In one sentence
Your lean mass divided by the square of your height: how muscular you are for your size, the way BMI says how heavy you are for your size.
Why it matters
BMI cannot tell a heavily muscled person from a heavy one; FFMI can, because it leaves the fat out. It is the number strength-focused members use to follow muscle over years, and the one that answers whether a diet kept the muscle. The reference points come from two places. In 157 male athletes, the FFMI of those who had never used anabolic steroids ran up to a well-defined ceiling of about 25 (normalized to a height of 1.8 m), and the pre-steroid-era bodybuilding champions sat at about 25.4, while steroid users ran well past it (Kouri 1995). In a Swiss reference population, typical adult values sat in the high teens to low twenties for men and the mid to high teens for women, falling with age (Schutz 2002). Both are shown as what they are: a ceiling observed in one sample and the spread of ordinary adults.
What moves it
Real lean mass, slowly; on a scale, hydration, immediately, as with everything derived from lean mass.
How Vitality Tracker uses it
Computed from the scale's lean mass (DEXA-corrected when you have a scan) and shown with the sex-specific reference points and their sources; charted as a trend; never scored.
How far to trust the number
As good as the lean mass behind it: a within-device trend from a scale, a reference-grade value from DEXA.
Sources
- Kouri EM, Pope HG, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med 1995;5:223–8. doi:10.1097/00042752-199510000-00003
- Schutz Y, Kyle UUG, Pichard C. Fat-free mass index and fat mass index percentiles in Caucasians aged 18–98 y. Int J Obes 2002;26:953–60. doi:10.1038/sj.ijo.0802037.