VO2 max is worth understanding, and the obvious next question is what actually moves it.
The training literature answers that reasonably clearly, including the part most articles skip: how unevenly people respond to identical training, and how little the number moves on the timescale you are probably watching.
Key takeaways
Interval training improved VO2 max by about 4.9 mL/kg/min and steady moderate training by about 1.9 in healthy young-to-middle-aged adults (Milanović et al. 2015).
Trials run 12 to 20 weeks. This is a months metric, not a weeks metric.
In 481 previously sedentary adults doing 20 weeks of supervised training, responses ranged from essentially nothing to gains of over 1,000 mL per minute, and about 47% of that variation ran in families (Bouchard et al. 1999).
Nearly everyone improves. How fast is partly luck.
What kind of training raises it most?
Intervals, by roughly a factor of two and a half, though both work.
A meta-analysis of training trials in healthy young-to-middle-aged adults found interval training improved VO2 max by about 4.9 mL/kg/min on average, against about 1.9 for steady moderate training (Milanović et al. 2015).
Set against the age reference tables, roughly 5 points is a substantial move. It is worth being careful about translating that into "years younger," though, because the rate of age-related decline is not constant: it runs about 3% to 6% per decade in your twenties and thirties and accelerates past 20% per decade after seventy.
Two things that result does not mean. It does not mean steady training is a waste; 1.9 points is a real gain, and it comes from work you are far more likely to keep doing. And it does not mean more intervals are better without limit, since the trials tested a couple of sessions a week alongside a base, not a diet of them.
How much does the base matter?
It is the foundation, and it is the least glamorous ingredient.
Most of what VO2 max measures is plumbing: how much blood your heart moves per beat, how well it is delivered, how efficiently muscle extracts and burns oxygen. Those adaptations are built by repetition rather than intensity.
Brisk walking counts, as do easy cycling, swimming and hiking. General guidance points at around 150 minutes of moderate activity a week, which sounds official until you notice it is roughly 20 to 30 friendly minutes a day.
The practical shape most trials used: a comfortable aerobic base most days, with intensity added on top once or twice a week rather than instead. Heart rate zones covers how to tell the two apart without a lab, and the talk test does that job better than any formula.
Why has my number not moved in weeks?
Because weeks is the wrong unit, and because your watch is smoothing.
The training trials that produced those gains ran 12 to 20 weeks. VO2 max reflects structural adaptation built cell by cell over months, so the underlying quantity genuinely holds steady on a weekly view.
Your device compounds that. Estimates are smoothed and typically only update when fresh outdoor walk or run data arrives, so the line moves like a glacier rather than a ticker.
The practical reading:
A single point up or down is noise. Conditions, terrain and how your watch sampled a given session all nudge it.
Several points across a season is genuine adaptation.
A slow slide during a sedentary stretch is honest feedback, and it is reversible.
The same logic governs walking heart rate, which responds faster and is a better week-to-week companion for exactly that reason.
Why do some people gain so much more than others?
Because trainability is itself partly inherited, and the spread is enormous.
In a study of 481 previously sedentary adults doing 20 weeks of supervised, standardised training, the average gain was solid but individual responses ranged from essentially no improvement to gains of more than 1,000 mL per minute. About 47% of the variation in how much people improved ran in families (Bouchard et al. 1999).
Read that carefully, because it is easy to garble. What is substantially inherited is how much you respond to training, not whether training works. Nearly everyone in that study improved. How fast is partly the luck of the draw.
Two people can follow the same programme for a season and see honestly different results with neither doing anything wrong. That is a strong argument for measuring against your own past self rather than anyone else, and a strong argument against reading someone else's transformation as a benchmark.
Age matters too. Everyone's aerobic ceiling eases downward across the decades. Training does not argue with that; it makes the most of what you have, and you can meaningfully become fitter than your past self at nearly any age.
Does the gain stick?
Better than you would fear, and it fades slower than it feels.
In endurance-trained subjects who stopped entirely, VO2 max fell about 7% over the first 21 days and then stabilised, settling around 16% below the trained value after 56 days. Much of that early loss was blood volume rather than a weaker heart, which is why comebacks feel disproportionately better in the first fortnight. We went into the detail in walking heart rate.
So a fortnight off is not a catastrophe, and rebuilding is faster than building was.
A realistic programme
Nothing here is novel, which is the point.
Most days: comfortable aerobic movement. Conversational effort, 20 to 30 minutes. This is the base, and it does most of the work.
Once or twice a week: short harder efforts. A hill taken with intent, or a minute at an effort where full sentences get choppy, then easy minutes to recover. Challenging, never punishing.
Give it a season. The trials ran 12 to 20 weeks, and that is the honest timeframe for judging anything.
Compare quarters, not weeks. This quarter against last, on the same kind of activity.
Expect your own curve. Given that nearly half the variation in response is familial, someone else's rate is not a target.
Frequently asked questions
How long does it take to improve VO2 max?
Months. Training trials in healthy adults typically run 12 to 20 weeks and found average gains of about 4.9 mL/kg/min from interval training and 1.9 from steady moderate training (Milanović et al. 2015). Weekly comparisons will show you noise.
Is HIIT better than steady cardio for VO2 max?
For this specific measure, yes, by roughly two and a half times in pooled trials. But those trials added intervals on top of a base rather than replacing it, and the steady-training gain was real. The best programme is the one you will still be doing in three months.
Why is my VO2 max not improving?
Most often because not enough time has passed, or because the estimate is being smoothed and only updates on fresh outdoor walk or run data. It is also worth knowing that response to training varies enormously between people, with about 47% of that variation running in families (Bouchard et al. 1999).
Is VO2 max mostly genetic?
Partly, but not in the way people usually mean. Your response to training is about half familial. That is different from fitness being fixed: in the 20-week study nearly everyone improved, just at different rates.
How much cardio do I need per week?
General guidance points at around 150 minutes of moderate activity weekly, roughly 20 to 30 minutes most days, with harder efforts added once or twice a week once the base is routine. Consistency is the active ingredient; intensity is the seasoning.
Knit shows patterns, not diagnoses. Talk to a clinician about readings that concern you.
