Here's a different, scientific approach to overtraining.
Volume training is not only a waste of time and effort, but is actually counterproductive. You draw from your energy reserves during training. And it doesn't take 5 minutes after your workout to restore the energy that was used up. It takes days. And It's far more important for your body to restore it's energy reserves than it is to use energy for building new muscle tissue because those energy reserves are necessary for survival, while building a 20 inch arm is not.
Imagine each set performed is like digging a hole into your energy reserves. The more sets performed, the deeper the hole and the longer that hole takes to fill up during rest. The first thing your body must do after the workout is not build a mountain, i.e., the new muscle growth on top, but fill the hole you've made below. That is, it must recover, overcome the deficit, compensate for the exhaustive effects of the workout. And so because it takes several days before that hole is filled up, it takes even longer to start building the mountain.
With that in mind, we're able to see just how and why HIT is as effective as it is. When volume is lowered, and energy reserves are spared, we are able to restore our energy reserves faster and enter the muscle building process sooner. Not only do we enter the muscle process sooner, but we also have a greater amount of energy which can be used to build new muscles.
Then there's the issue of intensity. The relationship of training intensity and duration exist on an inverse ratio. You can either train hard or long, but you can't do both. Honestly, I think people on high volume programs simply can't push themselves to the extent of absolute failure. I really do. If you did that, your body would force you to lower volume. And arguments like this is just for people to falsely reassure themselves that their lack of efforts in the gym is working. HA!
HIT is a workout program that was the result of one man's use of of a specific method of thought. One of logic and reasoning. Mike Mentzer was perhaps the greatest bodybuilder of all time. He dedicated his life to making a science out of bodybuilding. And that he did. It's called HIT. And I strongly suggest it.