Overload Principle
The overload principle requires a person exercising to provide a greater stress or demand on the body than it's usually accustomed to handling. For any muscle,including the heart, to get stronger,it must work against a greater-than normal resistance or challenge.To continue improve,you need further increases in the demands- but not too much too quickly.Progressive overloading gradually increasing physical challenge provides the bencfits of exercise without the risk of injuries.