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

## TL;DR

This study shows how high-intensity interval training affects blood biomarkers related to fitness and recovery, helping personalize training monitoring.

## Contribution

The study identifies specific biomarkers that correlate with training load, muscle soreness, and cardiorespiratory fitness during HIIT.

## Key findings

- Repeated HIIT decreases hemoglobin, hematocrit, red blood cells, and certain cytokines.
- Creatine kinase strongly correlates with training load and muscle soreness.
- VO2max correlates with specific cytokines like IL-5, -6, -10, -17F, -22.

## Abstract

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 (VO2max), 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 VO2max correlated with cytokines IL-5, -6, -10, -17F, -22. The present study revealed reliable biomarkers reflecting training load and VO2max, suitable for personalized monitoring of health and recovery and performance optimization.

•Repeated HIIT alters key blood markers linked to health and performance•Creatine kinase correlates with training load and muscle soreness•Several cytokines reflect cardiorespiratory fitness•Biomarkers enable personalized load monitoring of training and recovery

Repeated HIIT alters key blood markers linked to health and performance

Creatine kinase correlates with training load and muscle soreness

Several cytokines reflect cardiorespiratory fitness

Biomarkers enable personalized load monitoring of training and recovery

Kinesiology