Mike Israetel:
"Muscles grow from hard training, but when not training, they don’t shrink very quickly (very little within a week or less) and drop fatigue much faster than they shrink.
This means that lower frequency programs have fewer stimulative sessions, but each session is of higher stimulus magnitude and the program is sustainable for longer than with higher frequencies.
Thus, training even only 1x per week is perhaps 70% or 85% as hypertrophic as training 6x per week. Training 2x per week is probably 90%+ as effective as 6x per week, and even more so over long timecourses (months) as 6x per week tends to be much less sustainable. The growth of 3x per week is often statistically undifferentiable from 6x per week, and is likely much more sustainable, which essentially roughly equates the two in the long term. This means that the training frequency of a program is just not nearly as important to its effectiveness as is its Specificity, Overload, and Fatigue Management characteristics. Even if 1x a week training is used, robust growth will be seen in most cases and in the long term, and if 2-4x frequency is used, near-optimal results will occur if all other training principles are adhered to. Thus, the SRA principle that determines training frequency has great value but doesn’t compare in importance to the three fundamental principles that precede it."
From the Scientific Principles of Hypertrophy Training, due out sometime this year.