What does VO2 max mean? The oxygen your body uses at full effort

VO2 max is the largest amount of oxygen your body can take in, carry and burn in one minute while you work as hard as you can. It is not one organ's score. Breathing, oxygen transport in the blood and the work the muscles do all feed into it, and none of those steps fixes the figure on its own [7].

The number is written in millilitres of oxygen per kilogram of body weight per minute, usually shortened to mL/kg/min. A reading of 40 means the body used 40 millilitres of oxygen for every kilogram of body weight, every minute, at the limit. Dividing by weight is what makes a 60 kg runner and a 95 kg rower comparable at all. Researchers often convert the same measurement into METs, a unit where 1 MET is roughly the oxygen you burn sitting still; 1 MET equals 3.5 mL/kg/min.

A laboratory study of 150 adults aged 17 to 25 shows how wide the spread is. On a treadmill, 30 trained athletes averaged 52.4 mL/kg/min among the men and 41.0 among the women, against 33.4 and 25.1 in 120 non-athletes matched for age and sex [1]. The same athletes recorded lower figures on a stationary bike than on the treadmill, 45.2 against 52.4 for the men, so the protocol is part of the result [1].

A narrative review of cohort studies, trials and meta-analyses reported that fitness measured this way has the strongest and most straight-line relationship with all-cause death rates in preventive medicine, with each extra 1 MET of fitness, that 3.5 mL/kg/min step, linked to a 12 to 15% lower risk of dying [5]. The review did not give the underlying death rates, so what that percentage is worth to one person depends on how high their risk was to start with. It is an association drawn from observation, not evidence that raising the number causes the drop.

Training moves it. Fourteen healthy adults, seven of them around 68 years old and seven around 26, cycled for 45 minutes three times a week for 12 weeks and raised their VO2 max by 20% on average, older and younger together [9].

Oxygen has to get from the air into the inside of a muscle cell, and VO2 max is capped by whichever step on that route runs out first. Air enters the lungs, oxygen crosses into the blood, the heart pumps that blood to working muscle, and the muscle pulls oxygen out of the capillaries and into its mitochondria, the structures inside cells that burn fuel with oxygen. Physiologists split the chain in two: delivery, meaning how much oxygen-carrying blood the heart moves per minute, and extraction, meaning how much oxygen the muscle strips out of that blood as it passes through [7].

The 12-week cycling study measured both halves. Alongside the 20% rise in VO2 max came a 15% increase in how much blood the heart moved per minute at maximum and a 14% rise in how much it pushed out per beat, more capillaries supplying each muscle fibre, and more than double the activity of citrate synthase, an enzyme used as a stand-in for how much oxygen-burning machinery a muscle holds [9]. Maximum heart rate did not change. The people whose heart output rose most were the people whose VO2 max rose most, which makes delivery the likeliest bottleneck in healthy adults [9].

Delivery is not always the limit. When 15 endurance athletes exercised while breathing air with less oxygen in it, the ones who lost the most VO2 max lost it at the muscle, pulling 14% less oxygen out of their blood, while the ones who coped held extraction steady despite a larger fall in heart output [12].

In 20 patients whose lung arteries were blocked by old clots, every step of the chain was impaired at once, and peak oxygen uptake reached 56% of the level expected for their age, against 112% in healthy volunteers [8]. Surgery or balloon treatment raised it from 12.5 to 17.8 mL/kg/min by repairing delivery, and left extraction by the muscles exactly where it had been [8].

These are small studies, and most of them were not built to describe healthy people.

• The 150-person treadmill study was run at one Indian medical college on adults aged 17 to 25 [1]. It shows how far trained and untrained people sit apart at that age. It is not a reference range for a 45-year-old.
• The training study followed 14 people, 9 of them male [9]. Patterns picked out of 14 people are fragile, and individual results varied widely around the 20% average.
• The oxygen-chain study was done in 20 patients with blocked lung arteries [8]. A damaged chain shows how the chain can fail. It does not describe what limits a healthy person.
• The thin-air findings come from 15 trained endurance athletes [12]. Untrained people at sea level may be held back somewhere else.
• The mortality figure comes from a narrative review, where the authors picked which studies to include rather than following a pre-set search [5].

None of this tells you your own number. Estimates from wearables are a separate question: against a laboratory treadmill test in 30 people, Apple Watch readings came out about 6 mL/kg/min low on average, and individual readings fell both well below and well above the measured value [13].

A laboratory test means wearing a mask over the nose and mouth while walking, running or cycling at a load that climbs every few minutes until you cannot carry on. The machine measures the oxygen going in and the carbon dioxide coming out, and VO2 max is the highest oxygen figure recorded before the curve flattens.

Two results from the same person on the same morning can differ if one came off a bike and one off a treadmill [1], and a figure a watch infers from everyday walks and runs is a different kind of number again [13]. The comparison worth making is your own figure, measured the same way twice, months apart.