In the early 70s when he was Mr. Olympia, he worked out twice daily and hit body parts three times per week (not counting calves and abs, which he trained daily). Not everyone had the luxury of hitting the iron twice daily, but almost every bodybuilder was working body parts every other day. By the late 70s. this started to change. Splits that stressed muscles twice per week were the norm. And in the 80s, this was often stretched to twice every eight days via a popular three-on. one-off split (three training days, followed by one rest day).
In any split in which your workouts are spread over three consecutive days, typically one day is devoted to legs. Then the question is. how do you organize your upper-body work over the remaining two days to maximize rest for each muscle? If you hit chest and back one day and shoulders and arms the next, youre going to stress delts, biceps, and triceps on both days. This is because front delts and triceps assist on chest presses and dips and rear delts and biceps assist on almost every back exercise. Any split that results in this double-stress for shoulders and arms is going to severely reduce your growth-inducing recuperation for those areas.
The best solution to this quandary is the push-pull system. Exercises in which your elbow joints are straight at contractions (bench presses, shoulder presses, pushdowns. etc.) are pushers. Exercises in which your elbow joints are bent at contractions (T-bar rows, pulldowns, barbell curls, etc.) or in which you lift vertically (deadlifts, shrugs, etc.) are pullers. Pushers and pullers are divided into separate workouts. So. you work chest, front delts. and triceps in a push workout, and back, rear delts. traps, and biceps in a pull workout. This allows you to train body parts twice weekly or twice every eight days with sufficient time to recuperate between workouts.