Final answer:
The principle in exercise physiology that applies to the fact of 'use it or lose it' is called atrophy.
Step-by-step explanation:
The principle in exercise physiology that applies to the fact of 'use it or lose it' is called atrophy. Atrophy refers to the decrease in the size and strength of a muscle due to nonuse. When you don't use a muscle, it becomes smaller and weaker over time.The principle 'use it or lose it' refers to the concept that muscles will atrophy, or decrease in size and strength, if they are not regularly exercised.
This applies to various types of muscles, including skeletal muscle and cardiac muscle. For instance, in weight lifting, when you work out the skeletal muscle, it grows in size and increases in strength due to the resistance it encounters. Similarly, when engaging in cardiovascular exercises such as running, the heart, a type of cardiac muscle, beats faster and becomes more efficient.