What principle states that you train only that part of the body that is overloaded?

The principle that states you train only that part of the body that is overloaded is specificity. This principle emphasizes that training adaptations are specific to the muscles used during the activity, the type of training program employed, and the particular energy systems involved. For example, if a person performs resistance training focusing on the biceps, the adaptations—such as increased strength and muscle size—will be largely confined to the biceps, not affecting other muscle groups in the same way.

This principle guides fitness professionals in designing effective training programs tailored to individual goals, ensuring that the specific areas requiring improvement receive the appropriate focus through targeted overload. Understanding specificity helps in maximizing training outcomes by aligning the exercises with the desired fitness goals.