I had a problem with my picking arm getting stiff, too, specifically my shoulder. What solved it for me was dropping the bass until my right arm was straighter, picking up by the neck, and relaxing my grip on the pick. Basically, the bass is as low as it can get without my left arm making an obtuse angle. To the OP, what is the shape/thickness/material of the pick you're using? I use one of those rounded triangle ones from Clayton, 1.0mm. Still gets some pick scrape, but the thicker width gets more push behind the string. Hope that helps!