Why most AI body scans guess, and what "Not visible" means

Published September 4, 2026 · 3 min read

An AI physique grade is a coaching opinion produced by a vision model. Models are not measuring instruments, and most apps never check whether theirs gives the same answer twice. We did, on our own product, and published the result. This page is that result, what we changed, and what "Not visible" means when you see it on a FlexScan report.

The test we ran on ourselves (August 26, 2026)

Sixteen scans through the live FlexScan grader: one person, one front photo scanned six times at the default settings, plus darker, brighter and heavily compressed versions of it, an arms-up front, and a side. Every response was kept and every grade was compared. The person was a licensed stock model; no user photos were involved.

The same file scanned six times produced six different grade sets. Not one pair matched.
Chest swung from A+ to B, and core from A to B-, on byte-identical input, minutes apart. Four grade steps.
The grader invented grades for glutes in two of twelve front scans, on a photo where glutes are not visible. The "cannot see it" judgement itself was flipping.
The advice moved with the grades: across six identical scans, the "focus on this" list named five different muscle sets.
Lighting changes mattered less than that noise. Heavy compression was the only condition that pushed grades in one direction (core and legs down one to two steps).

Our own verdict at the time: not stable enough to sell "re-scan to measure progress". The run-to-run noise was bigger than any change a real person could produce in weeks of training.

What we changed

We pinned the model's randomness (temperature zero, a fixed seed). Re-run six times on the same photo: seven of the eight muscle groups came back identical every time; one group moved by one step in one of the six runs.
A one-step change is never reported as progress. The comparison screen requires a move bigger than the grader's own wobble before it draws an arrow, and it says so.
Back is never graded from a front photo. It is the one judgement that was stable in all sixteen scans, and the rule is now explicit for every group: grade only what is clearly visible, otherwise write "Not visible".
Every change to the grader is re-tested the same way before it ships. The test is part of the release gate, not a one-off.

What "Not visible" means on your report

It means the coach was not shown enough to grade that group and refused to make one up. From a front photo that is normally back and glutes; from a side photo, often core. It is not a bad grade and it is not counted against your score. You can tap the muscle on the map and rate it yourself, or add the missing angle next time. The example scan is a real report with two groups left "Not visible" on purpose, so you can see exactly what it looks like before you sign up.

Why we do not print a body-fat percentage

A photo has no depth, no density and no view of what a shirt covers. Some apps publish a comparison of their estimate against DEXA and quote a range, which is the honest version of that number. We would still rather grade what can be seen than print a percentage that looks like a measurement. If you want body fat, use a method that measures it, and photograph yourself the same way every time so the two agree about direction.

What we still cannot promise

A grade is an opinion about muscle development from pixels. Two coaches would disagree too.
Pose, distance and compression move grades. Shoot the same way every time or the comparison is comparing photography.
The model can change under us. That is why the sixteen-scan test is re-run, not remembered.
None of this is a medical assessment or a body measurement.