What Is VO2 Max? Why It Determines Running Ability

VO2 max is one of the most-cited numbers in endurance sports — but what does it actually tell you? If you’ve been studying running, you’ve likely come across the term maximal oxygen uptake. It’s widely recognized as one of the strongest predictors of long-distance performance.

Yet surprisingly few runners fully understand what VO2 max really means. I’ll admit that when I first started running, I had some misconceptions about it myself.

In this article, I’ll break down exactly what VO2 max is, why it determines running ability, and how your physiology shapes your aerobic ceiling. By the end, you’ll have a clear picture of what VO2 max truly represents — and why it’s only part of the performance equation.

What Is VO2 Max? Why It Determines Running Ability

The Definition and Units of VO2 Max

VO2 max (maximal oxygen uptake) is a measure of the maximum amount of oxygen your body can take in and use per minute. It’s expressed in ml/kg/min — milliliters of oxygen per kilogram of body weight per minute.

You may also see it written as VO2max or VDOT (the notation used in Jack Daniels’ Running Formula, short for “V-dot O2 max”).

Think of it as a measure of how much oxygen your body can consume during exercise — or equivalently, how much energy your aerobic system can generate at its absolute peak.

The higher your VO2 max, the more oxygen you can deliver and convert into energy, which translates to better sustained performance in marathon and long-distance running.

The diagram below illustrates what VO2 max represents in the context of oxygen use and energy production.

As the diagram shows, VO2 max is a broad measure that encompasses everything from lactate release to lactate utilization. In a wider sense, VO2 max also includes aspects of lactate threshold (LT).

One important point: VO2 max measures oxygen consumption, not the total energy ultimately produced. There are also efficiency losses when converting that energy into running speed.

This means a high VO2 max does not automatically guarantee strong marathon performance — other factors play a significant role as well.

Why a Higher VO2 Max Means Faster Running

At the population level, VO2 max correlates strongly with marathon performance. Higher VO2 max generally means faster marathon times.

However, as performance levels rise among faster runners, the relationship between VO2 max and marathon time actually weakens.

※The graph below is a conceptual illustration.

When the VO2 max–marathon correlation weakens at higher performance levels, it means other factors are playing a larger role.

The three determinants of running performance are VO2 max, lactate threshold (LT), and running economy ※5. At elite levels, LT and running economy become the primary differentiators.

Put another way: a high capacity to use oxygen doesn’t automatically translate to fast marathon times. If your running mechanics or energy utilization pathways are inefficient, energy is lost in conversion — and the gains from VO2 max are never fully realized.

What Determines Your VO2 Max?

VO2 max is determined by three main physiological factors.

The combined capacity of cardiac output, heart rate, and arteriovenous oxygen difference is what we express numerically as VO2 max.

Of these three factors, maximum heart rate is extremely difficult to improve through endurance training — unless you’re a complete beginner or have a compromised cardiovascular system.

The two factors that respond meaningfully to training are cardiac output and arteriovenous oxygen difference. Improving VO2 max requires developing both.

The Physiology Behind VO2 Max

Let’s take a deeper look at the physiological factors that determine VO2 max, breaking each component down in turn.

Breaking Down the Components of VO2 Max

To understand VO2 max more fully, let’s examine each determining factor in detail. Note that maximum heart rate is excluded here because it is largely unresponsive to endurance training.

As the figure shows, VO2 max is shaped by multiple interacting factors. Let’s work through each one.

How Training Changes Cardiac Output and Arteriovenous Oxygen Difference

The chart below shows how maximum cardiac output and arteriovenous oxygen difference change when a sedentary adult male follows an endurance training program.

Endurance Training and VO2 Max Role of Maximal Cardic Output and Oxygen Extraction, David Montero ¹, Candela Diaz-Cañestro, Carsten Lundby

A meta-analysis by Montero et al. (2015) ※3 covering 13 studies found that VO2 max gains from 5–13 weeks of endurance training were driven primarily by increases in cardiac output (central adaptation), with no significant improvement in arteriovenous oxygen difference in the short term. The peripheral adaptation — improvement in a-vO₂ diff — develops with longer-term training.

Over years of accumulated training, cardiac output tends to plateau relatively early, while arteriovenous oxygen difference continues to improve.

The sections below explain what drives each of these two factors.

What Determines Cardiac Output

Cardiac output is determined by three elements: cardiac muscle strength, preload, and afterload.

Cardiac muscle strength refers to how well-developed the heart muscle is. It responds to sustained exercise. The exercise intensity that maximizes cardiac muscle contraction is approximately 60% VO2 max — roughly the pace of a slow jog.

This means repeated easy jogging is sufficient to develop cardiac muscle strength over time.

Here’s a summary of the other two determinants of cardiac output — preload and afterload:

The increase in venous return is attributed to venous constriction, the muscle pump, and the respiratory pump — though the detailed mechanisms are beyond the scope of this article.

It’s easy to see how sustained endurance training progressively develops capillary density in muscle tissue, reducing afterload over time.

Arteriovenous Oxygen Difference: Muscle Blood Flow, Capillary Density & Mitochondria

The arteriovenous oxygen difference represents the gap between oxygen content in arterial blood (flowing from the heart to the muscles) and venous blood (returning from the tissues to the heart). In practical terms, it reflects the muscles’ ability to extract oxygen from the blood.

This difference is governed by muscle blood flow, capillary density, and mitochondrial content/function.

Increased sympathetic nervous activity during endurance exercise reduces vasoconstriction, boosting muscle blood flow. Sustained training also increases capillary density and improves both mitochondrial content and function.

It’s worth noting that mitochondrial content increases with greater total training volume, while mitochondrial function improves with higher training intensity.

For a deeper look at mitochondria in endurance running, see the related article below.

The combined effect of increased muscle blood flow, capillary density, and mitochondrial development raises the arteriovenous oxygen difference and, in turn, VO2 max.

Because capillary growth and mitochondrial adaptations take considerable time, meaningful gains in arteriovenous oxygen difference require months or years of consistent training.

Genetics and Individual Variation in VO2 Max

Just as some people are naturally faster than others, VO2 max is substantially shaped by genetics — approximately 50%, according to the research. Bouchard et al. (1999) ※1 estimated heritability at 47%.

There’s also wide individual variation in how much VO2 max responds to training. On average, endurance training improves VO2 max by 10–20% (Bacon et al., 2013 ※2 reported approximately 15–18% in a meta-analysis).

However, the HERITAGE Family Study (Bouchard et al., 1999) ※1 showed more than a 2.5-fold difference in training response within the same program, with genetic high-responders achieving substantial gains.

The chart below illustrates how genetic baseline and training response vary across individuals.

In this chart, person ⑤ starts with a low VO2 max and shows little training response, while person ① starts high and can improve substantially with consistent training.

How to Measure VO2 Max

There are two main methods for determining your VO2 max.

The direct method gives an accurate measurement but requires a laboratory with the appropriate equipment and involves a testing fee.

The indirect method estimates VO2 max from past race results. For most recreational runners, the indirect method is far more practical.

Among indirect methods — including the 12-minute run test and shuttle run — the most comprehensive approach is the VDOT calculation developed by Dr. Jack Daniels. Using your previous race results, you can estimate your VDOT (= VO2 max) across a wide range of distances.

The best race distance for estimating VO2 max is the 3,000m or 5,000m — with 3,000m being slightly more accurate. This is because VO2 max is defined as the intensity sustainable for approximately 11 minutes, which corresponds to the effort at vVO2max.

Research by Billat et al. (2000) ※4 found that the typical sustainable duration at vVO2max is around 5–8 minutes, and the 3,000–5,000m range aligns well with this window.

For most recreational runners, finishing 3,000m takes roughly 9–12 minutes — right in that target zone.

How to Improve VO2 Max: The Key Principles

Any aerobic training can improve VO2 max to some degree. But low-intensity jogging eventually provides insufficient stimulus, and VO2 max gains plateau.

The key conditions for effective VO2 max training are:

Training effect is determined by the combination of intensity × duration per session × total accumulated time. For a full breakdown of interval design, rest duration, and the research behind effective protocols, see the article below.

References

※1 Bouchard C, An P, Rice T, Skinner JS, Wilmore JH, Gagnon J, Pérusse L, Leon AS, Rao DC (1999) “Familial aggregation of VO2max response to exercise training: results from the HERITAGE Family Study” Journal of Applied Physiology

※2 Bacon AP, Carter RE, Ogle EA, Joyner MJ (2013) “VO2max Trainability and High Intensity Interval Training in Humans: A Meta-Analysis” PLOS ONE

※3 Montero D, Diaz-Cañestro C, Lundby C (2015) “Endurance Training and VO2max: Role of Maximal Cardiac Output and Oxygen Extraction” Medicine and Science in Sports and Exercise

※4 Billat VL, Slawinski J, Bocquet V, Demarle A, Lafitte L, Chassaing P, Koralsztein JP (2000) “Intermittent runs at vVO2max enables subjects to remain at VO2max for a longer time than intense but submaximal runs” European Journal of Applied Physiology

※5 Bassett DR Jr, Howley ET (2000) “Limiting factors for maximum oxygen uptake and determinants of endurance performance” Medicine & Science in Sports & Exercise

※6 Swain DP, Abernathy KS, Smith CS, Lee SJ, Bunn SA (1994) “Target heart rates for the development of cardiorespiratory fitness” Medicine and Science in Sports and Exercise