"Isolation" isn't a word that should be associated with muscles. They all work in synergy (together) ~ Yeah, you can put more emphasis on one muscle than another, but there is never a time when ONLY one muscle is working.... EVER!
Muscles that are formed in groups (triceps, biceps, chest, quads, calves, forearms, delts, abs, back) are prime examples of muscles that cannot possibly be *isolated*.
This is a very loose term when it comes to bodybuilding and anatomy.