Improving your cardio: what really makes VO2max go up

To boost your cardio, interval training works a little better than continuous endurance, especially intervals of a few minutes at high intensity. The first gains are visible within a few weeks, and if you are not progressing, it is most often because the dose is too low. Here is what twenty-one studies in healthy adults measure.

The "cardio" in question here is VO2max: the maximum oxygen uptake that your body can use during exercise. If your question is rather whether cardio interferes with muscle gain, it has its own article: Cardio and bodybuilding: should you choose?

Interval or continuous?

Both work. Across 28 studies and 723 adults aged 18 to 45, high-intensity interval training (HIIT) increases oxygen uptake by 5.5 mL per kilo per minute compared to non-exercising individuals, and 1.2 mL more than continuous endurance. A review of 24 meta-analyses, gathering 429 studies and nearly 13,000 participants, finds the same order of magnitude: +3.25 to +5.5 mL compared to no training, +0.52 to +3.76 mL compared to moderate-intensity continuous training — in healthy adults, overweight individuals, older adults as well as athletes.

The advantage of interval training is therefore real, but modest. In women, a meta-analysis dedicated to them finds no difference at all: about +3.2 mL in both cases. What made the difference was the number of training sessions and the length of the intervals.

Which intervals?

In 40 moderately trained men, for eight weeks, two interval training sessions increased VO2max much more than long runs at 70% of maximum heart rate or at the 85% threshold: +5.5% for 15-second intervals, and +7.2% for four sets of four minutes. The heart pumped about 10% more blood with each beat.

"All-out" sprints also work, but less well in athletes. In 48 well-trained men, the 4 × 4 minute protocol increased VO2max by 6.5%, compared to 3.3% for eight 20-second sprints and a non-significant gain for ten 30-second sprints. The same protocol in 81 well-trained women gives +7.3% for the 4 × 4 minute protocol, and no increase for the 20-second sprints. A meta-analysis of 37 studies in sedentary or active adults under 45 goes in the same direction: +0.51 litres per minute on average, and +0.8 to +0.9 litres in the nine studies with the longest intervals. In elite athletes, a meta-analysis finds the clearest effect when recovery between two intervals lasts at least two minutes.

In beginners, sprints remain an option that takes little time: in healthy young adults, they increase VO2max by about 3.6 mL (roughly 8%), with no significant difference compared to continuous endurance, and across nineteen studies lasting two to eight weeks, VO2max increases by 4.2 to 13.4%. Even low-volume interval training improves cardiorespiratory fitness more than continuous training — without changing fat mass, however.

What if you already do a lot of endurance?

For endurance athletes, the question becomes the distribution of intensities over the week. In 48 well-trained athletes, for nine weeks, "polarized" training — a lot of easy, a little very hard, almost nothing in between — increased VO2max by 6.8 mL (11.7%), time to exhaustion by 17.4%, and maximal power by 5.1%. High-volume blocks and threshold-focused blocks did not provide any additional significant improvement.

In twelve sub-elite runners followed for five months, those who spent 80.5% of their time in the easy zone and 8.3% in the hard zone gained 157 seconds on their test run, compared to 121.5 seconds for those who spent more time at moderate intensity. The meta-analysis on the subject leans in the same direction (a moderate effect on time trials), but is based on only four studies.

How long before seeing an effect?

A few weeks. In twenty sedentary women, two interval training programs increased VO2max by about 22% in 12 weeks. But the most intense one had already achieved about 60% of the progress after the first three weeks, compared to 20% for the less intense one. A meta-analysis comparing men and women finds identical gains between the sexes (+3.50 mL in men, +3.34 mL in women), and more effective programs beyond four weeks.

Age is no barrier: in middle-aged and healthy older adults (14 studies, 429 participants), interval training increases VO2max by 1.10 mL more than moderate continuous training. And in the 2015 meta-analysis, those who were less fit at baseline, and those with the longest programs, made the greatest progress.

Why some progress more than others

In the HERITAGE study, 481 sedentary adults from 98 families followed the same twenty-week program. VO2max increased by an average of about 400 mL per minute, but with huge variations from person to person — 2.5 times greater between families than within the same family. The authors estimate the heritability of the response at up to 47%.

Other works strongly qualify this reading. A meta-analysis of individual participant data from 1,879 participants finds no solid evidence of true "talent" differences for progress: most of the gaps come from measurement errors and what people do outside the program. And the more a program helps progress on average, the more everyone is likely to benefit. After six weeks of short sprints, in 117 adults, 18% were likely non-responders, and 49% had a gain deemed clinically significant.

In short

To fit one or two interval sessions into your week without sacrificing the rest, the workout generator builds a custom program for you, and the exercises page details the movements.

Every figure in this article comes from a study you can open: the underlined mentions lead to the summary of the work it is drawn from, written from the abstract alone. Informative content, which does not replace the advice of a health professional