Why VO2 Max Isn’t the Flex Most People Think It Is

Everyone obsesses over VO2 Max.

It is the number people compare, the stat watches estimate, and the metric that gets casually dropped into conversations as shorthand for how “fit” someone is. Higher is better. Bigger is faster.

Except it isn’t.

VO2 Max does matter. It is a useful physiological marker and a reasonable predictor of potential. But it has been elevated far beyond what it actually represents, and in doing so it has distracted people from the metrics that really determine performance.

Compounding the problem is the fact that most people do not even know what their actual VO2 Max is.

The vast majority of VO2 Max values people reference are not measured. They are predicted. Estimated by algorithms using heart rate, age, sex, and a series of assumptions layered on top of one another. Some are better than others, but none are direct measurements of oxygen uptake.

They are guesses.

At best, these predictions can loosely track changes over time under controlled conditions. At worst, they give people a false sense of precision and confidence in a number that was never properly assessed in the first place.

And yet that number is often treated as definitive.

Even when VO2 Max is measured accurately in a lab, it still represents only one piece of the performance puzzle.

At its most basic level, VO2 Max is the maximum amount of oxygen your body can take in, transport, and deliver to working muscles during maximal exercise. That is it. It is a measure of supply. A ceiling.

It tells you how big the engine is.

What it does not tell you is how efficiently you can run that engine, how long you can sustain power, or how much of that oxygen you can actually use.

This is where fractional utilization comes in, and why thresholds matter far more than VO2 Max itself. Fractional utilization describes how much of your VO2 Max you can sustain before fatigue forces you to slow down.

That fraction is defined by your thresholds.

Two athletes can have the same VO2 Max and perform at completely different levels because one can operate close to their ceiling for extended periods, while the other cannot. The engine is the same size. The usable portion is not.

Your first ventilatory threshold, often referred to as VT1 or aerobic threshold, represents the highest intensity you can sustain while remaining predominantly aerobic. Below this point, fat oxidation is high, carbohydrate demand is low, and the cost of producing energy is relatively low. You are using a modest fraction of your available capacity, but doing so efficiently and repeatably.

As VT1 improves, you can do more work at a higher fraction of that capacity without accumulating excessive fatigue. Easy becomes easier. Moderate becomes sustainable. Training volume becomes something you absorb, not something you barely tolerate.

Your second ventilatory threshold, VT2, marks the upper limit of sustainable aerobic work, and this is where fractional utilization really separates athletes. Some can operate close to their ceiling for long periods. Others fall apart well below it, regardless of how high that ceiling looks on paper.

This is why thresholds matter more than the ceiling itself.

They tell you how much of your engine you can actually use.

From a physiological standpoint, improving thresholds means improving peripheral adaptations. Mitochondrial density increases. Capillary networks expand. Enzymes involved in aerobic metabolism become more active. Fat oxidation improves. The same oxygen produces more usable energy at a lower relative cost.

As a result, the fraction of VO2 Max you can sustain goes up.

That is performance.

This also explains why chasing VO2 Max directly so often misses the point. Maximal intervals may nudge the ceiling upward, but if fractional utilization does not improve alongside it, very little changes in real world application. You just end up with a bigger engine that you still cannot use effectively.

Threshold-focused training flips that equation.

Time spent around VT1 improves efficiency and raises the fraction of your capacity you can use comfortably for long periods. Work around VT2 increases tolerance to sustained high output and shifts the point at which fatigue accelerates. Together, they expand the usable portion of your aerobic capacity.

Interestingly, when you train this way, VO2 Max often improves as a result. As fractional utilization increases, the demand placed on oxygen delivery rises. Stroke volume adapts. Cardiac output improves. The ceiling lifts as a consequence of better use of what is already there.

You do not chase VO2 Max directly. You earn it by improving how much of it you can actually use.

The problem with glorifying VO2 Max as the headline metric is that it encourages the wrong behavior. People train too hard, too often, guided by a number that is frequently estimated, poorly contextualized, and only loosely connected to real-world performance. Sessions drift above thresholds without purpose. Recovery suffers. Consistency breaks down.

Elite endurance athletes have understood this for decades. Their VO2 Max numbers are impressive, but not always exceptional. What separates them is how close to that ceiling they can operate, and how long they can stay there.

A high VO2 Max is interesting. It tells part of the story.

But performance is not about how big the engine is.

It is about how much of it you can actually use.

Build your thresholds. Improve your fractional utilization.

The rest usually takes care of itself.