A muscle is an organ system, which includes all of the muscles of a living organism. If the concept in conjunction with other body parts used, such as the names of abdominal muscles or back muscles, so the statement refers to the muscle groups of the body section, and their interaction.

Instructions
Stand with good posture (abdominal muscles tightened up, chest out, back straight). Greife the Expander / Dynaband with palms facing the bottom. Keep the shoulder blades together (withdrawn) and keep this position throughout the exercise.

Under Strength is a physical training, which focus on increasing the power capabilities (and not the stamina, speed or agility) sets.