Muscle mass is a term for the bulk of muscular tissue in a person's body. There are three different kinds of muscle in the human body, but muscle mass almost always refers to skeletal muscle. Skeletal muscle is the most visible and directly contributes to strength and power.
Skeletal muscle makes up a large percentage of a person's overall weight and body composition. When skeletal muscle growth is encouraged by training and nutrition, it produces distinct changes in the body's appearance. Those with a large amount of muscle mass appear bulkier and more toned. The toning effect is especially pronounced if the person has a low amount of body fat.