PHYS-46 · Principle
Smart scale: body fat up 2% overnight. Real?
No. It measures resistance to a current and estimates fat by equation; a change can be missed by several points, so 2% is noise. Watch calories, sleep stages and VO2max are estimates.
How the body works, agreed across the reviews below; a study entry shows what one experiment measured.
Practical Application
Smart scale: body fat up 2% overnight. Real? No. It measures resistance to a current and estimates fat by equation; a change can be missed by several points, so 2% is noise. Watch calories, sleep stages and VO2max are estimates.
What it means for training: name the reading's class first; act only on a measured reading on one device read as a trend, and give an estimate's error and ask (SYNTH-91).
Mechanism
Estimated: watch VO2max is modelled from heart rate against running speed, or from resting heart rate, HRV and personal data, and missed one person by about ±10 mL/kg/min after runs (REF-1253); sleep is inferred from movement, heart rate and HRV, about 17 min high on total sleep (REF-1252), with stages 50-65% in agreement (REF-364); calories, probably from movement, wrist heart rate and GPS, read 17-29% high on two watches (REF-1254).
Body fat: bioimpedance gives fat-free mass and body water through population-specific equations, valid in people without fluid or electrolyte disturbance (Kyle 2004); 15 devices repeated themselves well but missed a laboratory model by −3.5 to +11.7 points on average (73 non-athletes, REF-1257).
Common misreading
"My watch VO2max dropped 2 points, so I am losing fitness"; "I burned 3 000 kcal, so I eat 3 000"; "40 min of deep sleep means poor recovery".