# Submaximal exercise cardiac output is increased by 4 weeks of sprint interval training in young healthy males with low initial Q-V O2: Importance of cardiac response phenotype

PMID: 30673702
Journal: PloS one
Published: 2019-01-23
Authors: Bentley RF, Jones JH, Hirai DM, Zelt JT, Giles MD, Raleigh JP, Quadrilatero J, Gurd BJ, Neder JA, Tschakovsky ME

## Question

Does baseline inter-individual variation in the cardiac output-to-oxygen uptake relationship determine cardiovascular adaptation to 4 weeks of sprint interval training, and do changes in submaximal cardiac output affect skeletal muscle oxygenation and perceived exertion?

## Summary

This uncontrolled 4-week sprint interval training study tested whether young healthy recreationally active men with different baseline cardiac output-to-oxygen uptake responses adapted differently. The protocol was supervised cycling using 8 x 20-second sprints at 170% of peak VO2 work rate, four times per week. Peak VO2 increased in both lower and higher cardiac-response phenotypes, but submaximal cardiac output relative to oxygen uptake increased only in men who started with a low cardiac output response. Skeletal muscle oxygenation and perceived exertion did not improve despite the higher cardiac response.

## Population

- Healthy recreationally active young adult males with less than 3 hours per week of structured exercise.
- Sample size: 22
- Age: 21 +/- 2 years
- Sex: Male
- Fitness level: Recreationally active; group peak VO2 44.0 +/- 6.3 ml/kg/min at baseline.
- Health status: Healthy; no smoking history, cardiovascular disease, or hypertension.

## Methodology

- Uncontrolled 4-week pre-post sprint interval training study with post hoc lower and higher cardiac-response phenotype groups.
- 4 weeks
- Laboratory and hospital exercise testing setting; supervised cycling sprint interval training.

## Protocol

- 4-week supervised cycling sprint interval training.
- Modality: Stationary cycle ergometer.
- Work intervals: 20 seconds.
- Recovery: 10 seconds loadless active recovery.
- Sets or repetitions: 8 sprints.
- Intensity: 170% of work rate at peak VO2; 80 rpm target.
- Session duration: 5-minute warm-up plus 4 minutes of interval exercise; cooldown not reported.
- Frequency: 4 sessions per week.
- Program length: 4 weeks.
- Progression: No progression reported.

## Outcomes

### Peak VO2
Status: improved
Both lower and higher cardiac responders increased peak VO2 after SIT.

Both P < 0.01; lower responders 39.7 +/- 6.7 to 44.5 +/- 7.3 ml/kg/min; higher responders 47.2 +/- 4.4 to 52.4 +/- 6.0 ml/kg/min.

### Q-V O2 slope
Status: mixed
Lower cardiac responders increased Q-V O2 after training; higher cardiac responders were unchanged.

Lower P = 0.02; higher P = 0.5; post-training groups no longer differed, P = 0.3.

### Submaximal cardiac output and stroke volume
Status: mixed
Lower cardiac responders increased cardiac output at 120, 160, and 185 W because stroke volume increased; higher responders had a smaller mixed response.

Lower delta Q: 120 W P = 0.01, 160 W P = 0.04, 185 W P = 0.001; lower delta SV: 120 W P = 0.04, 160 W P = 0.03, 185 W P = 0.003.

### Skeletal muscle oxygenation and perceived exertion
Status: no clear change
Skeletal muscle saturation, leg RPE, and whole-body RPE were unaffected by SIT.

Skeletal muscle saturation F test P = 0.9; leg RPE F test P = 0.7; whole-body RPE F test P = 0.1.

## Practical Insights

- A very short supervised cycling SIT protocol improved peak VO2 over 4 weeks in young healthy men.
- Baseline physiology may influence how a person adapts to SIT, so broad claims about identical HIIT responses should be avoided.
- Improved cardiac output response did not reduce perceived exertion or improve measured muscle oxygenation in this study.
- Direct replication for consumer programming is limited by the need for cycle ergometry and peak VO2-based intensity prescription.

## Limitations

- Small sample.
- No non-training control group.
- Post hoc phenotype grouping.
- Male-only young healthy recreationally active sample.
- Attendance among completers not reported.
- Some cardiac output measures above 160 W depended on corrected Finometer estimates.

## Safety And Adherence

- One dropout cited headaches as negative side effects associated with training.
- One dropout did not feel well enough to complete the post-training progressive exercise test.
- Participants were young healthy males without smoking history, cardiovascular disease, or hypertension.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/30673702/) (pubmed)
- [DOI](https://doi.org/10.1371/journal.pone.0195458) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6343875/) (full text)

## Agent Guidance

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