Oh brother. I never said not to do isolation in the first place. If their arms are lagging, there's a possibility they're not capillary dense. In that case, sure, go do some high rep bicep curls. There's a time and place for everything, isolations included. The majority of mass is built with compound movements, you can't deny that. Look at powerlifters. My suggestion was merely a SUGGESTION to your IMAGINARY person.