In one sentence
The lean mass in your arms and legs divided by the square of your height, the research world's standard measure of whether you carry enough muscle for your size.
Why it matters
Arms and legs are almost all muscle, bone and skin, with no organs to confuse the picture, so their lean mass is the cleanest measure of skeletal muscle a scan or a scale can give. The European consensus on muscle loss with age uses it, with reference points of 7.0 kg/m² for men and 5.5 for women by DEXA, below which muscle quantity is called low (Cruz-Jentoft 2019). The same consensus puts strength first: low grip strength is the finding that matters, and a low index confirms it. VT shows the index as context for that conversation, not as a verdict.
What moves it
Resistance training and protein, slowly; age, slowly in the other direction, with the loss concentrated in the legs after the fifth decade (Janssen 2000); and, on a scale, hydration, immediately.
How to measure it well
A DEXA scan gives the reference value. A scale with a five-segment reading (each arm, each leg, the trunk) gives an estimate, and the consensus notes that bioimpedance equations differ by device, so DEXA cut-points do not transfer to a scale's figure directly.
How Vitality Tracker uses it
Computed from the five-segment scale reading or from a DEXA report, kept per device, charted as a trend, and shown with the consensus reference points and their source; it feeds the clinician report's muscle context. The founder's scale reports 9.4.
How far to trust the number
Estimate from a scale; laboratory-grade from DEXA. The scale's trend is believable; its level against the DEXA cut-points is not.
Sources
- Cruz-Jentoft AJ, Bahat G, Bauer J, et al. Sarcopenia: revised European consensus on definition and diagnosis. Age Ageing 2019;48:16–31. doi:10.1093/ageing/afy169
- Janssen I, Heymsfield SB, Wang ZM, Ross R. Skeletal muscle mass and distribution in 468 men and women aged 18–88 yr. J Appl Physiol 2000;89:81–8. doi:10.1152/jappl.2000.89.1.81.