>Are fullbody workouts better than bro splits for optimal muscle growth?
Yes.
However, you could hit full body not 3x a week, but 6x a week just as well, which would the best scenario, quite certainly.
The thing is, 5x5 is most common full body program, and it's a novice program, and as so it has a 3x week frequency in order to keep the layout simple enough that anyone with can follow through without a coach or, at least, formulated worksheet.
If you look into experts books and article (such as Tsatsouline, Sheiko, Poliquin etc) you'll notice they too propose full body workouts, at various frequencies with one remarkable condition: varying progression/intensity, from one cycle to another, if not IN cycle.
That's easily the most overlooked aspect of Tsatsoulines Power To The People book (and in the case of Sheiko, it's considered "too hard to do without a coach"), but it's probably the most rewarding if followed through. All that hitting light weights high reps everyday, hitting heavy weights low reps everyday, getting proper rest and alternating programming styles, everything is pretty much taken in account but not named as so IF give enough attention to proposed progression on the Power To The People book, which is in fact very simple read (simpler than SS, to say so).
As of current, I'm running my own derivative of Russian Bear program. It's not much different than the original, except that in between the "active" days (3-5x week, by the book), I fill in the blanks with either very low rep high intensity work or very high rep very low intensity work, while based around 3 main lifts (bench press, back squat, and narrow grip lat pull down), plus light accessory work (namely, high rep standing cable crunches and neck curls). By doing so, I've been able to keep slow but consistent progress in the main muscle groups without any hindrance due to my medical rotations tight hours schedule.
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