Glossary
The terms, without the hype.
Every term starts with a short, direct answer, then explains what the number measures, where it goes wrong, and where to go deeper. Each one lists the sources it relies on.
Body composition
Bioelectrical impedance (BIA)
Bioelectrical impedance analysis (BIA) sends a small, painless electrical current through the body and estimates body composition from how much the body resists it — lean tissue, which holds more water, conducts better than fat. It powers most smart scales and gym body-composition machines.
Read definition →DEXA scan
A DEXA (dual-energy X-ray absorptiometry) scan passes two low-dose X-ray energies through the body and uses how differently fat, lean soft tissue, and bone absorb them to estimate body composition. It's the most common reference method for body fat — but it's still an estimate, not a direct measurement.
Read definition →FFMI (Fat-Free Mass Index)
FFMI (Fat-Free Mass Index) is your fat-free mass in kilograms divided by your height in metres squared. It works like BMI, but counts only the part of your weight that isn't fat, so it tracks muscularity relative to your frame instead of total size.
Read definition →Lean body mass
Lean body mass is the part of your body weight that isn't fat: muscle, bone, organs, connective tissue, and water. It's larger than your muscle mass, and it's estimated — not weighed — by subtracting estimated fat mass from total weight.
Read definition →Skinny fat
Skinny fat is an informal term for having a normal body weight or BMI while carrying a relatively high body fat percentage and relatively little muscle. The closest research term is normal weight obesity: normal BMI with high body fat.
Read definition →Progress tracking
Training & recovery
Deload
A deload is a planned, temporary reduction in training stress — typically about a week of noticeably lower volume, intensity, or both — used to let accumulated fatigue dissipate so the next block of hard training is productive.
Read definition →HRV (Heart Rate Variability)
HRV (heart rate variability) is the variation in time between consecutive heartbeats, measured in milliseconds. Higher-than-usual HRV generally reflects a more recovered nervous system, but the absolute number is highly individual — it's only meaningful compared with your own baseline.
Read definition →