By Brad Schoenfeld:
"Bottom line: Research shows a benefit to training muscles frequently throughout the course of a week. Although evidence is somewhat limited, it would appear that at least twice-weekly stimulation of a given muscle is beneficial to maximize lean muscle growth. This can be accomplished with an upper body/lower body split carried out four days per week (i.e., two days on/one day off, two days on/two days off) or a three-way split (i.e., push/pull/legs) performed six days per week (i.e., three days on/one day off). It’s also possible that periodizing training frequencies might provide a means to maintain the novelty of the training stimulus. Accordingly, consider integrating full-body workouts into your programming over the course of a training cycle to enhance the hypertrophic response."