You're referring to 2 different kinds of fatigue. CNS fatigue and muscle/tendon/joint fatigue are, whilst related, completely different.
You can train a lot and not really affect the CNS. Bodybuilders on body part splits, whilst doing lots of sets and reps, typically don't do a lot to drain the CNS.
For example, think of a concentration curl for a set of 12 reps vs a heavy set of 5 on deadlifts. The concentration curl will hurt, no doubt. It'll burn more than the deadlift. You could do that every day though with no problems. However, the deadlift is a strain, and it can kill your strength for a couple of days afterwards even if you can't feel any muscle fatigue. That's CNS fatigue. Try deadlifting to 1-5RM a few days in a row and you'll get it. CNS fatigue is what you feel when your personal life is turned up side down and you're emotionally drained after a long day of sh*t hitting the fan. CNS fatigue really refers to the emotional toll lifting takes on you.
Some lifting has no emotional cost, like a concentration curl. But a concentration curl can fatigue the hell out your biceps.
Beginners rarely need to worry about over training and aren't strong enough to inflict a significant amount of CNS fatigue to warrant considering it. If they do feel it*, they most likely just need more sleep and food...
*early signs are lack of motivation, drop in appetite, becoming a bit of an a$$hole for no apparent reason.. However, sometimes and most of the time with beginners, a big feed and an early night will bring you back to feeling fresh again.
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