A: Heavier weights and lower reps will produce more growth—in a couple of fast-twitch fiber types. Scientists believe there are at least five (some even say nine) different fast-twitch fiber types, some being slightly more endurance oriented than others. So a set with four to six reps plus X Reps may do great things for one or two of those; but to get at the others you’d want to include slightly higher-rep sets (or include drop sets and supersets in combination with X Reps, as outlined in The Ultimate Mass Workout e-book). That will help you hit as many fast-twitch types as possible.
Technically speaking, fiber makeup dictates which rep range should dominate for an individual. For example, if you have more pure fast-twitch fibers, lower reps should dominate in your routine, but you shouldn’t neglect other rep ranges because you want to hit as many fiber types as possible to max out growth. In other words, the more fibers you get to grow, the bigger your muscles will be.
In our cases, Jonathan, being more mesomorphic (muscular/athletic), responds to lower reps, but he also includes drop sets and supersets to hit a variety of fiber types as well (those techniques bring in an endurance component). Steve, on the other hand, responds better to higher reps, as he’s an ectomorph (hardgainer/skinny) with more endurance-oriented fast-twitch fibers as well as lots of slow-twitch fibers. Heavy straight sets don’t do much for him, so he relies more on drop sets and supersets, while still keeping the reps in the six-to-12 range. That means if he does a drop set of 8(6) reps—eight reps, reduce the weight and immediately do six reps—he’s doing 14 reps. That’s two lower-rep sets back to back, giving him the best of both worlds. Add X Reps to one of those sets and he gets even more time under tension, overloading a number of fiber types. It’s a very efficient way to train for more muscle mass.
Remember, it’s not about just hitting fast-twitch fibers with low reps. There are a number of different fast-twitch fibers that vary in work capacity. We attack that problem by doing heavy work but also getting more endurance-oriented anaerobic work by using supersets and drop sets along with X Reps.