Yeah man, when I said "extending the set" I meant doing breakdowns, negatives, forced reps, etc... I'm still not convinced about the "real value" of set extenders, but I am convinced about using HEAVY RESISTANCE. I base this idea of mine on my experience with the leg press, 6 and a half pounds gain of muscle in 5 weeks on this belief. My leg press protocol relied entirely on progressive overload and heavy resistance, and I didn't use any "set extenders". I actually had a phone consultation with Mike Mentzer shortly before he passed, and he told me that set extenders were not necessary, and were even counter-productive.
You mention a couple of things that Mentzer said. He got the most progress with Rest Pause. "Set extenders" a term I've never heard him use, is counter productive. As I mentioned before, I was a big fan of Mentzer and he and Jones were most influential in my thinking. I've never heard these things. I use to see him regularly in the late 1980s-early 90s at Golds in Venice where I was introduced to him by Don Ross. He live about two miles from where I did in Redondo Beach, CA. It was Mentzer that actually spelled it out to me explaining force reps and then when that can no longer be done to continue with negatives. Of course, intensity variables, as well as weight training in general, can be counter productive if abused. Maybe that is what he meant.
The whole point of even lifting weights is to put the body under an over load. A stress that it is not accustom to with the idea of stimulating an adaptive response. As long as you are doing things that are already easy how will it stimulate that response? Of course, as I mentioned before, unless one goes on a gh15 mutation cycle, you pretty much adapt to any training protocol and progress stops. In fact, progress will stop even under the gh15, or anybody elses, protocol.
I use these intensity variables not because I think I'm going to get bigger muscles. I use them because it forces you to train harder. It is my belief that training hard puts you in better shape and toughens you up. When I do a workout just going to positive failure, doing rep after rep until I can't possibly complete another one, is nothing for me. I don't work up a sweat. I don't breath hard. It's like a vacation for me. In fact, when I'm feeling worn down and need a break I'll take a week where I just go to positive failure. And even then I feel a bit guilty because I don't feel like I'm even working out and start getting that fire going again where I want to torture myself. That's why it's no surprise to me that the vast majority of the people you see in the gym never looked like they have ever picked up a weight in their life. They spend day after day, week after week, year after year, in the gym going through the motions but don't look any different than the average skinny fat slob you see on the street. When I get to the gym I spend a few minutes on the bike to warm up and watch how people train. Virtually without exception, at least in my gym, no one comes even remotely close to pushing themselves. They terminate a set when they could have gotten at least another 5 or 6 reps.
As Jones said, "Below a certain threshold of intensity exercise will do little or nothing toward increasing strength and functional ability."