Austin is pretty much dead on. And yes, loukiss, since you mention daily training for strength in your original post, that’s largely a function of CNS fatigue governing that. But it can work well if you manage things for muscle growth as well.
And yeah, in my example of doing back 3 days in a row, it definitely was NOT low rep lol. It went basically: morning-heavy, night-light/isolation, morning-rowing for active recovery and activation, Night–light depending on the athlete’s readiness, very very occatsionally heavy, 3rd day was generally morning-rowing, night–isolation or heavy
Only one heavy day in there, things switched up depending on the gal’s readiness, but overall it was heavy, light, rowing, light, rowing, heavy or light.
But that’s beside the main point. Regarding CNS fatigue, there are several things to keep in mind. It’s a sort of in depth and complicated subject to really delve into, but there are a few good guidelines that can really make things understandable and allow you to make good decisions in a general way:
There are two primary kinds of exercises that fatigue the nervous system most are: compound exercises, but very specifically the ones with spinal loading: squats, deadlifts, good mornings. The others are really heavy explosive exercises like clean and jerk, and snatch. This is not to be confused with snatches done as a warm-up, or dumbbell snatches for back work–I am talking about them as main lifts in your focus. GENERAL RULE OF THUMB: the more muscles an exercise uses and the more it loads the spine, the more it has the POTENTIAL to fatigue the nervous system. So, deadlifts = squats > bench > overhead press > isolation stuff… you get the idea. Or competitive olympic lifts done really heavy
The intensity levels that fatigue the nervous system most–regardless of the exercise-- are: max strength (90-100% of your 1 rep max), then 85+%
The types of repetitions that fatigue the nervous system most are: FAILING A REPETITION!!, Forced reps, grinding reps (where the weight moves really slowly and you’re almost at failure but somehow manage to barely get the rep completed), drop sets to failure.
the OVERALL GUIDING PRINCIPLE THAT DETERMINES YOUR NERVOUS SYSTEM FATIGUE LEVEL IS: how close and how often you are training to your maximum capabilities in terms of heavy weight.
Think of it on a gradual slide—the more frequently you train an exercise, the less often you can fail a repetition or do forced reps, or grind a repetition out. Basically, the more frequently you train an exercise, the farther away from failure you have to be to successfully keep that frequency. Conversely the less often you train something, the more balls-out you can go: forced reps, beyond failiure, grinding, drop sets, the works.
This is why somebody only training squats once a week (or legs once a week), can absolutely trash themselves, but is incapable of doing it 2 or 3 times a week. This is also how olympic lifters can build big legs squatting 4-6 times a week for months on end: they’re on opposite ends of the slide.
Generally, working a lift or muscle group 2 times a week still allows you to do a lot in terms of pushing the fatigue levels, coming close to failure, some drop sets on occasion, etc. Working a muscle 3 or more times a week, not so much. 4 times a week or more you better not fail or grind a repetition out, you need to be powerful on all repetitions and not fail.