Toward Defining Aerobic Capacity

When we define aerobic capacity, the goal isn’t just to sound technical—it’s to make the definition useful.

A definition should give context and guide training, especially if the goal is to build true capacity.

In both scientific literature and coaching circles, aerobic capacity often collapses into a single number: VO₂ max—the maximal rate of oxygen consumption per minute, divided by body weight in kilograms.

It’s neat. It’s measurable. It’s everywhere.

But here’s the catch: VO₂ max measures power, not capacity.

Capacity is about the ability to do more work. Any definition worth keeping must not only describe it but also guide how we measure and expand it.

Why VO₂ Max Isn’t Enough

VO₂ max sets the aerobic ceiling—the theoretical upper limit of aerobic potential. But real performance isn’t about ceilings. It’s about how much of that ceiling you can actually use.

Two athletes can share the same VO₂ max. Same physiology on paper. Same efficiency.

But if one can sustain 85% of it and the other only 60% their real-world performances won’t even be close.

Same ceiling. Different capacity to produce work.

That’s why VO₂ max can’t define aerobic capacity on its own. It’s one piece of the picture, not the whole.

Two Anchors for Capacity

So what else matters?

Pair VO₂ max with lactate threshold (LT). Together, they create two anchors:

• VO₂ max – the ceiling.

• Lactate threshold (LT) – the floor.

And the anchors are made stronger by tying them to performance outputs:

• Velocity at VO₂ max (vVO₂max)

• Velocity at lactate threshold (vLT)

• Wattage equivalents in cycling and erg sports

This connects physiology to performance: how fast you can go, and how long you can hold it - aerobically.

A Metric To Target

This is where fractional utilization comes in—the percentage of VO₂ max an athlete can sustain at threshold.

An athlete with 80–85% is elite. That level of utilization doesn’t happen by accident—it must be developed.

Fractional utilization bridges the ceiling and the floor. It shows how much of your potential you can reliably bring into aerobic performance.

When developed to an elite level, it provides the closest thing to a guarantee of consistency in human performance: the ability to reproduce high-level output not just once, but repeatedly—across sessions, competitions, and under varying conditions.

A Working Definition

So what is aerobic capacity?

“Aerobic capacity is the maximum sustainable work an athlete can perform while the aerobic system dominates—defined by the interaction between vVO₂max and vLT, and expressed as a percentage of fractional utilization.”

This does two things:

1. Acknowledges the ceiling.

2. Classifies how much of that ceiling is usable.

Why It Matters

Why obsess over definitions? Because definitions dictate practice.

If aerobic capacity = VO₂ max, training collapses into VO₂-max workouts.

If aerobic capacity = the interplay between ceiling and floor, training shifts toward expanding metabolism and extending the work that can be sustained in longer time domains.

That’s a different lens. A different philosophy. And, over time, a different kind of adaptation.