I’m a big believer in measuring lean body mass. I think it’s the single best way to determine your level of fitness. To quote Art De Vany, “Your lean body mass is the metabolically active you. It is the tissue that allows you to function and think and live.”
Lean body mass is defined as, “anything that is not fat or water.” It is the total weight of your bones, muscles and organs (including the brain).
I think too many people focus on their total weight without considering the importance of body composition — the ratio of lean body mass to fat. It’s all too common for individuals to diet without including strength training as part of their program. This approach results in a high percentage of body fat even though the individual may appear thin.
If you include weight training as part of your fat loss program, you increase your lean body mass and raise your metabolic rate which helps keep the fat off because you burn more calories at rest. And, activities such as weight lifting and interval training promote hormone drives that keep you young in addition to helping you stay lean and fit.
While there are many methods for measuring your lean body mass, hydrostatic (underwater) weighing is considered the gold standard. But since this method is impractical most trainers use skinfold measurements.