The ability to perform 20 sit-ups in a row is categorized as muscle . the ability to lift a heavy box into a cart is categorized as muscle . the ability to lift a heavy child into bed is categorized as muscle . the ability to carry a book bag home from school is categorized as muscle . the ability to lift a heavy weight many times in a row is categorized as muscle .

People who utilizes the progression principle will ensure their exercise program increases in frequency, intensity, time, and type. the changes need to happen gradually in order to ensure they make gains in their fitness program. a person who doesn't utilize the progression principle can still gain health benefits from exercising, but will not continue to improve in cardiovascular fitness.