This is an excerpt from Science and Development of Muscle Hypertrophy by Brad Schoenfeld.
Modulating training frequency is an effective strategy to manipulate volume loads. There appears to be a benefit to higher training frequencies, at least over short-term training protocols. Thus, total-body routines represent an attractive option for maximizing training frequency for each muscle group. However, split routines allow for a greater volume of work per muscle group per session, potentially enhancing muscular adaptations via the dose - response relationship between volume and hypertrophy. A case can be made for periodizing frequency over time, altering the number of times a muscle group is trained weekly in accordance with individual response. This can be accomplished by alternating total-body and split routines (e.g., progressing from a cycle of 3 weekly sessions to 4 weekly sessions the next cycle, and then culminating in a cycle of 6 weekly sessions). In this way, the lifter can maximize hypertrophy while reducing the potential for overtraining.
Learn more about Science and Development of Muscle Hypertrophy.