# Six high-intensity interval training sessions over 5 days increases maximal oxygen uptake, endurance capacity, and sub-maximal exercise fat oxidation as much as 6 high-intensity interval training sessions over 2 weeks

PMID: 32565243
Journal: Journal of Sport and Health Science
Published: 2021 Jul
Authors: Atakan MM, Güzel Y, Bulut S, Koşar ŞN, McConell GK, Turnagöl HH

## Question

Can six HIIT sessions compressed into 5 days by using two same-day sessions every other day improve VO2max, submaximal exercise fat oxidation, and endurance capacity as effectively as six HIIT sessions spread over 2 weeks?

## Summary

This randomized training study compared the same six cycling HIIT sessions delivered either as two sessions per day across 5 days or as one session per training day across 2 weeks in healthy recreationally active young men. Both schedules improved VO2max and cycling time to exhaustion similarly. The 5-day double-session schedule also significantly increased total fat oxidation during a 60-minute submaximal cycling test, while the 2-week schedule's fat oxidation increase was not statistically significant.

## Population

- Twenty-eight healthy recreationally active males aged 20-30 years, randomly assigned to HIIT-D (n = 15) or HIIT-S (n = 13).
- Sample size: 28
- Age: 25 +/- 4 years overall
- Sex: Male
- Fitness level: Recreationally active, exercising 2-3 times per week but not systematically trained.
- Health status: Healthy non-smokers without exercise-limiting cardiovascular, respiratory, or metabolic illness.

## Methodology

- Randomized parallel-group exercise training study comparing equal-work cycling HIIT delivered over 5 days versus 2 weeks, with pre- and post-training physiological testing.
- HIIT-D completed six sessions over 5 days; HIIT-S completed six sessions over 2 weeks. Including pre- and post-testing, study duration was 3 weeks for HIIT-D and 4 weeks for HIIT-S.
- Exercise laboratory with cycle ergometer testing and supervised cycling HIIT.
- Randomized and controlled study design.

## Protocol

- HIIT-D.
- Modality: Cycle ergometer.
- Work intervals: 10 x 60 seconds at workload corresponding to peak power output reached at the end of the VO2max test.
- Recovery: 75 seconds cycling at 60 W between intervals.
- Sets or repetitions: 10 intervals per session.
- Intensity: Targeted 100% VO2max workload; mean session HR 90.8% +/- 1.6% of maximal HR.
- Session duration: Approximately 28.5 minutes including 3-minute warm-up and 3-minute warm-down.
- Frequency: Two sessions per day separated by 3 hours, every other day across 5 days.
- Program length: Six sessions over 5 days.
- Progression: None reported.
- HIIT-S.
- Modality: Cycle ergometer.
- Work intervals: 10 x 60 seconds at workload corresponding to peak power output reached at the end of the VO2max test.
- Recovery: 75 seconds cycling at 60 W between intervals.
- Sets or repetitions: 10 intervals per session.
- Intensity: Targeted 100% VO2max workload; mean session HR 91.5% +/- 1.0% of maximal HR.
- Session duration: Approximately 28.5 minutes including 3-minute warm-up and 3-minute warm-down.
- Frequency: One session per weekday training day, three times per week.
- Program length: Six sessions over 2 weeks.
- Progression: None reported.

## Outcomes

### VO2max
Status: improved
VO2max increased 6.1% +/- 5.9% in HIIT-S and 7.7% +/- 4.1% in HIIT-D, with no significant difference between protocols.

HIIT-S: 41.8 +/- 6.9 to 44.5 +/- 6.0 mL/kg/min, p = 0.001; HIIT-D: 45.2 +/- 6.6 to 48.7 +/- 6.1 mL/kg/min, p < 0.001; between protocols p > 0.05.

### Time to exhaustion
Status: improved
TTE increased by about 80% in both groups with no significant between-group difference.

HIIT-S: 13.2 +/- 5.1 to 23.8 +/- 10.7 min, +79.2%, p < 0.003, d = 1.2; HIIT-D: 17.6 +/- 6.9 to 31.7 +/- 17.9 min, +80.1%, p < 0.001, d = 0.56.

### Submaximal fat oxidation
Status: mixed
Total fat oxidation increased significantly in HIIT-D but not in HIIT-S.

HIIT-D: 19.6 +/- 7.5 to 22.6 +/- 7.1 g, +15.4%, p = 0.048; HIIT-S: 18.6 +/- 6.8 to 20.4 +/- 6.8 g, +9.2%, p = 0.290; d = 0.32.

### Submaximal carbohydrate oxidation
Status: improved
Total carbohydrate oxidation decreased significantly in both groups during submaximal cycling.

HIIT-S: 132.6 +/- 33.5 to 107.6 +/- 30.7 g, p = 0.014; HIIT-D: 118.4 +/- 18.6 to 107.4 +/- 23.2 g, p = 0.012; no group difference.

## Practical Insights

- For healthy recreationally active young men, six well-calibrated HIIT cycling sessions can produce rapid VO2max and endurance improvements.
- Compressing the same HIIT dose into 5 days did not reduce VO2max or endurance benefits compared with spreading it over 2 weeks.
- The protocol is informative for short training blocks but should be adapted cautiously for consumer apps because it used high-intensity cycling, laboratory calibration, and a narrow sample.

## Limitations

- Small sample.
- Male-only and young adult sample.
- Healthy recreationally active participants only.
- No non-exercising control group.
- Short-term post-test only.
- Laboratory cycling modality limits direct translation to bodyweight or home workouts.

## Safety And Adherence

- No adverse events were reported in the extracted article text.
- Participants with exercise-limiting cardiovascular, respiratory, or metabolic illness were excluded.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/32565243/) (pubmed)
- [DOI](https://doi.org/10.1016/j.jshs.2020.06.008) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8343121/) (full text)

## Agent Guidance

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