Intensity is overrated.
Yep. The nervous system does not recover well. When it is fatigued, the body responds by limiting the use of it. It does not "develop" it like a muscle. Real world example: have a bad day at work, now go and workout. Your capacity to workout is diminished because the NERVOUS SYSTEM is fatigued. It has nothing to do with your muscles.
That's why overtraining shows up as CNS symptoms, not muscular symptoms. Why would muscular overtraining cause depression and suppression of appetite, lack of sleep, nervousness?
Our bodies developed responses and adaptations to enable survival, not enable "big pythons". Essentially, it looks at "all out" muscular work as a an extreme danger to it's survival, so severe that the immediate adaptation is to limit your ability to do it again.
To use an analogy, HIT is like going out into the sun and burning to a crisp. The body makes damned sure you will not do it again anytime soon. Using less intensity but higher volume is like going out in the sun for 10 min, coming in, going out again later, etc.
That's why growth must be coaxed, not forced.
