Overload principle
The basis for the overload principle is that adaptations will not occur unless the demands of training are greater than the usual demands made on a particular physiological mechanism. When you increase the usual demands on as system, we say that the system is overloaded.
Although simple in definition, the overload principle is complex in application. Although the demands of the training must be sufficient to stimulate adaptation, they cannot be excessive or the training effect will be lost through injury or failing adaptation.