Exercise physiologist and fitness consultant Elizabeth Quinn tells About.com that overload requires the body to take on a greater than normal stress or load for training adaptation to take place. However, once the body adapts to a stimulus in training, it begins to require a different stimulus to continue its change.
Richard Weil at eMedicineHealth explains that progressive overload may involve lifting weights until the muscles become fatigued. However, when a certain amount of reps becomes easy, the exerciser must increase the amount of weight after the muscles adapt. The muscles must work for a longer period of time than usual to increase endurance. She further explains that a normal amount of exercise maintains the current fitness level, but removing stress or decreasing stress decreases that particular component of fitness.
Weil explains that progressive overload creates the greatest gains in strength training as it develops strength and tone. Weil also credits Milo of Croton, an ancient Greek athlete, strong man and wrestler, with first using the concept of progressive overload. According to legend, Milo would carry a baby calf on his back every day for years prior to the Olympic games. By the start of the Olympic games, Milo would still carry the calf that had grown into a full-size cow.