About 45 years ago Arthur Jones addressed the question about why so few are growing bigger muscles. According to his analysis muscleheads were not training with enough intensity. He said they were also training too long. Arthur advocated brief, intense workouts and even designed his Nautilus machines to make exercises more intense. What did we find? A few champions tried the High Intensity Training. Mike and Ray Mentzer were outspoken proponents of that way of training. Ellington Darden, PhD, wrote many books about HIT and is probably the best authority on that method today. Sergio Oliva was at his largest and most impressive size after training with Arthur.
Well, has HIT become the way to train to gain muscles? Nope. What everyone found was intensity wasn't the only factor. You needed volume as well. Arnold, Sergio and just about everyone else who got huge did it by doing volume.
So what is the factor that causes hypertrophy? It isn't intensity per se or even volume. What has to happen for the muscles to grow is something extraordinary. When you think about it such a system would soon become extremely difficult to do.
If we look at the current pros training in videos we don't see themselves training in any brutal fashion. Sure, they workout hard but we see just about everyone doing the same thing. Lots of sets and lots of different exercises.
Instead of asking what is the stimulus we can examine the aftermath of training. If and only if the muscles are sore the following few days is rapid growth occurring. Anything else is mere maintenance.
The goal then is to always try to tigger DOMS on the day following a hard workout. Then sustain the DOMS after each and every hard workout. You will see that there has to be a progression in some way.
More weight lifted, finishing the sets quicker, doing more sets. It really doesn't matter as long as the end result is always DOMS. Combine that with a diet that makes you gain a little weight and growth will occur.