In my experience with any of these downpicking / uppicking things, speed is the best test. I think common wisdom historically accepted that these were endurance or strength feats. But if you ever stumble across these techniques accidentally, you realize that when you do it right it suddenly feels fast and easy, for almost no reason.
I assume you are using some type of forearm or wrist-forearm technique, with a flexed posture? Because that’s the most common form for this. If so, try experimenting with loose / rapid downpicking motions on a single note, just to see if you can unlock where it feels fast and the upper arm tension goes away. You can also just do chordal downpicking. Same basic thing, just bigger, which is sometimes eaiser.
It’s not an exercise, you’re not trying to work through the pain. It’s a technique thing. When it’s right there is no tension. Just try to go fast like you’re in a hardcore punk band. If you can make that happen, that’s the basic feeling of these techniques. Then refine from there.