Fitness

VO2 max

Your standing aerobic ceiling, and where it sits against the population for your age and sex.

Empirical estimate

The runs anchor is an empirical estimator (see Aerobic index); the norm bands are peer-reviewed descriptive statistics from the FRIEND registry, which vary by test mode and edition.

Your VO2 max is your standing fitness ceiling. Where the Aerobic index is the fitness a single run showed, this is the size of your aerobic engine right now, the number your better runs keep proving over the last six weeks.

How we settle on one number

Every steady run gives us a per-run VDOT (that is the Aerobic index). We take the 85th percentile of those estimates over the last 42 days. The 85th percentile means one great run cannot spike your ceiling, but your genuinely better runs are what define it, not your average day.

Then we blend that with two other anchors and keep the highest of the three:

What your runs demonstrate (the 85th-percentile VDOT above).
A VO2 max reading you enter (for example from a watch).
A recent race result.

The highest wins on purpose. A conservative anchor (a cautious watch reading, an old race) should not drag down what your runs have already proven you can do.

VO2_max = max(runs, entered reading, recent race)

Where you land: the norm bands

A raw VO2 max means little on its own, so we place it against population norm bands for your age and sex: Poor, Fair, Good, Excellent, Superior, Elite. The bands come from the FRIEND registry (Kaminsky and colleagues), a large set of healthy adults measured by treadmill test, which is the same effort your running reflects. The medians quoted here are from the registry's updated (post-2019) standards.

Two representative 50th-percentile (median) values give the scale: men aged 20-29 sit around 49.5 ml/kg/min, women aged 20-29 around 40.6 ml/kg/min.

It falls with age, and it accelerates

VO2 max declines with age at roughly 0.4 ml/kg/min per year on average, but that average hides the shape. The decline is nonlinear and speeds up: about 3-6% per decade in your 20s and 30s, rising to over 20% per decade after age 70. Training slows the fall but does not stop it, which is why the bands shift down as you get older.

Representative FRIEND 50th-percentile VO2 max by age. Both lines fall, and the drop steepens with each decade.

Worked example

A 35-year-old whose recent runs demonstrate a VDOT of 48, who enters a watch reading of 46, and has a recent race worth 47:

Runs anchor: 48 ml/kg/min
Entered reading: 46 ml/kg/min
Recent race: 47 ml/kg/min
Ceiling: max(48, 46, 47) = 48

So their VO2 max ceiling is 48, set by what their runs prove rather than the more cautious watch or race figure. For a 35-year-old that lands in a strong band for their age.

Read the bands as representative, not exact

Norm values depend on the test mode (treadmill values run higher than cycle) and on which edition of the registry they come from. Treat the bands as representative of the population, not exact to the decimal. Your placement can move a little with the reference set without your fitness changing at all.

Honest caveats

The runs anchor rests on the per-run VDOT, which is an empirical estimator (see the Aerobic index), not a lab test. The norm bands are descriptive population statistics, a picture of where healthy adults sit, not a medical diagnosis.

So the two halves sit on different evidence: the estimate of your fitness is empirical, and the norm bands it is placed against are peer-reviewed descriptive statistics. Both are honest, and neither is a lab measurement of you specifically.