It doesn't make allowance for training a body part every 4, 5 and 6 days.
Personally I've always tried to train a body part every 5 or 6 days and have never used the week as a cycle guide.
In most routines there's some indirect work done on most body parts even on their "off" days. Indirect shoulder and elbow work can quickly overtrain and subsequently cause injury to these vulnerable areas. Anytime you pick up a weight the shoulders are doing quite a lot of work.
It's important to take this into consideration when designing a program.
Usall Once a week days:
Monday: Chest Biceps And somtimes forearms
Wednesday: Back/Traps Triceps
Friday: Legs Calves Shoulders.
I only do this^^^ if on a Monday I dont have enough time, to do
this twice a week routine:
Monday: Chest Back Shoulders (all compound excersises for 3x5)
Wednesday: Legs Calves Biceps Triceps. (using both high and low reps)
Friday: Chest back Shoulders. (More Isoaltion excersises with lower weights and high reps)
If you happen to be a guy whose arms are barely 14 inches..... you had better get you ass to the gym at least three times a week.... the good old MON, WED & FRI thing.
If you have pipes that are 16 and up then I consider you an intermediate and then you would show up at the gym four times a week..... working each body part only twice per week..... this is an example of what I mean....
Best work out I've ever had was the 5x5 where I squat three times a week. I stopped worrying about "body parts" and more about progressive load. Bodybuilder "body part" workouts did nothing but slow my results and hinder me for years.
lol @ quoting and responding to usernames that haven't posted in over 6 years.
As for the once/twice a week bodypart training, there are too many factors that go into it. What level you're at, what type of training you are doing, etc.
I don't use a calendar, I let my muscles tell me which gets trained. Once per week isn't enough for me, and twice is too many. It's probably more like once every 5 days for me. I don't see how anybody could train their entire body twice per week.