Zone 2 is the best way to raise your VO2 max
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Sort of. Steady, easy cardio builds most of your VO2 max, 4.9 points across 28 trials. But intervals added a bit more, and in already fit men, eight weeks of steady running at 70 percent of max heart rate didn’t raise it significantly while four 4-minute efforts did.
What we’d do
Do both. Make most of your week steady and easy, at a pace where you can still talk, because that’s where most of the gain comes from and you can keep it up for years. Then make one or two sessions a week hard, like four 4-minute efforts at 90 to 95 percent of your max heart rate. If you’re already fit and your number has stalled, the hard sessions are the part to add.
If you remember three things
- Steady training works. It raised VO2 max 4.9 points across 28 trials, against 5.5 for intervals.
- Fitter people need the hard minutes. In fit young men, steady running didn’t move it in eight weeks and the 4x4 raised it 7.2 percent.
- Zone 2 is just an easy pace. It’s 60 to 70 percent of your max heart rate on most watches, about 104 to 121 beats a minute at 50.
Why people believe it
Zone 2 is everywhere right now, from podcasts and longevity books to watch apps, all selling hours of easy cardio, slow enough to chat, as the foundation of fitness. Some go further and say it’s the best way to raise VO2 max, and that hard intervals are mostly for show.
There’s a real reason it caught on. Easy cardio is pleasant, it’s hard to get hurt doing it, and you can do a lot of it. And steady training really does raise VO2 max a lot. The mistake is the jump from “it works” to “it’s the best way”, and from there to “you don’t need anything else”.
A simple way to picture it
Think about getting stronger. Carrying groceries and taking the stairs every day keeps your legs strong, and if you’ve been sitting for years it’ll make a real difference at first. But at some point, if you want to lift more, you have to put a weight on the bar that’s hard to move for a few reps.
Steady cardio is the daily carrying. The 4x4 is the heavy weight for your heart. The first gets you a long way, and the second takes you further.
What the trials found
Steady against intervals, 28 trials
723 healthy adults aged 18 to 45, starting at 40.8 on average. Milanović and colleagues, 2015
Against no exercise, steady endurance training raised VO2 max 4.9 points and interval training 5.5. Head to head, intervals came out 1.2 points ahead, with bigger edges for longer intervals, longer programs and the older people in the trials.
Young and middle-aged adults, programs of weeks to months.
The same work, four ways
40 moderately trained men, three sessions a week for eight weeks, matched for total work. Helgerud and colleagues, 2007
Steady running at 70 percent of max heart rate and at 85 percent didn’t raise VO2 max significantly. The 4x4 at 90 to 95 percent raised it 7.2 percent, from 55.5 to 60.4, and short 15-second intervals 5.5 percent.
Fit young men, already around the 80th to 90th percentile for their age.
Five years in people in their 70s
1,567 adults aged 70 to 77, two sessions a week. Stensvold and colleagues, 2020
Both the steady group (50 minutes at about 70 percent) and the interval group held their fitness. The interval group ended 0.7 points higher.
Fit volunteers, and the comparison group exercised too.
Put together, steady training gets you most of the way, and in fit people the hard minutes are what keep it climbing.
Why the hard minutes add something
VO2 max is capped by how much blood your heart can pump, and training raises it mainly by raising that (Bassett and Howley, 2000). In the 4x4 trial, the men’s stroke volume, the blood pushed out per beat, rose about 10 percent after interval training, and their VO2 max rose with it. The steady groups did the same total work at a lower effort and didn’t get that change in eight weeks.
The effort level is part of the dose. Steady cardio is enough of a push for an untrained heart. A trained heart needs a bigger one.
The best case for zone 2
“Steady training raised VO2 max nearly as much as intervals in 28 trials, it’s easier to keep up, and it’s safer. Why bother with the hard stuff?”
Most of that is right. The pooled difference was only 1.2 points. Steady work got a bigger boost in younger people and in longer programs. Brisk walking alone raised VO2 max in 24 trials of people who had been inactive. And volume counts. In a 2017 study of 78 healthy adults, 69 percent of people training once a week didn’t improve, against none of those training four or five times a week.
If you’re starting from mostly sitting, easy cardio is the right first step, and it might be all you need for a while. The case breaks down for fit people whose numbers have stalled, which is exactly the group in the 4x4 trial. And it misses that you don’t have to choose. One or two hard sessions fit inside a week of easy ones.
What is zone 2, exactly?
On Garmin and most watches, zone 2 is 60 to 70 percent of your max heart rate, described as a comfortable pace with slightly deeper breathing, where conversation is possible. With max heart rate estimated as 208 minus 0.7 times your age:
| Age | Zone 2 | The 4x4 hard minutes |
|---|---|---|
| 30 | 112 to 131 | 168 to 178 |
| 40 | 108 to 126 | 162 to 171 |
| 50 | 104 to 121 | 156 to 164 |
| 60 | 100 to 116 | 149 to 158 |
Different apps define zone 2 differently, and some use a share of your heart rate reserve or a lab threshold instead. The talk test works whatever your watch says. If you can speak in full sentences, you’re in it. Your heart rate zones works out yours.
bodyfat.guide covers the other belief about easy cardio, the fat-burning zone, and the full plan is in how to raise it.
8 sources
- Milanović Z, Sporiš G and Weston M, Effectiveness of high-intensity interval training and continuous endurance training for VO2max improvements: a systematic review and meta-analysis of controlled trials, Sports Medicine, 2015europepmc.org
- Helgerud J and others, Aerobic high-intensity intervals improve VO2max more than moderate training, Medicine and Science in Sports and Exercise, 2007europepmc.org
- Stensvold D and others, Effect of exercise training for five years on all cause mortality in older adults, the Generation 100 study: randomised controlled trial, BMJ, 2020pmc.ncbi.nlm.nih.gov
- Montero D and Lundby C, Refuting the myth of non-response to exercise training: non-responders do respond to higher dose of training, Journal of Physiology, 2017europepmc.org
- Murphy MH and others, The effect of walking on fitness, fatness and resting blood pressure: a meta-analysis of randomised, controlled trials, Preventive Medicine, 2007europepmc.org
- Bassett DR and Howley ET, Limiting factors for maximum oxygen uptake and determinants of endurance performance, Medicine and Science in Sports and Exercise, 2000europepmc.org
- Garmin, Forerunner 245 owner's manual, Heart rate zone calculations, checked 2026-11-14www8.garmin.com
- Tanaka H, Monahan KD and Seals DR, Age-predicted maximal heart rate revisited, Journal of the American College of Cardiology, 2001europepmc.org
