+heat, respectively, all p<0.05). Improvements in jump velocity (-3 ± 5 vs. 8 ± 5%∆) and power (-2 ± 5 vs. 9 ± 6%∆) tended be greater in HIIT+heat (p=0.08-0.10). CONCLUSION: Short term HIIT combined with heat stress did not improve running performance or economy more than HIIT alone, but did significantly improve blood pressure, vascular stiffness, and tended to improve muscle function in thermoneutral conditions. Further work exploring longer training and/or greater heat stress in larger populations, or those with vascular dysfunction, is warranted.