# Acute and Chronic Effects of a High-Intensity Interval Training Shock Microcycle on Cell-Free DNA: A Randomized Controlled Trial

## TL;DR

This study found that high-intensity interval training (HIIT) causes large, short-term increases in cell-free DNA (cfDNA), but no long-term changes.

## Contribution

The study introduces a novel approach to measuring acute and chronic cfDNA responses to a HIIT shock microcycle in trained athletes.

## Key findings

- cfDNA concentrations increased significantly after HIIT and physiological exercise testing.
- Higher cfDNA increases were observed in afternoon HIIT sessions compared to morning.
- No chronic changes in cfDNA were observed during or after the HIIT shock microcycle.

## Abstract

This study aimed to evaluate acute and chronic exercise-induced changes in cell-free DNA (cfDNA) concentrations during a 7-day high-intensity interval training (HIIT) shock microcycle in trained endurance athletes. Thirty-five participants were randomly assigned to one of three groups: a HIIT-only group (HSM), a HIIT plus low-intensity training group (HSM + LIT), and a control group maintaining regular training. The intervention included 10 HIIT sessions (5 × 4 min at 90–95% maximum heart rate) over 7 days, with HSM + LIT completing an additional 30 min of low-intensity training after each session. Physiological exercise testing (PET) was conducted at baseline, 3-, 7-, and 14-days post-intervention. On days 2 and 7 during the intervention, HIIT sessions were supervised in both morning and afternoon, and venous blood samples were collected at rest, immediately post-exercise, and 30 min post-exercise to measure cfDNA for 90 and 222 bp fragments. Correlations between cfDNA and physiological exercise variables such as peak power output (PPO), running velocity at lactate threshold (LT), and VO₂max were analyzed.

cfDNA90 (10.4-fold, p < 0.001) and cfDNA222 (12.4-fold, p < 0.001) increased significantly after PET. In addition, cfDNA90 (17.1-fold, p < 0.001) and cfDNA222 (20.2-fold, p < 0.001) increased after HIIT, both remaining significantly elevated 30 min post-HIIT (both p < 0.001). cfDNA90 concentrations were higher in afternoon (22.4-fold) compared to morning HIIT sessions (17.2-fold, p < 0.001). A significant interaction effect was found between group and measurement point for cfDNA90 (p < 0.001) and cfDNA222 (p < 0.001), with higher concentrations in HSM + LIT compared to HSM 30 min post-HIIT. cfDNA90 showed moderate correlations with PPO (r = 0.48, p < 0.001), LT (r = 0.36, p < 0.001) and VO₂max (r = 0.30, p = 0.01). cfDNA222 correlated moderately with VO₂max (r = 0.34, p = 0.001) and slightly with PPO (r = 0.21, p = 0.05). No chronic changes in cfDNA were observed throughout the study period.

cfDNA is a reliable marker for detecting acute exercise-induced stress. However, the potential of cfDNA for detecting chronic adaptations in short-term, high-intensity interval training settings, such as a HIIT shock cycle, appears limited thus far.

Trial registration clinicaltrials.gov, NCT05067426. Registered 05 October 2021—Retrospectively registered, https://clinicaltrials.gov/ct2/show/NCT05067426.

The online version contains supplementary material available at 10.1186/s40798-025-00923-9.

Acute increases in cfDNA concentrations were observed after physiological exercise testing and high-intensity interval training (HIIT).No chronic cfDNA changes were observed during and after a HIIT shock microcycle.Acute increases in cfDNA concentrations correlated with physiological exercise variables e.g. peak power output, running velocity at lactate threshold and VO2max during acute exercise.Higher increases in cfDNA were found after HIIT compared to physiological exercise testing.Higher increases in cfDNA were found after HIIT in the afternoon compared to morning.Higher increases in cfDNA were observed in males compared to females.

Acute increases in cfDNA concentrations were observed after physiological exercise testing and high-intensity interval training (HIIT).

No chronic cfDNA changes were observed during and after a HIIT shock microcycle.

Acute increases in cfDNA concentrations correlated with physiological exercise variables e.g. peak power output, running velocity at lactate threshold and VO2max during acute exercise.

Higher increases in cfDNA were found after HIIT compared to physiological exercise testing.

Higher increases in cfDNA were found after HIIT in the afternoon compared to morning.

Higher increases in cfDNA were observed in males compared to females.

The online version contains supplementary material available at 10.1186/s40798-025-00923-9.