Every time I try to play relatively fast palm muted power chords, with all downstrokes, it gives me a sore, tired forearm and elbow. I've tried holding the pick lighter, relaxing my arm and using a softer picking motion, but nothing seems to be working. Is this one of those things that just comes with practice? or is there some special technique I need to use or something
Make the motion less involving your whole arm, and try to just move your hand. But just kepp practicing, and you'll build up endurance.
Its like trying to fingerbang your girlfriend in church, its all in the wrist.