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

Brief Explanations:

The body adapts to exercise over time. When it gets used to a certain level of difficulty and duration, it becomes more efficient and requires less effort. To continue making progress and seeing results, the exercise program must challenge the body more by increasing difficulty and duration. This principle is known as the overload principle in exercise science.

Answer:

The body adapts to exercise. To keep seeing results and making progress, the overload principle requires increasing difficulty and duration over time as the body becomes more efficient at a given level of exercise.