# The effect of fitness level on cardiac autonomic regulation, IL-6, total antioxidant capacity, and muscle damage responses to a single bout of high-intensity interval training

PMID: 30356659
Journal: Journal of Sport and Health Science
Published: 2018-07
Authors: Cipryan L

## Question

Does baseline fitness level modify cardiac autonomic, inflammatory, antioxidant, and muscle-damage responses to one HIIT session?

## Summary

Young healthy participants with different fitness levels completed one treadmill HIIT bout. All groups completed the protocol; better-trained participants showed faster cardiac autonomic recovery and less muscle-damage signal, while inflammatory and antioxidant responses also differed by fitness level.

## Population

- Young healthy individuals in well-trained, moderately trained, and untrained groups.
- Sample size: 30
- Age: Approximately 22.6-24.4 years by group
- Fitness level: Well trained, moderately trained, and untrained.
- Health status: Healthy; no disease, smoking, medication, or supplements.

## Methodology

- Acute exercise study with fitness-level groups and randomized exercise/control visit order
- Single HIIT session with 4 h recovery sampling
- Laboratory treadmill testing
- Controlled study design.

## Protocol

- Treadmill HIIT.
- Modality: Treadmill running.
- Work intervals: 2 min at 100% vVO2max.
- Recovery: 2 min passive recovery.
- Sets or repetitions: 6 intervals.
- Intensity: 100% vVO2max; mean HR about 83-85% HRmax.
- Session duration: 30 min.
- Frequency: Single session.
- Program length: Acute.
- Rest control.
- Modality: Seated/resting laboratory control.
- Intensity: Rest.
- Session duration: 30 min.
- Frequency: Single control visit.
- Program length: Acute.

## Outcomes

### HRV recovery
Status: mixed
Untrained participants had the most pronounced transient HRV suppression.

rMSSD decrease persisted most clearly at 1 h in untrained participants.

### IL-6/TAC
Status: mixed
Well-trained participants showed larger IL-6 and antioxidant-capacity responses.

### Muscle damage
Status: worse/safety concern
Muscle-damage markers rose more in less-trained groups.

Myoglobin changes were more noticeable in moderately trained and untrained groups.

### Completion
Status: improved
All participants completed the prescribed HIIT.

30/30 completed.

## Practical Insights

- Baseline fitness may affect how quickly users recover from hard intervals.
- A 6 x 2-min treadmill protocol is feasible in screened young adults but physiologically demanding.

## Limitations

- Acute-only study.
- Small fitness subgroups.
- Fitness group assignment was not randomized.
- Young healthy participants limit generalizability.

## Safety And Adherence

- No adverse events were reported.
- The session produced high lactate, high RPE, transient HRV suppression, and increases in muscle-damage biomarkers, especially in less-trained participants.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/30356659/) (pubmed)
- [DOI](https://doi.org/10.1016/j.jshs.2016.11.001) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6189275/) (full text)

## Agent Guidance

Preserve the paper-level scope of this note. Do not generalize beyond the population, protocol, measured outcomes, and limitations above.