When it comes to exercising it is about Overload, Progression, and Specificity.
Overload – is the physical stress placed on the body when physical activity is greater in amount and intensity than usual. The body’s structures and functions respond and adapt to these stresses. For example, aerobic physical activity places a stress on the cardio respiratory system and muscles, requiring the lungs to move more air and the heart to pump more blood and deliver it to the working muscles. This increase in demand increases the efficiency and capacity of the lungs, heart, circulatory system, and exercising muscles. In the same way muscle strengthening overload muscles making them stronger.
Progression – is closely tied to overload. Once a person reaches a certain fitness level, he or she progresses to higher levels of physical activity by continued overload and adaptation. Small, progressive changes in overload help the boy adapt to the additional stresses while minimizing the risk of injury.
Specificity – means that the benefits of exercise are specific to the body systems that are doing the work. For example, aerobic physical activity largely benefits the body’s cardiovascular system.