The principle of overload states that a greater than normal stress or load on the body is required for training adaptation to take place. The body will adapt to this stimulus. Once the body has adapted then a different stimulus is required to continue the change. In order for a muscle (including the heart) to increase strength, it must be gradually stressed by working against a load greater than it is used to. To increase endurance, muscles must work for a longer period of time than they are used to. If this stress is removed or decreased there will be a decrease in that particular component of fitness. A normal amount of exercise will maintain the current fitness level.
Also See: The Principles of Sports Conditioning