Which principle relates to gradually increasing the difficulty of an exercise program?

The principle that relates to gradually increasing the difficulty of an exercise program is progression. This principle emphasizes the need to enhance the intensity, duration, or complexity of workouts over time to continue improving fitness levels and achieve desired results. By gradually increasing the challenge presented by an exercise routine, individuals can effectively build strength, endurance, and skill without risking plateauing or injury.

Other principles in exercise, such as regularity, focus on maintaining a consistent practice schedule, while specificity pertains to training in a way that targets particular fitness goals or skills. Cross-training involves engaging in different types of exercise to prevent boredom and overuse injuries, but it does not specifically address the gradual increase in difficulty associated with progression. Thus, progression is essential for ensuring ongoing improvement and adaptation in physical fitness.