I would say that I am an extremely good guitarist and have been playing for many years. I have always been into really technical music, and have always spent a lot of time on my technique... sweeping, alternate picking, economy picking... etc... I have always focused on lead stuff, and I am big into neo classical metal.

Recently, I am just getting into post thrash and metalcore and such... and after trying to learn several riffs... like Two Weeks by All That Remains... I realize I cannot even play it up to speed. I have always thought of downpicking as something that you cannot really develop... but I think I may be wrong. I am a really weak guy, but that has never held me back in my legato and other leads techniques that require muscle... i supposed because thats what I put all my time into. Now that I start playing down picking riffs... I fail... the second riff in master of puppets, I cannot keep up with.... unless I alternate pick all of it which most people would find next to impossible.

Am I correct in my conclusion that i cannot keep up with these downpicking riffs because of muscles I have not used. Which muscles are these and what excursuses can I do to improve them?