Lean Muscle Mass. Lean muscle is a concept related to lean body mass, which is the content of the body minus fat. Lean body mass is used to calculate basal metabolic rate.Lean muscle is less of a scientific term and more a term of art that refers to muscle that is independent of, and not obscured by, fat.ean Muscle Mass. Lean muscle is a concept related to lean body mass, which is the content of the body minus fat. Lean body mass is used to calculate basal metabolic rate.