This study was designed to determine whether longer HIIT sessions are more effective for improving fitness and health than shorter HIIT sessions in teenagers who didn’t benefit from HIIT in a previous study that involved up to 4-minute sessions. To achieve this aim, the researchers compared 6 min, 8 min, and 10 min bouts of HIIT performed during school physical education classes and tracked changes in VO₂max, body fat, and blood pressure.

VO₂max increased across most intervention groups, and the 10-minute HIIT sessions showed the most consistent gains. Average changes in body fat and resting blood pressure were tiny and inconsistent between sexes and groups. However, on average, the participants did not have high body fat or high blood pressure at baseline, so it’s not particularly surprising that these variables didn’t change.

When the authors looked at individuals rather than group averages, they found substantial variability in responsiveness (improvement) between participants, with 53% to 68% of participants classified as “responders” depending on the outcome. Furthermore, there was a statistically significant increase in the proportion of participants who showed an improvement in either VO₂max or blood pressure with this HIIT intervention, compared to the previous study.

Ultimately, the findings show that exercise programs should probably be personalized to achieve the best results.

VO₂max, a marker of cardiorespiratory fitness, is a useful biomarker for future health.^[2]^[3] Exercise increases VO₂max, yet many teenagers don’t meet activity guidelines.^[4] School exercise programs that are brief, scalable, and safe could help improve teenagers’ health. School-based HIIT fits this idea.

A 2022 meta-analysis reported that school-based HIIT reliably improved fitness when compared to a nonexercise control group,^[5] whereas changes in body fat and resting blood pressure were smaller and more variable.^[5] However, HIIT did not appear to be more effective for improving fitness than other exercise interventions.^[6] Therefore, for general fitness gains in children and teenagers, the method of exercise probably matters less than just doing some exercise in general.

The strengths of this study include the accessible school-based protocol, heart rate monitoring during sessions, and the responder analysis that revealed individual variability, rather than hiding it within “average” changes. The consistency of VO₂max gains (especially with 10-minute sessions) supports a real benefit. However, VO₂max was predicted from a shuttle run test rather than measured, and sticking to the 10-minute HIIT protocol seemed challenging.

It’s also worth considering that the teenagers who benefited from HIIT in the original study were excluded from this new study. Therefore, it’s unknown how much the original “responders” would benefit from this new HIIT protocol.

VO₂max is associated with better health outcomes^[2]^[3] and many teenagers don’t meet activity guidelines,^[4] so including 10-minute HIIT sessions during school physical education classes seems like a smart way to increase physical activity levels and improve teenagers’ health.

Can greater benefits be achieved with longer sessions (12 minutes) or with more frequent classes (3 times per week or more)? Can HIIT improve body fat and blood pressure in teenagers with obesity and/or high blood pressure? These should be future research questions.

Longer HIIT sessions may turn exercise-nonresponder teenagers into responders Original paper