Effect of Heat Acclimatization, Heat Acclimation, and Intermittent Heat Training on Maximal Oxygen Uptake

Background: Maximal oxygen uptake (VO 2max ) is an important determinant of endurance performance. Heat acclimation/acclimatization (HA/HAz) elicits improvements in endurance performance. Upon heat exposure reduction, intermittent heat training (IHT) may alleviate HA/HAz adaptation decay; however, corresponding VO 2max responses are unknown. Hypothesis: VO 2max is maintained after HAz/HA; IHT mitigates decrements in aerobic power after HAz/HA. Study Design: Interventional study. Level of Evidence: Level 3. Methods: A total of 27 male endurance runners (mean ± SD; age, 36 ± 12 years; body mass, 73.03 ± 8.97 kg; height, 178.81 ± 6.39 cm) completed VO 2max testing at 5 timepoints; baseline, post-HAz, post-HA, and weeks 4 and 8 of IHT (IHT 4 , IHT 8 ). After baseline testing, participants completed HAz, preceded by 5 days of HA involving exercise to induce hyperthermia for 60 minutes in the heat (ambient temperature, 39.13 ± 1.37°C; relative humidity, 51.08 ± 8.42%). Participants were assigned randomly to 1 of 3 IHT groups: once-weekly, twice-weekly, or no IHT. Differences in VO 2max , velocity at VO 2max (vVO 2 ), and maximal heart rate (HR max ) at all 5 timepoints were analyzed using repeated-measure analyses of variance with Bonferroni corrections post hoc. Results: No significant VO 2max or vVO 2 differences were observed between baseline, post-HAz, or post-HA ( P = 0.36 and P = 0.09, respectively). No significant group or time effects were identified for VO 2max or vVO 2 at post-HA, IHT 4 , and IHT 8 ( P = 0.67 and P = 0.21, respectively). Significant HR max differences were observed between baseline and post-HA tests ( P < 0.01). No significant group or time HR max differences shown for post-HA, IHT 4 , and IHT 8 ( P = 0.59). Conclusion: VO 2max was not reduced among endurance runners after HA/HAz and IHT potentially due to participants’ similar aerobic training status and high aerobic fitness levels. Clinical Relevance: HAz/HA and IHT maintain aerobic power in endurance runners, with HAz/HA procuring reductions in HR max .