I've been practicing old Slayer songs for years now to keep my chops up. One thing I was doing for a long time was I would tense up my shoulder to blast out fast downpicking chugs, but I could never build up endurance that way, and I was confused as to why. After some research and experimentation, I came to the conclusion that if I just play straight from my wrist utilizing ONLY my forearm muscles, starting slow, feel the burn, take breaks, speed it up, rinse and repeat, I was able to maintain speed without fatigue. The same goes with alternate picking. How you hold your pick matters a LOT, too, so don't overlook that.