Understanding and improving VO2max, what the number really says about your endurance

What VO2max actually is

VO2max is maximal oxygen uptake, meaning the amount of oxygen your body can use per minute at most in a state of maximum exertion. It is usually given relative to body weight, namely in millilitres of oxygen per kilogram of body weight per minute.

Why relative and not absolute? The absolute value in millilitres per minute does not take body weight into account, which is why tall and heavy people sit systematically higher there. For sports in which you move your own weight, meaning running, cycling or cross-country skiing, the relative value is therefore more meaningful and comparable between people.

The value bundles three processes into one number, namely the uptake of oxygen through breathing, its transport via the heart and circulation, and its use in the working muscle. By the Fick principle, VO2max is the product of cardiac output and the oxygen difference between arterial and venous blood, meaning of pump capacity and extraction. Because lung, heart and muscle work together, the quantity is called cardiorespiratory fitness in research, meaning the capacity of breathing and circulation in combination.

What the value says and what it does not

VO2max does not directly show your endurance performance capacity. It is the ceiling, not the performance itself. In sports science it is therefore called the gross criterion of endurance performance.

The reason is a practical one. The intensity at which you actually reach your VO2max can only be held for about five to eight minutes, and exactly how long is highly individual (Billat and Koralsztein 1996 report a mean of around six minutes with a spread of about 25 percent between individuals). For a race over 8 or 42 kilometres that is irrelevant.

What is decisive is a second quantity, namely what percentage of your VO2max you can call on continuously. This limit of sustained performance, often called the anaerobic threshold, describes the intensity you can hold for about 30 minutes. The standard criterion is the maximal lactate steady state, meaning the highest load at which blood lactate rises by no more than 1.0 millimoles per litre between minute 10 and minute 30 of the test (Kindermann, Anaerobe Schwelle). How high this share sits is regarded as an indicator of your training status.

In practice this means that two people with identical VO2max can run at different speeds. Someone who calls on 85 percent of their ceiling continuously beats someone with the same ceiling and 75 percent. Anyone looking only at the number misses half the equation.

Which VO2max values are normal

The most robust reference is the FRIEND registry, the largest collection of cardiopulmonary exercise testing results in the United States. For heart-healthy men aged 20 to 29 the median there is 48.0 millilitres per kilogram per minute, for women of the same age group 37.6 (Kaminsky et al. 2015, treadmill, 4611 men and 3172 women).

Two things for context before you compare your value. First, these are percentile values from a reference population, not medical normal values. A percentile says which share of the comparison group sits below your value, so at the 10th percentile one tenth. There is no binding German guideline that sets VO2max normal values. Second, the tables from different sources differ considerably, because they rest on different samples and test protocols.

The value falls with age, and measurably so. Across five decades it falls in the FRIEND registry of 2015 by 9.2 percent per decade in men and by 10.3 percent in women; the 2022 version gives around 4.0 millilitres per decade. Between the 20 to 29 year olds and the 40 to 49 year olds there are therefore a good ten millilitres in the men's median, namely 48.0 against 37.8 (Kaminsky et al. 2015).

That is the part you do not train away. What you can shift is your position within your age group, and that room is the larger one. Among men aged 20 to 29, the FRIEND registry of 2015 has almost 30 millilitres between the 10th and the 90th percentile, namely 32.1 against 61.8, so considerably more than the good ten millilitres between two decades. Trained people also visibly flatten the age curve. Ageing does cost something in real terms, but training gains more within the same age group than two decades cost.

Why the number on your watch is not your VO2max

Watches do not measure VO2max. They estimate it from heart rate and pace using formulas that were tested on small, very specific groups.

One widespread method uses only the ratio of maximum to resting heart rate. Uth et al. 2004 validated it on 46 well-trained men aged 21 to 51, there with an estimation error of 2.7 millilitres per kilogram per minute, which they rate as favourable compared with other indirect tests. Transferability to other groups would, according to the authors, first have to be validated directly. For everyone who is untrained, female or older, that is the decisive limitation.

Another method, the Cooper test, estimates the value from the distance covered in twelve minutes. Both give a ballpark figure, not a measurement.

How large the deviation is is shown by a validation study of the Apple Watch Series 7 against cardiopulmonary exercise testing, in which the mean absolute error was 15.8 percent and agreement with the laboratory value was weak, with an intraclass correlation of 0.47 instead of 1.0 for perfect agreement (Caserman et al. 2024). The authors additionally describe a bias towards the middle, meaning overestimation at low fitness and underestimation at very good fitness. If your watch places you as a trained person below your laboratory value, that is to be expected and not a defect.

Two rules follow from this. Use the number as a trend under the same conditions, because whether it rises or falls over months is meaningful. And do not compare it with the number from another watch or from a laboratory test.

Everyday factors also distort the estimate. Heat, gradient, an infection or poor sleep change heart rate at the same pace, and the algorithm reads that as changed performance capacity.

How to have your VO2max actually measured

The standard is cardiopulmonary exercise testing. On a treadmill or ergometer, maximum exertion is brought about while a breathing mask measures the oxygen content of the exhaled air and the breathing volume. From that and your body weight the relative VO2max follows.

An even more precise method determines the oxygen content in arterial and venous blood, but requires catheters under load and therefore practically never occurs in routine diagnostics.

Ask yourself what you need the value for. For clean training zones, performance diagnostics make sense; for the question of progress, the trend from your watch is enough.

How to improve your VO2max

VO2max rises most reliably through the combination of plenty of easy volume and a clearly limited hard share. What does not work is training permanently in the middle range, because that is too hard for good recovery and too easy for a strong stimulus.

The effectiveness of individual intensities has been disputed among coaches for decades. What can be observed is the practice in endurance sport. Seiler and Kjerland 2006 analysed the training of nationally competitive junior cross-country skiers and found around three quarters of sessions clearly below the anaerobic threshold and 15 to 20 percent clearly above it, while the range in between was barely used. How high that leads at the top end is shown by data from Tønnessen et al. 2015 from Winter Olympics between 1990 and 2013, where medal winners in distance cross-country skiing sat at about 84 millilitres per kilogram per minute, the women at 72.

For someone in full-time work with four sessions a week, that translates as follows.

• Three easy sessions at a steady pace at which you can speak in full sentences. How to find this range without a laboratory is covered in the article on zone 2 training.
• One hard session with intervals in the range you can only hold for a few minutes. Four to six repetitions over three to five minutes with a similarly long recovery are classic, which fits well with the five to eight minutes that are possible at VO2max at all.
• Patience with a realistic time frame. In controlled studies, gains are often measurable after four to six weeks, most clearly from a low starting level. Milanović et al. 2015 find, across 28 controlled studies with 723 participants, 4.9 millilitres per kilogram per minute more from endurance training and 5.5 from interval training. What does take months is reliably reading it off the watch, because the estimation spread of the devices is of the same magnitude as the early effect.

A second lever is often overlooked. Because the value is expressed relative to body weight, relative VO2max also changes when weight changes and absolute oxygen uptake stays the same. That is mathematics, not improved endurance, and the two should be kept apart.

What VO2max has to do with your blood values

VO2max depends on how well your body transports oxygen, and that is exactly where two laboratory values come in that no training plan replaces.

Iron. Iron is a component of haemoglobin and therefore directly involved in oxygen transport. Ferritin is the storage protein for iron, so its value in blood shows how well the stores are filled. A low store value goes unnoticed for a long time, because the signs such as fatigue and heavy legs are hard to distinguish from overtraining. Anyone who runs a lot and still stagnates is therefore better off having ferritin measured and placed in context medically, together with a blood count and inflammatory markers, because ferritin rises during inflammation. A performance benefit is only established where a deficiency has been demonstrated, for instance in iron-depleted women (J Nutr 2019), while a meta-analysis on iron status in sport gives a more nuanced picture (Sports Med 2024). This is a pointer for diagnostics and explicitly not a recommendation to take iron on your own initiative, because without a demonstrated deficiency iron is not harmless.

Recovery. Resting heart rate and heart rate variability show whether the next hard session makes sense. Heart rate variability is the fluctuation in the time intervals between two heartbeats. If your resting heart rate rises over several days and variability falls, you are better off moving the session rather than forcing it. Because VO2max only grows through a few hard stimuli, your recovery decides how many of them take effect.

That is not an argument against training diligence, but one for knowing your own values. Further articles on this are collected under performance and training.

What a high VO2max means in everyday life

A high VO2max means, first of all, only that your body can use more oxygen under maximum exertion. It becomes interesting because, as a measure of cardiorespiratory fitness, it is widely studied and is linked to health endpoints.

How strong this connection is we described in the article on zone 2 training with the underlying cohort data. Here the sober version is enough. These are observed associations from observational studies, not a promise for the individual case.

In practice the recommendation stays the same, whether your goal is a personal best or health across decades. Plenty of easy volume, few hard stimuli, real recovery in between.