1. Definition: Muscle mass refers to the content of all muscles in human body, including skeletal muscle, cardiac muscle and smooth muscle. Skeletal muscle, also known as voluntary muscle, is completely dominated by people's autonomous consciousness and is the main component of muscle quality.

2. Composition: In muscle mass, skeletal muscle accounts for more than 98% of the total muscle content, while cardiac muscle and smooth muscle cannot be dominated by human consciousness.

3. Use: Muscle mass is generally used as data for evaluating physical fitness, while skeletal muscle is mainly used to protect bones and stabilize joints.

4. Growth mode: The total amount of human muscle mass and whether it grows or not depends on the weight and development of skeletal muscle. Skeletal muscle can increase its capacity through exercise and reasonable diet.

To sum up, although skeletal muscle mass and muscle mass are both part of human muscles, they are different in definition, composition, use and growth mode.