Different fitness levels significantly influence exercise program design, as individuals with varying backgrounds and abilities have distinct needs and goals. Beginners may require a focus on technique and basic movements, while more advanced individuals can handle complex exercises and higher intensities. Trainers design programs that consider strength, endurance, flexibility, and mobility tailored to each client's level. Additionally, modifications and progressions are essential for ensuring safety and effectiveness, promoting adherence by keeping workouts engaging and appropriately challenging for each individual.