This study investigated whether implementing heat training during the final week of a 3-week altitude training camp affects the hematological adaptations and variables monitored during chronic hypoxia. Twenty-three well-trained triathletes (19 males, 4 females) underwent 21 days of altitude training camp in spring (Live-High-Train-High, 1850 m). From day 14 to day 21, participants were split into two groups: a control group (HYPOXIA), which remained in the same condition, while the second group (COMBINED) had a ~1 h cycling session per day conducted in the heat (36°C ± 1°C, 40% ± 5% RH). Training load was the same for both groups. Hematological responses (including hemoglobin mass (Hbmass) and plasma volume (PV)) were assessed. Monitoring included physiological responses at rest (nocturnal oxygen saturation (SpO2) and heart rate, Hooper and Spiegel questionnaires, hydration status) and during exercise (incl. power, SpO2 through 45 min cycling at fixed heart rate). Both Hbmass (3.2% ± 3.1% for HYPOXIA, 3.7% ± 2.9% for COMBINED, p < 0.001) and PV (p = 0.003) increased during the camp, without interaction between groups (p = 0.266). Exercise SpO2 decreased between the end of the second and the third week (p = 0.034, d = 0.583) in the COMBINED group. No difference between groups was detected for the other physiological or perceptual parameters. Adding heat training during the third week of an altitude training camp did not affect the increase in Hbmass or monitoring variables at rest. However, it did not confer any additional PV expansion and was associated with a reduced exercise SpO2.
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