For HYPERTROPHY (not strength) I've recently come up to the conclusion (via discussions with some Ph.Ds and knowledgeable people) that the most important thing is hitting failure and that the load (thus the number of reps) is actually irrelevant. I am writing an article on this subject.
A recent study found equal hypertrophy with 3 sets at 30% to failure and 3 sets at 80% to failure. So when training for hypertrophy it would seem that you can actually select the weight you want and just go to failure... so the number of reps will vary depending on how fast failure is it.
For strength it's not the same thing because neural factors come into play a lot more.
Now, neural stress from exercise is mostly related to 1) the amount of force you need to produce 2) the complexity of the exercise 3) the psychological strain of the movement (for example having a very heavy bar in your hands or on your back represents a high psychological strain).
So if you want to limit to the minimum neural stress from hypertrophy training you should...
- Train to failure
- Use the weight that allows you to feel the muscle working the best (bet quality of contraction)
- Select non stressful exercises (mostly isolation exercises or cable/machines) FOR THE HYPERTORPHY-SPECIFIC WORK
- Stay away from very heavy loads for your hypertrophy work (I would recommend not going above 75% or your 8-10RM)
Now that obviously means that you will have to do your strength work (heavy lifts) separately