This systematic review and meta-analysis pooled 86 studies covering 239 high-intensity interval training (HIIT) protocols to find which workout designs keep people closest to their maximal oxygen uptake (VO2max) for longest, a measure called time at VO2max, or “red zone” time. Work intervals of 2 minutes or longer produced more red-zone time than shorter intervals in between-study comparisons, and varying intensity within each interval (especially starting fast) added about 2.75 minutes per session. Whether recovery between reps was active or passive made no difference. However, workout structure explained only about 6 to 7 percent of the variation in red-zone time, the head-to-head comparison of long versus short intervals was essentially a tie.
If you have ever ground through a set of intervals and wondered whether the suffering was paying off, a new analysis offers a partial answer. The payoff, it suggests, depends less on how hard you go than on how you arrange the effort.
Researchers led by the University of Essex pooled 86 studies covering 239 different interval workouts in running, cycling, rowing, swimming, kayaking and cross-country skiing. Rather than tracking fitness over months, they measured something more immediate: how many minutes each workout kept people near their VO2max, the ceiling of the body’s aerobic engine. Exercise physiologists call this the red zone, and many coaches treat it as the active ingredient of interval training. The logic is that the heart, lungs and muscles adapt most when pushed to their limit, so more time there should mean a stronger signal.
Three practical patterns emerged. First, intervals of two minutes or longer kept people in the red zone longer than short sprints or one-minute efforts. Long-interval sessions averaged a little over 9 minutes at or above 90 percent of VO2max, compared with about 5.5 minutes for shorter formats. Part of that gap reflects the fact that long-interval sessions also contained roughly twice as much total hard work.
Second, varying the effort within each interval helped. Starting each rep hard and then easing off, or adding surges to a steady effort, added close to three minutes in the red zone compared with holding a perfectly even pace. A fast start drives oxygen uptake up quickly, and the surges appear to recruit more fast-twitch muscle fibers, which raises oxygen demand.
Third, and perhaps most liberating, recovery style did not matter. Easy jogging or pedaling between reps did not keep oxygen uptake higher than simply standing or walking. In sessions taken to exhaustion, passive rest let people complete more reps and accumulate more total time at high intensity.
The catch is that red-zone time is a stand-in, not an outcome. The authors state that the link between minutes near VO2max and real fitness gains is shaky. In one study, cyclists doing short intervals improved VO2max by 6.3 percent despite spending less time in the red zone than a long-interval group that gained only 3.3 percent. Workout design explained only about 6 to 7 percent of the variation in red-zone time; the rest came down to the individual, the equipment and how researchers crunched their data.
There is also a tension inside the paper. When long and short intervals were compared directly in the same people, the long-interval advantage essentially vanished. Well-built short-interval sets, such as 30 seconds hard and 15 seconds easy repeated in blocks, matched long intervals.
Why should anyone not chasing race times care? VO2max is one of the strongest predictors of lifespan in large population studies, and in one major clinic cohort, low fitness carried a death risk comparable to smoking. Raising it in midlife is among the most dependable interventions available. Yet nearly everyone in these studies was a fit young man. Women made up about one in ten participants, and almost nobody was older than 50.
The take-home is sensible but modest: longer, front-loaded intervals with easy rest are a solid default, but the red zone is a guide, not a guarantee.
Actionable Insights
Use 3 to 5 minute work intervals as your default (the classic 4x4 format: four 4-minute hard efforts with 3 minutes easy). Across studies these produced about 9.3 minutes at or above 90 percent of VO2max, versus about 5.5 minutes for shorter formats, a moderate effect (Hedges’ g of 0.87 to 1.06). Put plainly, a typical long-interval session beats a typical short-interval session on red-zone time roughly 73 to 77 percent of the time.
Start each interval hard for the first 30 to 60 seconds, then settle. This added about 165 seconds (between 97 and 223 seconds) of red-zone time, a moderate effect (standardized difference 0.80; wins about 71 percent of the time).
Rest however you like. Active versus passive recovery made essentially no difference (standardized difference of -0.15, statistically indistinguishable from zero). In sessions to exhaustion, passive rest added about 5.5 minutes of high-intensity time because people could complete more reps.
If long reps feel unbearable, blocks of 30 seconds hard and 15 seconds easy are a reasonable substitute.
Why bother: each 1-MET gain in fitness (3.5 ml/kg/min) is linked to about 13 percent lower all-cause mortality in observational data. For someone at 35 ml/kg/min, that is a 10 percent improvement. Get medical clearance first if you are older or have heart disease; this paper contains no data on either group.
Context and Source
- Open Access Paper: Time spent at or near VO2max during high-intensity interval training: a systematic review and meta-analysis
- Institutions: University of Essex (lead); Edinburgh Napier University; Mountain Bike Centre of Scotland; University of Regina; independent researcher, Juiz de Fora
- Country: United Kingdom (lead), with Canadian and Brazilian contributors
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Journal: BMC Sports Science, Medicine and Rehabilitation, 2026
Impact evaluation: The impact score of this journal is 3.4 (2025 Journal Impact Factor, Clarivate JCR released June 2026, up from 2.8 the prior year), evaluated against a typical high-end range of 0-60+ for top general science, therefore this is a Low impact journal. Within sports science and rehabilitation it is a respectable mid-tier, fully open-access specialty title, and its JCR listing places it in the upper half of its category.