Developing separation of muscle groups isn't something that's given—it's something you have to earn. Today you'll hit your wheels once more with 100 sweet reps of leg curls and extensions. The goal of this program is to get shredded, so sticking with the weight you used last week is fine as long as you're still feeling challenged.
Take a page from Arnold's book and chase the pump: "A bodybuilder knows that when he pumps his muscle it means growth. So therefore he knows when he pumps up well, that is progress. And that satisfies him because he feels the progress in his body. Therefore the pump feels good. It's actually the best feeling a bodybuilder can have."
Hit that pump for Arnold!