I believe that volume and intensiveness both provide a stimulus for growth and a certain amount of each is required to get growth.
For example if you do a lot of volume but all your sets are stopped 4-5 reps short of failure you will get minimal gains. Similarly, if you train to failure or even beyond, but only do 1 set per muscle per week, you will also get minimal gains.
I believe that past the beginner stage you need at least 6 sets/muscle per week and your work sets to be at least 7/10 as far as intensiveness is concerned.
And from there, there is an inverse relationship between intensiveness and volume REQUIRED.
For example if your work sets are a 9 or 10 on average you will only need around 6 work sets per week per muscle (direct or indirect stimulation). If it’s at an average RPE of 7 you will likely need 12 or maybe even 15 weekly work sets for best results.
While no studies have looked into the relative effectiveness of various intensiveness/volume pairings (and it would be hard to do anyway because its hard to externally evaluate intensiveness) I believe that…
- 6-8 weekly work sets/muscle at an RPE of 9-10
- 9-11 weekly work sets/muscle at an RPE of 8
- 12-14 weekly work sets/muscle at an RPE pf 7
Are all pretty much equivalent provided that one uses a similar loading/rep range.