Background High-intensity interval training (HIIT) with respiratory-restricting masks is used to simulate hypoxic training and potentially enhance physiological adaptations. However, the acute hematological, metabolic, and inflammatory responses to masked versus unmasked HIIT in youth soccer players remain unclear. This study aimed to compare these responses during simulated 6000-ft masked and unmasked HIIT. Methods Twelve male amateur soccer players completed two HIIT sessions in a randomized crossover design: one with a simulated 6000-ft training mask and one without. Each session consisted of identical high-intensity running intervals. Heart rate and oxygen saturation were monitored continuously. Blood samples were collected pre- and post-exercise to assess lactate, hemoglobin, hematocrit, platelet count, C-reactive protein, glucose, total cholesterol, free-fatty acids, insulin, cortisol, and creatine kinase. Paired statistical tests were used to compare pre- to post-exercise changes and between-condition differences. Results Both masked and unmasked sessions increased heart rate and reduced oxygen saturation compared with baseline, with no significant between-condition differences (p > 0.20). Masked HIIT induced greater lactate accumulation (5.11 ± 1.59 vs. 4.34 ± 1.32 mmol·L?¹; p = 0.015) and larger increases in hemoglobin (p = 0.006), hematocrit (p = 0.013), and platelet count (p = 0.015), indicating enhanced hemoconcentration. C-reactive protein rose only after masked exercise, though the between-condition difference was non-significant (p = 0.513). Post-exercise glucose was higher in the masked condition, whereas unmasked HIIT elicited larger increases in total cholesterol (p = 0.002) and free-fatty acids. Insulin decreased similarly after both protocols (p < 0.05), and cortisol and creatine kinase remained unchanged (p > 0.05). Conclusions The simulated 6000-ft training mask augmented metabolic acidosis, hemoconcentration, and acute inflammatory stress without additional cardiovascular or arterial oxygen desaturation compared to unmasked HIIT. These findings suggest that such masks may provide a manageable yet heightened physiological stimulus, potentially useful for targeted training adaptations, provided hydration and recovery are carefully monitored.
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