I mean ADHD is executive function dysregulation. An overtired kid can have a lot of behaviors and be able too tired to work. I don't find myself focused after a long day of work.

I think what you mean is that what physical and mental exertion may do is train your body and increase your stamina which improves your focus long term. Sure it may help develop the neural pathways. Much like learning to read is hard at first but then becomes easy.

Also people with ADHD can hyper-focus on the things that interest them.

I am not sure what you mean by or how you would test varying physical and mental exertion in any reliable way. If we make kids run a mile and then rest and give them a math test what does mean?