Methodology

What the test produces

The test estimates a fitness age: the age at which a person of your sex would sit if their estimated fitness equaled yours. It's an informational, wellness-oriented estimate. It is neither a diagnosis nor medical advice, and measures no biological marker.

The term “biological age” used on our marketing pages is a common phrasing. The product and this page use “fitness age”, which describes exactly what is calculated.

Source

The calculation is based on a non-exercise VO2max estimation model from the HUNT study, conducted in Norway:

Nes BM, Janszky I, Vatten LJ, Nilsen TI, Aspenes ST, Wisløff U. “Estimating VO2peak from a nonexercise prediction model: the HUNT Study, Norway.” Med Sci Sports Exerc. 2011;43(11):2024-2030.

The corresponding public reference tool is the CERG (Cardiac Exercise Research Group) Fitness Calculator at NTNU.

Variables used

• Age and sex. The model has two distinct equations; sex only selects the right one.
• Physical activity index. The product of the frequency, duration and intensity of your sessions.
• Waist. Measured if you know it, otherwise derived from your body mass index — the result is then flagged as partly estimated.
• Resting heart rate. Counted by you over 15 seconds. If you choose to estimate it, the result is flagged accordingly.

No clinical data is collected: no blood pressure, no lab work, no document, no photo, no genetic data.

How the fitness age is derived

Your estimated VO2max is compared to that of a person of your age and sex whose activity, waist and resting heart rate equal the population medians. The gap is converted into years using the equation's age coefficient, then capped at ±15 years around your real age, with a floor of 18.

The calculation is a deterministic, versioned function (current version: v1.0.0). The same answers always give the same result. No language model is involved in the number, or in the choice of your priority lever.

Limits

• A VO2max estimated without an exercise test carries a standard error of 5.7 ml/kg/min in men and 5.14 in women. Translated into age, this margin far exceeds a few years: a fitness age from a single measurement is a fitness marker, not a precise measurement. The ±5-year range shown next to the number is an indicative zone, not a statistical confidence interval.
• That's the point of the product: what matters is not the absolute value of a single measurement, but how it changes as you follow your program. A marker you watch move over time beats an isolated number you believe is exact.
• The ±15-year cap avoids absurd values at the extremes. It doesn't reduce the uncertainty, it limits its visible effects.
• The model was established on an adult Norwegian cohort. Its transfer to other populations is reasonable but not proven.
• Answers are self-reported. An approximate waist or overestimated activity shifts the result.
• That's why the product emphasizes change measured over time rather than the absolute value of a single measurement.

Complementary axes

Sleep, mental load, alcohol, nutrition and circadian rhythm are assessed by simple scales and shown separately. They don't enter the fitness-age calculation. Their dimensions draw on public-domain instruments — PSS-4, AUDIT-C, MEDAS — from which we reuse the measured dimensions, without copying their wording and without diagnostic use.

Sources

• Nes BM, Janszky I, Vatten LJ, et al. “Estimating VO2peak from a nonexercise prediction model: the HUNT Study, Norway.” Med Sci Sports Exerc 2011;43(11):2024-2030.
• Loe H, Rognmo Ø, Saltin B, Wisløff U. “Aerobic Capacity Reference Data in 3816 Healthy Men and Women 20–90 Years.” PLoS ONE 2013;8(5):e64319 (open access).
• The comparison profile — men: waist 94 cm, resting pulse 66 bpm; women: 80 cm and 70 bpm — gives a reference VO2max consistent with this cohort's measured averages, to the precision a fitness marker requires.