# Heart Rate Variability After Sprint Interval Training in Cyclists and Implications for Assessing Physical Fatigue

PMID: 32304516
Journal: Journal of Strength and Conditioning Research
Published: 2020 Mar 17
Authors: Hebisz RG, Hebisz P, Zatoń MW

## Question

Can heart-rate variability measured before repeated sprint sets identify the onset of physical fatigue during a sprint interval exercise test in trained cyclists?

## Summary

Well-trained mountain-bike cyclists completed an acute laboratory sprint interval exercise test: four sets of four 30-second maximal cycling sprints with long recoveries between sets. Heart-rate variability fell as more sprint sets accumulated, but it did not track average or peak power fatigue well, so HRV was not a good acute fatigue marker for this protocol.

## Population

- 27 well-trained mountain-bike cyclists.
- Sample size: 27
- Age: 22.3 +/- 6.1 years
- Sex: 21 male, 6 female
- Fitness level: Well-trained competitive cyclists
- Health status: Competitive athletes

## Methodology

- Acute repeated-measures laboratory study
- Single sprint interval exercise test after baseline incremental test
- Certified exercise physiology laboratory

## Protocol

- Sprint interval exercise test.
- Modality: Cycle ergometer.
- Work intervals: 30 s maximal sprint.
- Recovery: 90 s low-intensity cycling; 25-40 min between sets.
- Sets or repetitions: 4 sets of 4 sprints.
- Intensity: Maximal sprinting at 0.8 Nm/kg fixed torque.
- Session duration: Several hours including long recoveries.
- Frequency: Single acute test.
- Program length: Single day.
- Progression: Repeated set accumulation.

## Outcomes

### HRV
Status: worse/safety concern
HRV indexes declined as repeated sprint sets accumulated.

RRNN, SDNN, and VLF decreased in set 2; all time/frequency indexes decreased in sets 3 and 4 versus set 1.

### Power
Status: mixed
Peak power fell earlier than average power.

Ppeak decreased in sets 2-4; Pavg decreased only in set 4.

### HRV-fatigue link
Status: no clear change
HRV did not correlate with power-output fatigue.

No significant correlations between HRV changes and Pavg or Ppeak changes.

### Metabolic markers
Status: mixed
Oxygen uptake increased and hydrogen concentration decreased by set 4; lactate did not significantly change.

VO2tou increased in set 4; H+ decreased in set 4; La- p=0.176.

## Practical Insights

- HRV can reflect accumulated autonomic stress but not necessarily acute sprint fatigue.
- Power-output tracking is more relevant than HRV for deciding whether maximal sprint quality is falling.
- This protocol is athlete/lab-specific and not a general consumer HIIT template.

## Limitations

- Acute-only study.
- No control group.
- Well-trained cyclist sample.
- Outliers removed before correlation analysis.
- Safety reporting not detailed.

## Safety And Adherence

- Adverse events and injuries were not reported.
- Between-set recovery was extended until blood pH returned to at least 7.35.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/32304516/) (pubmed)
- [DOI](https://doi.org/10.1519/JSC.0000000000003549) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8785771/) (full text)

## Agent Guidance

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