In one sentence
Your weight split in two: the kilograms that are fat, and everything else, which is muscle, bone, organs, skin and the water in all of them.
Why it matters
The two halves answer different questions. Fat mass is the quantity most people want to change. Lean mass is what sets your resting metabolism: the more of it you carry, the more energy you burn doing nothing, which is why Vitality Tracker's energy engine builds its resting-metabolism estimate from lean mass when it has a trustworthy figure. During a diet the pair tells you what you are actually losing; a falling weight with a steady lean mass is fat going, a falling weight with a falling lean mass is muscle and water going with it.
What moves it
Both are arithmetic on body fat, so both inherit its noise. Lean mass inherits more, because about 73 percent of lean mass is water (Kyle 2004), so a liter of hydration shows up as a kilogram of "lean" overnight. A kilogram of real muscle takes months of training to add.
How to measure it well
The scale's routine (How to use a bioimpedance scale); a DEXA scan for a calibrated split when you want the real number.
How Vitality Tracker uses it
The energy engine uses lean mass for its resting-metabolism prior when your scale's body fat has been corrected against a DEXA scan; without that anchor it falls back to an equation that needs only height, weight, age and sex. Lean mass divided by height squared gives the fat-free mass index; the arms-and-legs part of it gives the appendicular skeletal muscle index. Both halves are kept as series per device.
How far to trust the number
Estimate, with the same within-device consistency as body fat: the trend is believable, the level is the scale's guess until a DEXA scan corrects it.
Sources
- Kyle UG, Bosaeus I, De Lorenzo AD, et al. Bioelectrical impedance analysis, part I: review of principles and methods. Clin Nutr 2004;23:1226–43. doi:10.1016/j.clnu.2004.06.004
- Vitality Tracker, How Vitality Tracker estimates your energy expenditure, method note v1.0.