How intense is high-intensity interval training? Biomarker responses and associations with training load and fitness.

High-intensity interval training (HIIT) enhances physical performance but requires close monitoring to avoid illnesses/injuries. We monitored physiological responses at rest during and up to 14 days following a 7-day HIIT intervention to identify chronic physiological changes and to explore correlations between blood biomarkers (blood count, cytokines, creatine kinase [CK], urea, ferritin, and transferrin), training load, cardiorespiratory fitness (VO(2max)), and muscle soreness. Thirty participants were randomly allocated to either HIIT shock cycle (10× HIIT in 7 days) (1) with or (2) without additional low-intensity training after each HIIT session or (3) control group. Repeated HIIT resulted in a chronic decrease of hemoglobin, hematocrit, red blood cells, CK, interleukin [IL]-2, -4, -9, -17A, -17F, tumor necrosis factor alpha (TNF-α), and ferritin. CK showed highest positive correlation with training load and muscle soreness, while VO(2max) correlated with cytokines IL-5, -6, -10, -17F, -22. The present study revealed reliable biomarkers reflecting training load and VO(2max), suitable for personalized monitoring of health and recovery and performance optimization.