Curiosity with scepticism is healthy. Did you notice one of the guys who most needs help 'yawns' in this thread. How typical of knuckleheads to dismiss what others say and cling to notions that result in hardly any improvement at all. That is the problem. Too many think they know what they are doing. The results say otherwise.
There are two ways we could approach the problem of diminishing returns from training. Over a certain size the number of years spent in the gym mean nothing at all. No further growth occurs and bodybuilders subscribe to the belief that steroids and other drugs are the only way they will get any bigger. In addition, they attribute the success of some to genetics when in truth no one has a clue about who has potential, and how much size is possible. If Larry Scott was a young fellow today could he be Mr Olympia? I wonder. The odds are that he could not. However, it might be possible. He put an awful lot of muscle on a rather small frame. Larry was and is the master of bodybuilding technology. Because he had a small frame he had to discover the most effective methods to build his body. The question is whether he found the optimum system or just part of it.
Okay, we have some factors in hypertrophy that must be there. Resistance leading to mechanical tension on a target muscle and some sort of duration and frequency. Exactly how much is required, how long and how often has never been determined. It is believed that each person might be unique and therefore has to find his optimum protocols, etc. If it is true that each person is an individual and needs special routines and protocols then we might as well abandon any help from science. When you cannot predict results you cannot know the process and hence hypertrophy would not be knowable by science. Or if knowable would be too complex to be of use to individuals.
Why do so many toil for years and years and stay more or less the same? That is something that has never been studied and therefore all we can do is conjecture. Sure, diet and the repeated bout effect can undermine the progress of most of us. Are there strategies that can overcome the repeated bout effect? We can eliminate diet by assuming one should keep abreast of the latest proven research and maintain an optimum diet for growth.
What amuses me is to talk to people about training and then see the people doing the same thing over and over while looking the same. Surely that is lunacy. Let me describe a situation with an actual bodybuilder. This bloke is trying to build his calves up. We have had a few discussions about strategies and methods. His calves do not look any bigger than before and this has been going on for several years now. He is not stupid and considers what I say. Today I had another chat with him. He had the seated calf machine loaded up with about 180 Kg or 400 pounds. The heavy frame adds another 100 pounds at least. Here he was doing sets with 500 pounds of resistance. To be able to move that amount of weight and do several sets he was resting several minutes between sets. Maybe 5 minutes. I told him that isn't going to make his calves much bigger? Why? Well, it seems to me he is doing strength training and there is little reason for his calves to get any bigger. So the calves do the right thing and stay the same size. We talked about eating and I told him he can't expect more size if he is not gaining weight. I suggested he wear a heavy vest around and walk up lots of hills every day for a few months and see what happens.
The point is simple. If you are training hard and not growing rapidly you are doing something wrong or failing to do something and are probably not eating enough, either. So, if you return to the gym and keep doing the same thing that is lunacy. Oh, I am sure trainees justify what they are doing in the belief that gains come slowly and therefore are imperceptible. It takes years, or so everyone says. My point is if you are not growing from each workout then do something different. That is a requirement. Sure adding weight might help but you cannot keep adding weight indefinitely. If you are adding weight and not growing then that is pointless. You might not be putting enough mechanical tension on your muscle. You might have to try a different exercise or different protocols or both. Somehow you have to figure out how to grow rapidly. When you can do that for one muscle it is easy to apply those principles to other muscles.
Oh, for what it is worth, HIT and HST do not work beyond a certain point. Something is wrong with the theory behind those methods. It isn't intensity that causes hypertrophy although sometimes it can stimulate some growth. Nope, that isn't a sufficient factor. HST tries to distill a method from science. If only the scientists were trying to cause as much growth as possible. Well, they have had some success with fowl and cats and perhaps some rats. Humans have been competely ignored as far as trying to cause substantial growth in their muscles. It is a pipedream that HST is going to be the optimum method and that is what we find. The forums of these believers are repetitive and hardly anyone is still growing. The methods seem to work on some people for a while then growth stops. Thus, these believers wonder what to do next and start changing things in the prescriptions. I guess that is trial and error and you wonder how a supposedly scientific method still needs it. The truth is those methods are not themselves scientific. There has been little or no research to support those beliefs and methods. It really reduces to charlatanry.
I have no obligation to describe specific methods and prescriptions. Suffice is to point out the folly of what so many believe. This is, afterall, a discussion board of opinions regarding bodybuilding, etc.