Mass is nothing more than bigger muscles, so work your muscles, not your joints." Keep that in mind every time you pick up a weight, and the bodypart you're training will tell you how many reps are best for that particular exercise. If you feel those reps in your muscle, then you're doing the right amount, but if you feel those reps in your joint, then you're using too much weight and doing too few reps.
In my earlier years, my reps were quite low, six or fewer, and very explosive, since that is the only way to move extremely heavy weight. Explosive reps, however, incur a lot of stress, sometimes doubling the effective weight on the muscle. Let's say you're squatting with 600 pounds. If your reps are consistently paced and tightly controlled, your legs will be under a steady resistance of about 600 pounds through the full range of motion of the repetition; however, if at some point in the rep you suddenly explode upward, that effective weight (the total amount of force placed on the muscle and joints) can become as much as 1,200 pounds, creating dangerous stresses on unsuspecting muscles and joints. Also, during a slow and controlled rep with a consistent 600-pound stress, all of the muscle fibers have balanced control of the weight; but with the explosion, that balance is thrown off and some of the muscle fibers lose their tension in their desperation to regain control. Some fibers might even strain or tear from the overload shock.
When I felt myself approaching that danger point, my solution was to lower the weight just enough to take that explosive element out of the movement, so the effective weight, balance, tension and control remained constant. Since I'm doing more full-range reps, I am also progressively pushing my leg muscles further into fatigue; whereas, if I had to struggle with heavier weight and fewer reps, I would reach failure before I would reach peak fatigue.
By Dorian Yates SIX-TIME MR. OLYMPIA