I wonder if there is a quota of muscle building. For example : If I train my arm only, will it be more efficient than if I train my arm plus my leg the same day ?
Protein synthesis and adaptations are sectorial. If you train only your left arm only that arm will grow.
Training other muscle parts will not impair the capacity of growing in the muscle itself, but can just fatigue the part if the exercise uses more than one muscle group ( actually is the case for every exercise ).
The misconception is that you can isolate a target muscle. This is not correct and actually is not detrimental but beneficial.
There's not a rate/quota for muscle building. It depends on a moltitude of variables and cannot be predicted with ease.