Hard work. Intelligent training. Good heredity. Will or desire.
I am not as strong as those guys but I am not far off and I am clean. I have always trained heavy and hard and have been lucky enough to have decent heredity for strength. I also have strong passion and desire for strength and my will to have it plays a strong role.
Chris this isn't a flame. But just because you can single, double or triple a shit ton on the deadlift DOES NOT mean that you are strong in the sense that these guys are strong. These guys, as well as the likes of Ronnie and Branch, possess an other-worldly sort of what I call "functional strength," in that they can do multiple, heavy sets of very heavy, very compound movements. The deadlift doesn't test strength or fortitude like the squat does. Squats are dreadful. Fucking bent-over rows, if done through a full range of motion, with significant weight, for multiple sets of multiple reps (4 sets of 12, for example) ARE A BITCH. I first remember Porter Cottrell saying this regarding the bent-over row, and he's right. When you can do back, chest, shoulders, legs, all that shit in a week, and in that week perform numerous sets of full reps of REAL exercises, then you are indeed a fucking beast. These guys are like universal soldiers or something. Just relatively unstoppable in relation to the rest of the population. I mean, how much of a beating does your body take doing these workouts day in, day out. It's incredible. You will never see a guy possessing this sort of overall strength have a physique that is nothing less than incredibly hearty/solid. I believe it to be the training that initially enables them to gain the ability to metabolize both the incredible amounts of macronutrients and yes, I'm sure in some cases juice. Have you ever tried shooting even 750 grams of test a week without training incredibly hard and heavy? You just can't do it. You can't stand it for long enough to get any kind of real gains from it. You can't eat and you can't sleep and you want to come out of your own skin or jump in front of a fucking bus or something. ALL DRUGS is never ALL DRUGS. There is a very delicate synergy of variables required and hitting on that magic combination is the hardest thing. Once you have hit that magic equilibrium, however, from everything I've seen, your gains come VERY fast. It takes equal parts heart, know-how, and daring. Couple this with a very acute appreciation for your own being afforded these efforts and THEN you have the true art of bodybuilding. This is what many of the most jaded trolls here are missing. THESE are the points that need to be elaborated on most in the various bodybuilding media outlets, IMO.
