Skeletal muscle mass

The muscle attached to your skeleton, which is the muscle you can train. It excludes heart and smooth muscle.

This is the tissue resistance training builds and that age and inactivity take away, and it tracks with strength, metabolic health and how well you recover.

It moves slowly, so judge it over months and expect the noise between two readings to exceed a real month of gain. More muscle also raises creatinine and lowers the eGFR estimated from it, with no kidney problem at all.

Worth mentioning an unexplained loss, especially with fatigue or weakness.