Answer:
The principle of progression in endurance training implies that there is an optimal level of overload that should be achieved, as well as an optimal time frame for this overload to occur. The Progression Principle instructs that the overload process should not be increased too slowly, or improvement is unlikely to occur. However, overload that is increased too rapidly can result in injury issues or muscle damage. Thus, exercising above the target zone is counterproductive and can be dangerous and potentially result in injuries.
The Progression Principle states that there is a perfect level of overload in-between a too slow increase and a too rapid increase.