# Effect of self-paced sprint interval training and low-volume HIIT on cardiorespiratory fitness: the role of heart rate and power output

PMID: 39973903
Journal: Frontiers in physiology
Published: 2025
Authors: Hesketh KL, Shepherd SO, Wagenmakers AJM, Cocks M, Strauss JA

## Question

Do self-paced SIT and LV-HIIT performed without encouragement improve cardiorespiratory fitness, and do heart-rate or power responses explain adaptation?

## Summary

In 82 previously inactive adults, both 6-week self-paced sprint interval cycling and low-volume HIIT cycling improved VO2peak and cardiometabolic markers, with no meaningful difference between protocols. Higher heart-rate responses did not produce superior VO2peak gains, while higher power output during LV-HIIT was associated with larger VO2peak improvement.

## Population

- Previously inactive adults, mostly women, with low coronary heart disease risk and BMI <=32 kg/m2
- Sample size: 82
- Age: 28 +/- 10 years
- Sex: 26 male, 56 female
- Fitness level: Previously sedentary; excluded if >150 min/week moderate-intensity exercise
- Health status: Low CHD risk; pregnant/breastfeeding participants excluded

## Methodology

- Randomized counterbalanced crossover training study
- Two 6-week interventions separated by 4-week washout
- Laboratory training at Liverpool John Moores University
- Randomized and controlled study design.

## Protocol

- Self-paced SIT.
- Modality: Wattbike cycling.
- Work intervals: 30 seconds.
- Recovery: 120 seconds active recovery.
- Sets or repetitions: 4-8 intervals.
- Intensity: >80% predicted HRmax, all-out recommended.
- Session duration: 10-20 minutes plus 5-minute warm-up.
- Frequency: 3 sessions/week.
- Program length: 6 weeks.
- Progression: 4 intervals week 1, 5 week 2, 6 weeks 3-4, 7 week 5, 8 week 6.
- Self-paced LV-HIIT.
- Modality: Wattbike cycling.
- Work intervals: 1 minute.
- Recovery: 1 minute active recovery.
- Sets or repetitions: 6-10 intervals.
- Intensity: >80% predicted HRmax.
- Session duration: 12-20 minutes plus 5-minute warm-up.
- Frequency: 3 sessions/week.
- Program length: 6 weeks.
- Progression: 6 intervals week 1 progressing to 10 by week 6.

## Outcomes

### VO2peak
Status: improved
Both SIT and LV-HIIT increased VO2peak, with no protocol difference.

SIT +3.1 +/- 0.4 and LV-HIIT +2.7 +/- 1.2 mL/kg/min; p<0.001.

### Cardiometabolic
Status: improved
Body fat percentage, aortic pulse wave velocity, and 120-minute glucose tolerance improved across training.

Body fat p=0.002; aPWV p=0.002; OGTT 120-min glucose p=0.024.

### Heart rate
Status: no clear change
Higher heart-rate tertiles did not show superior VO2peak changes.

Low versus high HR groups p>0.05.

### Power output
Status: mixed
For LV-HIIT, the highest peak power tertile had greater VO2peak improvement.

Low 1.8 +/- 4.1, medium 1.9 +/- 3.3, high 4.3 +/- 3.6 mL/kg/min; p=0.020.

## Practical Insights

- Self-paced hard cycling intervals can improve VO2peak without verbal encouragement.
- Heart-rate targets may help guide effort but do not fully represent training stimulus.

## Limitations

- No non-exercise control group
- Lab-based sessions
- Cycling-only
- No long-term adherence follow-up

## Safety And Adherence

- Article did not report adverse events or injuries.
- Participants were screened as low CHD risk by Framingham risk score, BMI <=32 kg/m2, and pregnant/breastfeeding participants were excluded.
- All sessions were lab based with HR monitoring and HR feedback; withdrawal criteria were <90% session completion or <2 sessions/week. Intensity compliance included peak HR >80% HRmax in 81% of SIT and 94% of LV-HIIT sessions.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/39973903/) (pubmed)
- [DOI](https://doi.org/10.3389/fphys.2025.1484722) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11835828/) (full text)

## Agent Guidance

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