How to raise VO2max without losing strength

To raise VO2max, train your aerobic system consistently and add hard intervals: both steady training and intervals work, and intervals work a bit better. In the best-known trial, four 4-minute efforts at 90 to 95% of max heart rate, three times a week for eight weeks, lifted VO2max 7.2% in moderately trained men.¹ Lifting alongside doesn't stop that from working, and the strength cost of adding intervals is small if you keep them away from your heavy leg days.

What actually raises VO2max#

The cleanest summary is a 2015 meta-analysis of 28 controlled trials in healthy adults aged 18 to 45.² Compared with people who didn't train, steady endurance work raised VO2max by about 4.9 ml/kg/min and high-intensity interval training by about 5.5. Head to head, intervals came out ahead by 1.2 ml/kg/min, with a confidence interval that runs from 0.3 to 2.1.

VO2max gain by training type

	Low (ml/kg/min)	High (ml/kg/min)	Mark (ml/kg/min)
Endurance training vs no training	3.5	6.3	4.9
Intervals vs no training	4.3	6.7	5.5
Intervals vs endurance training	0.3	2.1	1.2

Mean effect and 95% confidence limits from Milanović 2015 (28 controlled trials, 723 adults, mean baseline 40.8 ml/kg/min).

Read that chart the right way round. The big win is training at all. Intervals add a bit on top. That matters if you lift, because it means the easy running you already do isn't wasted, and you don't need to turn every run into a suffer-fest to move the number.

The people in those trials were 25 on average and started at 40.8 ml/kg/min. The same analysis found bigger interval gains in people who started less fit,² so if you've been training for years, expect less than the averages.

A second meta-analysis, focused only on intervals, pooled 37 studies of sedentary or recreationally active people under 45 and found an average gain of 0.51 litres per minute (95% CI 0.43 to 0.60).³ The subset that used longer intervals showed bigger gains, around 0.8 to 0.9 litres per minute, with a marked response in every subject. That's the closest thing to a hint that the length of the work bout matters.

Why hard intervals work#

VO2max is limited mostly by how much oxygen your heart and blood can deliver to working muscle, not by how much the muscle can extract, and when training raises VO2max it's mainly through a bigger maximal cardiac output.⁴ That lines up with what the 4x4 trial measured: alongside the VO2max gain, the interval groups' stroke volume went up about 10%.¹

The practical upshot is that intervals train the delivery side by holding you near your ceiling for minutes at a time. Easy running does a lot of other useful things, but it doesn't spend long up there.

It also explains why you don't strictly have to run. If the limit is mostly central, any mode that uses a lot of muscle and drives your heart rate to 90 to 95% of max should train it. That's an inference from the physiology, not a head-to-head trial, so if you race on your feet, do at least some of your hard work on your feet.

Longer reps, or shorter?#

Both work. In Helgerud's trial the 15-second and 4-minute versions both beat steady running, with the 4x4 slightly ahead (7.2% against 5.5%), but the abstract doesn't report a significant difference between those two.¹

The meta-analyses lean towards longer work bouts. Against steady endurance training, interval programs with longer repetitions added about 2.2 ml/kg/min more (plus or minus 2.1), and programs with more work relative to rest added about 1.6 (plus or minus 1.5).² In the interval-only meta-analysis, the subset with longer intervals showed the biggest gains.³ None of that is precise, and the confidence limits are wide. But if you're picking one session, the 4x4 is the one with the best track record.

The 4x4, in detail#

Helgerud's 2007 trial is worth knowing properly, because a lot of "best VO2max workout" advice is a loose version of it.¹

Forty moderately trained men were split into four groups and trained three times a week for eight weeks. The trick was that every group did the same total amount of work, measured as oxygen used, just at different intensities:

Only the two interval groups improved VO2max significantly more than the others. The 15/15 group went from 60.5 to 64.4 ml/kg/min (5.5%), the 4x4 group from 55.5 to 60.4 (7.2%), and stroke volume, the amount of blood the heart pushes per beat, rose about 10% after interval training.

Two details people skip. These men were already fairly fit, so this isn't a beginners-only effect. And the slow and threshold groups weren't lazy: they did the same total work. The difference was intensity.

The way I explain it: the interval is the work, the jog between is the recovery, and the rest period is doing as much work as the effort itself. Cut the rest short and you're not training the same thing anymore. In the 4x4 the 3 minutes aren't standing still either. They're an easy jog at about 70% of max.

Turning it into numbers#

You need your real max heart rate for this, not 220 minus your age. The highest your watch has seen in a genuinely all-out effort is a reasonable stand-in.

Mine is 194, the highest my watch has recorded. That puts the 4x4 work bouts at about 175 to 184 bpm and the recovery jogs at about 136. The whole session, four work bouts and the recoveries between, runs about 25 to 28 minutes before your warm-up.

Getting it wrong#

There are three easy ways to get it wrong. The first is running the recoveries too hard. In the trial they were an easy jog at 70% of max, so the next work bout could actually hit 90 to 95%.¹ The second is going out too fast on the first rep and fading, which turns a 4x4 into one hard rep and three survival efforts. Aim for four reps at the same pace or the same heart rate. The third is guessing your max heart rate from an age formula. In 3,320 healthy adults with a verified max test, the best age formula still had a standard error of 10.8 beats per minute,⁵ so a guessed max can put every target off by 10 beats or more in either direction. The highest heart rate you've seen at the end of an all-out race is usually a better number.

If four reps at that intensity is new to you, three is a reasonable way in. Add the fourth when three feels controlled.

Doing it while you lift#

Here's the part the VO2max articles skip. Does adding intervals cost you strength?

A 2018 meta-analysis looked at exactly this: high-intensity interval training added to resistance training, against resistance training alone.⁶ Muscle growth and upper-body strength gains were the same. Lower-body strength gains were a little smaller, an effect size of minus 0.25, which is small and only just statistically significant.

The broader concurrent-training picture agrees. A 2022 meta-analysis of 43 studies found adding aerobic work didn't compromise maximal strength (SMD minus 0.06) or muscle growth (minus 0.01), though explosive strength took a small hit, mostly when both were done in the same session.⁷ For trained lifters specifically, another meta-analysis found lower-body 1RM gains were blunted when endurance and strength work shared a session (minus 0.66) but barely at all when they were done in separate sessions (minus 0.10).⁸ The interference effect article goes through these in more depth.

Bike or run?#

This is where the evidence argues with itself. An older meta-analysis found running, not cycling, caused the strength and size losses, and that more frequent and longer endurance work cost more.⁹ The 2018 interval meta-analysis found a trend the other way: cycling intervals hit lower-body strength harder than running intervals, though neither subgroup reached significance.⁶ And the 2022 one found no difference between the two at all.⁷

So pick on practical grounds. If your legs handle impact badly the day after squats, a bike or rower interval is fine. If you race HYROX, you'll want most of your hard work on your feet anyway.

Where it goes in your week#

Here's a real week from my log, 14 to 20 September 2026: three runs of about 7 km on Monday, Wednesday and Friday, lifting on Tuesday, Thursday and Saturday, nothing on Sunday.

One week from Ken's log, 14 to 20 Sep 2026

	Run (min)	Lift (min)	Total (min)
Mon	35	0	35
Tue	0	40	40
Wed	34	0	34
Thu	0	24	24
Fri	35	0	35
Sat	0	43	43
Sun	0	0	0

Session minutes by day from Ken's hybridAF training log (runs from his watch, lifting from the watch's strength records), read 2026-10-22.

Every run over 5 km I logged from early September to mid October looks like that: 5.4 to 8.4 km, average heart rate between 145 and 166, and none of them logged as an interval session. That's a fine base. It's also exactly the shape the research says leaves a little VO2max on the table.

The fix in a week like that is a swap, not an addition. Turn one of the three runs into a 4x4. Which one depends on your legs. Put it on a day after an upper-body or light lifting day, so you start it fresh, and keep it at least a few hours from heavy squats or deadlifts, ideally on a different day. The same-session cost in Petré's data was large; the separate-session cost was close to nothing.⁸

Keep the other runs easy. Polarized training, mostly easy with a small dose of hard, beat other intensity mixes for VO2peak in a 2024 meta-analysis, though only modestly (SMD 0.24, 95% CI 0.01 to 0.48) and with no difference in time-trial performance.¹⁰ The 80/20 article covers that debate.

One session a week is a starting dose, not the Helgerud protocol, where the groups did their sessions three times a week. If you're also lifting three or four times a week, one hard session is a sensible place to begin. Add a second only if your lifts hold and you're recovering. The weekly split guide shows how to lay out three to six days.

What to expect#

Nobody can promise you a number. Endurance training studies often show modest changes and very limited responses in some people, which is part of why Bacon's group went looking at intervals in the first place,³ and every trial here reports a group average, not what happens to you. What the evidence does support: consistent aerobic training plus a weekly dose of hard intervals should move VO2max over a couple of months, more if you're starting low, less if you've trained for years.

Measure it the same way before and after: the same 5K course, similar weather, or the same watch on the same kind of run. If you want a reference point, the VO2max by age chart shows where you sit now. The rest of the engine articles are on the engine hub.