In one sentence
The scale's estimate of how much of you is muscle, in kilograms, and that figure as a share of your weight.
The catch, first
Most consumer scales call all of your lean soft tissue "muscle": skeletal muscle, but also the heart and organs, the water in them, and everything that is not fat or bone. That is why the founder's scale shows 64.0 kg of "muscle" in a 74.87 kg body, a muscle rate of 85.5 percent. Skeletal muscle proper, the muscle you train, is about a third of body weight: measured by whole-body MRI in 468 adults, it averaged 38 percent of body mass in men and 31 percent in women (Janssen 2000). Read the scale's muscle figure as "lean soft tissue", and read its rate as a mirror of body fat (a lower fat share is a higher muscle rate), not as a separate fact about your muscles.
Why it matters
Muscle is the tissue that falls with age, by about the fifth decade in absolute terms and earlier as a share of weight (Janssen 2000), and the tissue that protects function, metabolism and recovery later in life. The trend in lean soft tissue during a diet is worth watching for the same reason as lean mass: it tells you whether you are losing what you meant to lose.
What moves it
Hydration, immediately, because lean tissue is mostly water; real muscle, slowly, with resistance training and enough protein. A kilogram gained overnight is water.
How to measure it well
The scale's routine; for a real skeletal-muscle figure, a DEXA scan's appendicular lean mass (arms and legs), which is what the research measures use.
How Vitality Tracker uses it
Stored from the full scale reading, kept per device, charted as a trend; the arms-and-legs part of the five-segment reading feeds the appendicular skeletal muscle index. Nothing in VT scores on the scale's muscle figure.
How far to trust the number
Estimate. A research bioimpedance equation predicts MRI-measured skeletal muscle to within about 2.7 kg (9 percent) in healthy adults (Janssen 2000), and a consumer scale with its own undisclosed equation does no better than that. The direction over months on one scale is the usable part.
Sources
- 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
- Janssen I, Heymsfield SB, Baumgartner RN, Ross R. Estimation of skeletal muscle mass by bioelectrical impedance analysis. J Appl Physiol 2000;89:465–71. doi:10.1152/jappl.2000.89.2.465.