So I'm not sure how many people subscribe to this idea but CnS fatigue is not a chronic thing. There is evidence of transient central fatigue in the motor system but I don't know of any reason to think that the central nervous system will somehow fail (in healthy, nonpathologic situations) to excite muscle tissue adequately due to chronic over training. Safety factors at the neuromuscular junction are too high for this to occur. Central fatigue effects may last a few hours at most but likely do not accumulate over days

I agree, it's not like there is some neurotoxin causing fatigue...

your body will adapt to volume of training as well how about that!

I could go for inflammatory structural damage at the level of the muscle but the nervous system will not fail so easily

There are a number of positions on PNS and CNS fatigue, but there isn't a ton of scientific research on the topic to give a conclusive answer as one might hope. There is acknowledgement of its existence in scientific literature though.

I can say in the past that I have undergone what is normally called CNS fatigue or overtraining, but I suppose it could be attributed to a number of things that I don't have an answer for (inflammation, as mentioned, being one of them).

When I was in my most intensive stages of off-season training, I would do a light lift/mobility workout in the early morning followed by a throwing session, go to class, complete a second throwing session, then a heavy lifting session. There was a lot of volume with high intensity mixed in on heavy lifting days. That was the schedule for Mon/Wed/Fri. Tues/Thurs were similar, but the heavy lifting sessions were replaced with plyometric sessions, then either Saturday or Sunday would just be mobility, a light plyometric warm-up, and a single throwing session. This usually went on through October, November, December and part of January. Each year, by mid December, things would start to crash. Everything just felt heavy, I was slow, couldn't move well, everything hurt. Nutrients were in surplus and there didn't appear to be catabolism (can't be sure). There just wasn't enough recovery to allow for the adaption to that volume and intensity. The CNS was believed to play a roll in this as it didn't appear to be a gradual drop off as you might see with inflammation - it was just like the body didn't want to respond to stimulus as it would only days previous. So, the training volume and intensity was cut down for 1-2 weeks, and then everything seemed to bounce back to normal. It only seemed to happen like this once a year as volume was cut down once the in-season came in, so overtraining didn't seem likely. This is anecdotal of course, but it brought me to believe that overtraining is indeed a possibility, as I didn't have much other explanation for what I experienced on multiple occasions over a few years. As you said, it may last a few hours at most, but if you are training 3-4 times a day, 5 days a week, that may be enough to make it noticeable.

Eventually, in my last heavy phases of training, we would actually use a hand dynamometer as a way to try measure CNS response (this may be an old school way of doing things, but it seemed to apply to the daily training state). Some days, even weeks, the results were significantly down, so that was our indicator to cut back the volume for short periods. It served as well as there weren't big crashes in the response to training stimulus that year.

Doing research paper for physiology. Neural adaptations to resistance training. I'll let you know

Doms-like effects and structural muscle damage are a totally different ball game and there are ways to assess things like that. Did you know that acutely following muscle damage and for about 24h after there is significant increase in surface EMG which actually indicates increased neural drive submaximally (presumably to compensate for damaged mass and recruit higher threshold fibers earlier)? If anything this indicates a substantial reserve in nervous system power. Hand dynamometer mean nothing to the CnS. The only way to assess descending drive from the brain and spinal cord is electrophysiologically. There is too much going on post NMJ to take hand dynamometer torque as measure of voluntary activation.

If you give me a few hours I can probably get you the names of some of the bigger researchers in the field so you can look through their papers specifically. I work in a lab that deals fairly specifically with this topic, and I'm working on a project relating to the topic as well.

I didn't know about the EMG, but I never delved deeply in to the subject. The hand dynamometer was more as a gauge for training response than for an actual CNS test I suppose. The purpose was for the actual planning of the training than for the root physiological reason of the fatigue - there is undeniable fatigue and it can be observed, but CNS fatigue or overload may be the incorrect term that is used by laymen. I can't say that it is possible to easily ascertain the actual scientific cause of the fatigue without testing in a lab, so a practical approach in the field is beneficial for real world use in training manipulation.

I don't have the answers obviously, nor do I necessarily have the correct explanations as to why it happens, but I do know that nervous system response and fatigue does play a role in training and have been around enough elite athletes to have seen and heard discussion of such. I would be interested in seeing this stuff tested and observed in populations that have highly acute senses of the states of their bodies. I have seen so many instances of up and down days when it comes to response to stimulus, that I can't decide what else to attribute it too.

I can only relate with my own experience since I don't have the lab to test these things, and I know that with science, anecdotal and untested information isn't good enough for an explanation. I just see what is going on in the field. I know the throwing events in track and field as that is where my experience has been for over a decade - in that time I have trained with and observed world champions, olympic medalists, and world record holders. At that level of performance, their strength and power has reached a point that training in that aspect generally at a point of diminishing return and they stay around that level. Their main improvements and focus then come from the neural connection with technical changes, rhythm, and response to stimulus (such as ground contact time for force application, application of force to the implement, velocity in varying parts of the technique, etc). That is generally how they talk about their training - how it feels, how they can respond to movement, how they can cause movement, etc. So when there are days or weeks where that response is down, they refer to it as nervous system fatigue. That may be the incorrect way to label it, but when people talk about CNS fatigue, I think that is what they are referring too. When someone is highly in tune with their body, they can tell the difference between being tired, having soreness/doms, and actually feeling like your body won't respond to stimulus the same way. I just don't know what else I would call the later of those things.

Yea. Mine have to be all primary sources. So the papers has to include a methods and data section