Muscle mass refers to a measure of body composition. Muscle mass is the total amount of muscle in the body, including skeletal muscles, smooth muscles, and cardiac muscles. In many of the studies we review, two other metrics—lean mass and fat-free mass—are often used as approximations for muscle mass, although they're technically different. Your lean mass is your weight minus your fat mass. It includes your bones, organs, and water, although bone is sometimes excluded. Fat-free mass differs by excluding essential fats found in cell membranes; these are included in lean mass.