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Interval workouts alternate high-intensity work periods with low-intensity recovery intervals. Aerobic interval workouts target the cardiovascular system by increasing one's maximum stroke volume (volume of blood the heart pumps with each beat) and cardiac output (volume of blood the heart pumps each minute), along with increasing muscle power by recruiting fast-twitch muscle fibers – thus sculpting the body; the more intense the workout, the greater and longer the post-workout elevation in metabolism, which burns more calories the rest of the day.