It's funny you should bring that up...everybody's always up in arms (no pun intended) about the need for isolation. "Give arms their own day!" They did a study on major league baseball players. They put them on identical weight training programs, except one had arm isolation and the other didn't. The groups had equal arm growth. So I think isolation is a bit less necessary than everyone makes it out to be.
I stopped dedicating a day to arms because they were growing too fast. I was getting bad stretch marks all over because my arms are genetically my most responsive body part.
Well even after I stopped doing direct bi's and tri's my arms kept growing and the stretch marks were getting worse. My arms get plenty of work from back and chest exercises. Now all I can seem to remember getting out of my arm day was bad tendonitis pain.
yea, i dont think i will ever have a day dedicated to only arms again either. i like how it's goin now, just be happy if i can start putting on some weight.