What's the ideal rep range for building muscle size? 6-25 Reps, Arnold's magic reps were 25,15,12,10,8,6 increasing weight with each set. How the reps are performed is also important. Maximum focus on contraction of the muscle fibers is most important, I'll touch on some methods in the ADVANCED section later. For now the key element to hypertrophy training is the cumulative effect of exhaustion over the total number us sets, not just exhaustion per set. You want to push each set to a high level of exhaustion, this is where that "One More Rep" mentality comes in for a lot of lifters.
Unlike Strength training a high number of sets actually can have an adverse effect as the body isn't able to recover from depleted glycogen storage quickly. Somewhere between 3-7 sets is your ideal range. Your sets will typically be longer, time under tension should be your focus for building muscle size. Your individual number of sets will depend on your fitness level. If you've completely fatigued your muscles in 3 sets, it would be wise to rest and switch to a different muscle group.
How long should I wait between sets? Building muscle size requires the muscle fibers to be under tension frequently. Your rest will be relatively short 30-60secs during your sets, this is enough time for you to catch your breath, and the blood and nitric oxide in the muscle will begin to dissipate. However, at less than a minute of rest you're able to come right back and keep the muscle fibers under tension. I think it's important to note here that this short rest for hypertrophy has never been scientifically proven, it's just the general rule of thumb with trainers and lifters. There isn't anything saying a longer rest will have a negative effect, refer back to reps where I said the key is working the muscle to a high level of exhaustion is important.
I'm going to cover a few Hypertrophy training methods that can add some variety to your training.
First, CHEAT REPS: Cheat reps are legal in hypertrophy training, in fact when wanting to target a specific muscle or when you've reached fatigue in doing full reps. Ex: During the bench press come down fully, but only come up about half way. This will target the pectoral major as opposed to the tricep activation in the later part of the movement.
SUPER SETS: Super sets are your friend when training hypertrophy, linking 2 or 3 exercises together allows you to get more work done in less time. You will pick two exercises of different muscle groups (often opposing) and instead of resting between sets you jump between the multiple exercises. This gives rest to the muscles between sets, but keeps your heart rate up. I'll note here that hypertrophy training is typically not taxing on the CNS.
DROP SETS: Drop sets are the ultimate in working the muscle to exhaustion, here you'll start at a high weight AMRAP, and without rest immediately drop the weight and continue another AMRAP set. In my opinion drop sets are highly effective for hypertrophy training, however they can quickly lead to over training and should not be used in high frequency.
ISOKINETIC TRAINING: "Isokinetic" means same speed down as up. Also referred to as "SLOW TRAINING". Performing the exercise with the same controlled lowered speed through the eccentric and concentric portions of the movement have been shown to increase gains in hypertrophy. This reinforces our statements earlier about focusing on muscle contraction and time under tension.