Which principle of fitness development suggests that one should increase physical activity gradually?

The principle of fitness development that suggests one should increase physical activity gradually is progressive overload. This principle is fundamental for effectively enhancing physical performance and achieving fitness goals. Progressive overload involves systematically increasing the demands placed on the body during exercise. This can be achieved by increasing the intensity, duration, frequency, or type of activity over time.

By gradually increasing physical activity levels, the body adapts positively to these increases, leading to improvements in strength, endurance, and overall fitness. This gradual approach also helps to reduce the risk of injury, as it allows the body's musculoskeletal system to adapt to the new levels of stress without becoming overwhelmed.

In contrast, the other principles address different aspects of fitness. Individual differences take into account that each person has unique responses to training, specificity emphasizes training for particular goals or activities, and reversibility underscores that fitness gains can be lost if training is stopped. While all these principles are important for a comprehensive understanding of fitness, they do not specifically highlight the necessity of gradually increasing physical activity as effectively as progressive overload does.