Final answer:
The "use it or lose it" principle states that if a person stops exercising, they will lose the adaptations gained from exercise. Muscle atrophy occurs when muscles are not regularly used, leading to a decrease in muscle size and strength.
Step-by-step explanation:
The "use it or lose it" principle refers to the concept that if a person stops exercising, they will lose the adaptations they have gained from exercise. When muscles are not regularly used, they can undergo atrophy, which is a decrease in muscle size and strength. This atrophy occurs because muscle cells die and are replaced by connective tissue and adipose tissue, which do not possess the same contractile abilities as muscle tissue.