MariposasLean Body Mass (LBM)
Lean body mass is the total weight of everything in your body that isn't fat. That includes muscle, bone, water, organs, connective tissue, and even the glycogen stored in your muscles. The number matters in training because it gives you a clearer picture of your physical composition than scale weight alone. Two people can weigh exactly the same and have completely different amounts of muscle and fat. The common mistake is treating LBM as a synonym for muscle mass. Muscle is the biggest variable component of LBM, but if you're dehydrated or have low bone density, your LBM shifts too, even if your actual muscle tissue hasn't changed at all.
Example
A 180-pound man with 20% body fat has roughly 144 pounds of lean body mass and 36 pounds of fat. If he cuts down to 170 pounds but loses mostly water and muscle rather than fat, his LBM might drop to 135 pounds even though he looks lighter on the scale. That scenario is exactly what most trainers are trying to help clients avoid by tracking LBM alongside total weight.