Effects of Heat Acclimatization, Heat Acclimation, and Intermittent Exercise Heat Training on Time-Trial Performance

Background: The purpose of this study was to investigate effects of heat acclimatization (HAz) followed by heat acclimation (HA), and intermittent heat training (IHT) on time-trial performance. Hypothesis: Time-trial performance will improve after HA and will further improve with twice a week of IHT. Study Design: Interventional study. Methods: A total of 26 male athletes (mean +/- SD; age, 35 +/- 12 years; body mass, 72.8 +/- 8.9 kg; peak oxygen consumption [VO2peak], 57.3 +/- 6.7 mL center dot kg(-1)center dot min(-1)) completed five 4-km time trials (baseline, post-HAz, post-HA, post-IHT4, post-IHT8) in the heat (ambient temperature, 35.4 degrees C +/- 0.3 degrees C; relative humidity, 46.7% +/- 1.2%) on a motorized treadmill. After baseline time trial, participants performed HAz (109 +/- 10 days) followed by post-HAz time trial. Then, participants completed 5 days of HA, which involved exercising to induce hyperthermia (38.50 degrees C-39.75 degrees C) for 60 minutes. Participants were then divided into 3 groups and completed IHT either twice per week (IHTMAX), once per week (IHTMIN), or not at all (IHTCON) over an 8-week period. The exercise used for the IHT matched the HA. Four-kilometer time trials were performed after 4 weeks (post-IHT4) and 8 weeks of IHT (post-IHT8). Results: Time trial was faster in post-HA (17.98 +/- 2.51 minutes) compared with baseline (18.61 +/- 3.06 minutes; P = 0.037) and post-HAz (18.66 +/- 3.12 minutes; P = 0.023). Percentage change in time trial was faster in IHTMAX (-3.9% +/- 5.2%) compared with IHTCON (11.5% +/- 16.9%) (P = 0.020) and approached statistical significance with large effect (effect size = 0.96) compared with IHTMIN (1.6% +/- 6.2%; P = 0.059) at post-IHT8. Additionally, IHTMAX (-2.2% +/- 4.2%) was faster than IHTCON (3.6% +/- 6.9%) (P = 0.05) at post-IHT4. Conclusion: These results indicate that HA after HAz induces additional improvement in time-trial performance. IHT twice per week shows improvement after 8 weeks, while once per week maintains performance for 8 weeks. No IHT results in a loss of adaptations after 4 weeks and even greater losses after 8 weeks.