What is the least amount of training needed to produce maximal muscle gains? It's a question long debated among both bodybuilders and exercise scientists. Many bodybuilders follow the dictum that "more is better." They believe that the greater the training volume, or the amount of exercises, sets, and repetitions in a workout is what ultimately determines how much muscle you gain. This has some scientific basis in that training volume does appear to equate with muscular hypertrophy or growth. But volume doesn't take into account training intensity levels. Proponents of high-intensity training, which is marked by lower training volume and frequency, but much high levels of intensity to the point of training to momentary muscular failure, note that the level of training intensity limits the amount of training volume possible. Or to put it simply, the harder you train the less training you will be able to do.
Ideas about the ideal amount of training to promote gains in muscular size and strength have evolved over the years. For many years the standard amount of training thought to maximize muscle growth involved training the whole body, three times a week, such as on Monday, Wednesday, and Friday. The intervening days were rest days to allow maximal recuperation from the training. Many great physiques of the 40s and 50s were built in this manner. This includes such celebrated bodybuilders as Steve Reeves, who won both the 1947 Mr.America contest and the 1950 Mr.Universe contest while training three days a week. The notion that training the whole body three times a week was best came from rehabilitation studies, using weight training as a modality to treat injures suffered during the war. Such techniques were first used on soldiers who returned from World War 2 with numerous injuries. Providing these soldiers with whole-body routines that focused on three times a week training produced significant improvement in these injured men as they regained their lost strength and muscle mass. Most of them were not bodybuilders, but bodybuilders did serve in the war and noticed the effects that whole-body training produced on their injured peers. As such, they adopted the same style of training to their personal workouts. Training more than three times a week was considered overtraining, since even back then it was apparent that muscles grew not during training, but during the rest periods between training.
Somewhere along the line, likely in the early 50s, the idea of split training became popular. While whole-body workouts three times a week did appear effective for promoting muscular gains, it was arduous to train the entire body in one workout. And thus the concept of split routines emerged. Split routines involved an increased frequency of training that involved training fewer muscles each workout, but with a greater training volume. Because you were training fewer muscles in a split routine, this allowed you to do a greater number of exercises, sets . . .