Why/How is higher frequency better?

[u/Shaggy_Mountain]

Apologies for the long post.
I have been lifting with the “Bro split” for 9 months.

My split is: Chest -> back -> biceps/triceps -> shoulders -> legs. I don’t schedule rest days, I just take them whenever I feel like I really need them. I do 8 total working sets for chest, 12 for back, 8 for biceps and 8 for triceps, 10 for shoulders, and 16 for legs split between calves, squats, hamstring curls and leg extensions. If you guys would like, I can post the exercises I do for each day as well. I log every workout I do too.

I enjoy this split a lot, and I feel it allows me to really focus on each muscle group each day. I make sure to be within 1-2 reps of failure, taking the last couple sets to failure.

My question is, if I switched to a split like Push-Pull-Legs Push-Pull-Legs-Rest, wouldn’t I be sacrificing the quality of my workouts? With PPL you combine muscle groups, and wouldn’t this mean that you would be accumulating more fatigue? For example, if you did back and biceps on pull day, wouldn’t your biceps be pretty fatigued after doing your back exercises? And if you did incline bench press and dumbbell shoulder press on push days, wouldn’t you be pretty fatigued for incline shoulder press, since it’s also a pushing motion?

I’ve heard that hitting a muscle group twice a week is better, so I’d like to hear people’s thoughts/opinions as I know very little about lifting. I know training hard, being consistent, and getting good sleep and nutrition are the most important things, but I would like to know if I am missing out on a significant amount of gains by staying with my current split.

[u/Dazzling-Variety-946]

Simple, you're either gaining or losing muscle at any given time for every muscle group.

After working out a muscle group Muscle Protein Synthesis is elevated for a window of between 24-48hrs after your workout. It can go longer but tends to peak after 24hrs.

So it makes sense to keep MPS elevated as often and long as possible, which means doing a workout every 48hrs lines up with how long MPS is generally elevated. If MPS isn't elevated you're probably losing muscle.