Picture a reader who got a DEXA scan in 2023. The machine said 23.6% body fat. That number now lives in their phone, their identity, and three conversations they've had this week with their partner. They've compared it to their wearable's smart-scale reading, their gym's skinfold result, and a figure they found online that called 23.6% "athletic." The original machine was a Hologic. The wearable is a smart scale. They are not comparable. The number has a confidence interval of plus or minus three percent. The number is three years old.
That reader is not unusually unwell. That reader is a cautionary example.
Inside this one
- Reliability vs validity — the distinction the testing industry sells you the confusion of
- The screening reality: most healthy adults don't need clearance to start exercising
- The actual injury-prediction performance of the FMS (AUC 0.587)
- Grip strength as a cheap, well-validated mortality predictor
- The body composition hierarchy: DEXA, BIA, skinfolds, BMI — and what each one can and can't say
- A baseline-setting framework that won't turn into daily anxiety
The standard it is held to
Every factual claim in this volume names its source, and every claim carries a confidence grade — settled, strong, emerging or contested. Where the evidence is thin, the text says so. How it’s made, in full → · What we’ve got wrong →