Muscle mass is the volume of skeletal muscle tissue. The largest organ system by mass, and the most metabolically active.
Mechanism: Muscle drives glucose disposal (insulin sensitivity), serves as the body's amino acid reservoir during illness, generates myokine signaling, and stabilizes joints. Loss begins at 30, accelerates after 50.
Use case: Resistance training 3-4x/week with progressive overload. Protein intake 1.6-2.2 g/kg/day. The hypertrophy floor is real: above ~1.6g/kg, returns flatten.
Caveats: Cardio alone doesn't build muscle. Long endurance work can be neutral or negative for hypertrophy without dedicated lifting. After 60, the resistance-training case becomes urgent, not optional.