Explain why exercise programs must increase in difficulty and duration over time to remain effective.

When you start with the program, your new to it. its hard for you at first but eventually gets easier. once you get better at the difficulty, they must raise it so that your not stuck doing the same difficulty the whole time. its kind of like taking a class to walk all of your life when the whole time you already knew how to instead of raising the difficulty to running and then sprinting. hope this helps

Eventually your body gets used to your current "max", when your body gets used to it, it stops putting as much effort and strenuous work into the excersize, so to increase in strength you must increase weight. That is just a weightlifting example it goes for all forms of excersize