Definition: The Principle of Progression implies that there is an optimal level of overload that should be achieved, and an optimal time frame for this overload to occur. Overload should not be increased too slowly or improvement is unlikely. Overload that is increased too rapidly will result in injury or muscle damage. Exercising above the target zone is counterproductive and can be dangerous. For example, the weekend athlete who exercises vigorously only on weekends does not exercise often enough, and so violates the principle of progression.
The Principle of Progression also makes us realize the need for proper rest and recovery. Continual stress on the body and constant overload with result in exhaustion and injury. You should not (and can not) train hard all the time. Doing so will lead to over training and a great deal of physical and psychological damage will result.
Also see: The Principles of Sports Conditioning
