Muscle Mass
Muscle tissue only
Muscle mass refers to muscle tissue. Depending on the measuring method, it may include skeletal muscle only or a broader muscle estimate.
Lean Body Mass
Everything except fat mass
Lean body mass includes muscle, but also bone, water, organs, blood, connective tissue, and other fat-free tissue.
Why People Confuse Them
Lifters usually care about building muscle, so lean mass often gets talked about as if it means muscle. That shortcut can be useful in casual conversation, but it is not technically exact.
Hydration, glycogen, digestion, and measurement method can all influence lean mass estimates. That is why short-term changes in LBM are not always pure muscle gain or loss.
Why Lifters Still Care About LBM
Lean body mass is still useful because it removes estimated fat mass from total body weight. That makes it easier to see whether body composition is moving in the right direction.
FFMI uses lean body mass relative to height, which is why it can be more useful than body weight alone for people who lift.
A Practical Way to Use Both
Use muscle mass measurements, if you have a reliable method, to track muscle-specific progress.
Use lean body mass and FFMI to understand broader body composition changes over time.
Do not overreact to one reading. Body composition metrics work best when you compare trends across weeks and months.