Long‐term passive heat acclimation enhances maximal oxygen consumption via haematological and cardiac adaptation in endurance runners

Long-term heat acclimation has been shown to elevate haemoglobin mass (Hbmass) and enhance maximal oxygen consumption (VO2max). To date, however, all evidence derives from exercise-induced heat stress, making it unclear whether passive strategies, which allow athletes to maintain training intensities, can produce similar adaptations. Moreover, while heat-induced increases in (VO2max) are often attributed to haematological changes, adaptation of other convective components of oxygen transport may underpin these improvements. To investigate whether 5 weeks of hot-water immersion (HWI) would enhance (VO2max), 10 well-trained runners (nine male; (VO2max) = 64.5 ± 8.1 mL min−1 kg−1) completed a within-subject, counterbalanced cross-over intervention involving HWI (5 × 45 min week−1; ≥40°C) and a time-matched control, alongside habitual training. Haematological and cardiac adaptations were assessed using carbon-monoxide rebreathing (in duplicate) and four-dimensional (4D) speckle-tracking echocardiography, respectively. HWI increased Hbmass (+33 g, P < 0.001; [95% confidence interval (CI) 18 to 49]), blood volume (+284 mL, P < 0.001; [95% CI 113 to 455]) and left ventricular end-diastolic volume (LVEDV; +10 mL, P < 0.001; [95% CI 6 to 13]), without altering 4D global peak longitudinal strain, diastolic-strain rate or diastolic-filling rate. These changes facilitated a 2.7 mL kg−1 min−1 improvement in (VO2max) (P < 0.001; [95% CI 1.4 to 4.1]) and a 0.8 km h−1 increase in treadmill speed at (VO2max) (P < 0.001; [95% CI 0.5 to 1.1]). Best subset regression analysis identified Hbmass as the strongest independent predictor of (VO2max) (β = 3.8, P < 0.001), with cardiac adaptations contributing additional explanatory value (R2 = 0.825 fixed effects; 0.961 full model). Our findings demonstrate that passive heat exposure elicits coordinated adaptations to multiple convective components of the oxygen transport chain, and represents a physiologically effective and sustainable strategy for enhancing (VO2max) in well-trained populations.