I’m no scientist, but I can explain the difference (which is indeed relevant) from my own experience. And yes, heavy high-intensity training fries the CNS more quickly than light or moderate-intensity training will.
When I get home after having done really heavy sets of a heavy compound movement (say, 5 or 6 sets of 4 or 5 reps, going to or close to failure) like deadlifts or squats, my whole body feels “fried.” This has nothing to do with muscular fatigue or soreness. This is a general sense of weakness, tiredness, lack of motivation to get out of my chair and get into the shower, feeling like my entire body is tapped out, almost tingling, couldn’t even dream of having any sex drive at that moment . . . almost feels like the overall state of weakness that your system is in when you’re coming down with something (cold or flu or something), only without the sinus issues.
Sometimes after a heavy workout like that, or a combination heavy workout plus cardio (cardio can do it to me as well), the CNS fatigue will last for the entire following day (I work out in the evenings). Even though my muscles might not be sore at all, my “system” just feels weak, fried, tired, doesn’t want to get off the couch, I don’t have the energy/motivation, etc. Caffeine can artifically pep you up in the meantime, but it’s clearly artificial energy – you can clearly feel that your nerves are still frazzled, and you don’t yet feel normal.
Yep, this makes total sense to me. Experiencing it now. Taking a nap and making sure my nutrition is dialed is helps me to feel ‘recovered’ as well.