Or just give out some sort of drug that acts as artificial adrenaline, maybe? It would numb pain and improve physical performance, though it wouldn't create invincible hulks. The idea of giving soldiers drugs to dull pain/make them tire less/improve performance is not a new one. I think I read somewhere about fighter pilots in WWII actually doing methamphetamine before dogfights. Improved their reaction times and lessened their fear, apparently.