I have this problem and I cannot overcome it. I don’t know why but my right arm gets fatigued and needless to say that ends up harming my picking technique. Sometimes I don’t even realize it’s like this because it has been for a while, so this fatigued sensation becomes “normal”. I believe that this is the reason my technique sometimes gets worse the next day after practicing a lot of alternate picking.
Now, I use the conventional playing posture, so my right arm has to be a little suspended in order to pick above the pickups and not far to the left, above the neck. I rest my forearm on the body of the guitar but not so much that it’s practically limp. That’s because the more weight I rest on the body, the harder it is to move my picking hand across the strings because of the friction between the skin of my forearm and the wood. If I’m using a long sleeved t-shirt, on the other hand, my arm just slips down straight towards the floor.
This issue has been a pain in the ass for the longest time. I don’t even need to do any motion for my arm to start building fatigue, I can just have it completely still in that position and after a few seconds I can start feeling it, ever so slightly. I seem to have plateau’d on alternate picking speed because of it. Don’t know what to do about it, so I’m hoping you guys might have an idea.