The indirect effect of exercise, in that growth somewhere produces some growth everywhere, the magnitude is based on the mass of muscle involved, in that the closer the neighbor the greater the effect.
as a muscle works intensively a chemical reaction occurs that spills over and effects the entire body.
since there is a limit to your overall recovery ability, and since many of the body's chemical functions affect the entire body, it should be evident that training everyday is a mistake.
which makes it apparent that the effect is greatest when all major muscles are worked at the same time.
All I'm saying, you can biuld large arms without working without working your legs - but you will biuld them much larger and much quicker, if you also exercise your legs.
ill go further to state that most trainees react to the lack of progress by procuring their own chemicals, supplements and shit, instead of using their heads.
"hard-work" should be its own anti-overtraining, if the intensity is high as it should be and workouts brief you would not want to return for days.