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What is CNS Fatigue?


I've come across multiple articles on this site stating that overtraining causes CNS fatigue or that working
to failure really taxes the nervous system. What exactly does this mean?

I thought the CNS is the brain, spinal cord and spinal cord's "links" (not sure of the scientific word) to body's muscles. So, what gets tired? The message relay process? Can definitely feel an overall fatigue -- especially after doing heavy dls for 2-3 consecutive days (near end of PTP cycle). Curious what's going on.


Too much stress on your CNS results in, what I would call, signs that tell your body to "slow the fuck down!".

These signs can be as simple as the feeling of not wanting to exercise out or an obvious decrease in strength.

Other symtoms as a result of working your CNS too hard: a raise in morning body temperature (something as small as a 2-5degree change), increased tiredness or regression of CNS functions.


What is Nervous System Fatigue and How do I Prevent it?

Nervous system fatigue can be grouped into 2 categories, peripheral and central.

Central nervous system (CNS) fatigue is neural fatigue originating in the brain, brain stem, spinal cord, or spinal nerves. The exact mechanism for CNS fatigue remains largely unknown but it appears that acute CNS fatigue may occur as a result of decreased reflex sensitivity and / or less than optimal output from the motor cortex. Chronic CNS fatigue on the other hand is likely caused by increased inhibitory drive to the alpha motor neurons. Motivation and emotional state have also been linked to both acute and chronic CNS fatigue, but it is not known if this is a cause or effect relationship.

The demand that is placed on the CNS is a product of both the volume and intensity of training. CNS fatigue can be brought on by doing a high volume of low intensity training or by comparison a low volume of high intensity training. Insufficient balancing of work and rest periods coupled with overambitious training will likely lead to CNS fatigue. The best way to think of the effect various activities have on the CNS may be to put them on a continuum. The more intense something is (as a % of maximal effort / speed), the lower the volume that can be done until the CNS becomes fatigued.

Unlike CNS fatigue, peripheral nervous system (PNS) fatigue is localized to a given body site. For example, when running the 100m, PNS fatigue is likely to occur in the knee and hip extensor muscle groups. There are several possible mechanisms for PNS fatigue, ranging from the accumulation of inorganic Phosphate and H+, to the failure of the sarcoplasmic reticulum to release sufficient Ca++ due to signaling problems from the T-tubules, to insufficient manufacture and release of Ach at the neuromuscular junction. Unlike in the case of CNS fatigue, all of these mechanisms are acute and should not produce long lasting fatigue.


It's been shown (in an article that CT posted about a year ago for example) that after a few very heavy workouts, the natural firing rate of muscles gets locked in at a new, self protective level and that the rate can only be re-set with lighter training. For a beginner it might not matter. Later, you might need 1 lighter workout for every 2-3 heavy ones. Eventually, highly trained individuals need about a 1:1 ratio of intense to lighter training. Not training at all requires much longer to reset the levels (2-3 weeks versus being able to reset every week with a lighter workout). Lighter happens to be about 50-70% depending on the nature of the movement.

Also, as far as CNS, if you sleep well I believe you should wake up "sleepy" because you got into deep sleep. If you are stressed, you may be ready to jump out of bed because you are so wired and never really got to sleep in the first place. If you sleep 7 1/2 hours, its fine to wake up tired, but if you feel like you need a nap after 6-7 hours, you are not sleeping well at night.


No-one has ever demonstrated to my satisfaction any scientific basis for the concept of "CNS fatigue". There does not currently exist any valid, reliable test for assessing the state of the CNS (at least with regard to physical performance). I'm not saying that "CNS fatigue" doesn't exist, becuase it probably does, but I believe that most of the ideas we see getting tossed around on the subject are largely or purely inferred. Unfortunately, due to the scientific nature of the subject, mere inference doesn't cut it IMO.

I don't really see any need for the distinction of "CNS fatigue" anyway. Either you're ready to perform or you're not, and I don't see why it matters what the source of fatigue is. Regardless of why you're fatigued, the answer is rest.

I also don't see any basis for the claim that training to failure stresses the CNS more than anything else. Failure is related primarily to muscular fatigue. If I had to infer, I would suppose that heavy lifting and explosive lifting (not to failure) are more stressful to the CNS due to the higher firing rates involved.


What iam not understanding is how the system can get "fatigued" even though nuerotransmitters are so readily replenished in the CNS. CW recently mentioned that CNS recovery is actually quick, but the muscle tissue breakdown is what requires us to rest longer periods.... ???


Ever heard of education? people like to understand the reasoning behind things.


Why the angry response? I'm not saying that it isn't interesting, just that it probably wouldn't make any difference in your training if you knew.

And, it's wrong to draw conclusions based on huge inferential leaps that border on outright speculatory just because you want answers. Sometimes-often, in the field of training- the answers are beyond anyone's understanding.


I am experiencing that right now. I suspect that I am overtrained right now and may need more than just sleep/rest to get past it. I am using Power Drive daily regardless of whether I'm doing my dynamic effort days, lightened up on my loads and volume, and sometimes just taking a rest instead of training. I feel worn down a lot of days and take naps on my lunch break. It's getting better, and may resume sled dragging this weekend to break up the motor patterns that may have been overused/overtaxed by my training.

I've been training the Big Three, and toss in power cleans, hang cleans, and snatches if I feel like I may not have trained the speed hard enough. Speed training, even at 45% can wear your ass down if you push hard enough! Overcoming the fatigue can be a bitch if you're hard headed like me.


I understand that you feel people are drawing conclusions based on speculation, but i dont think the initial thread starter had that in mind. People seem to be curious in the subject for interest purposes, not necessarily foundation work for all their training.



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. :wink:

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.

Make sense?


I normally train in the 10-12 rep to absolute failure on most exercises but lately I've been mixing it up going as low as 3 reps on some compound exercises. Although I've made some steady gains due to the fact my body isn't used to low reps and was basically shocked I plan on returning to higher reps next week.

Point is I actually do feel a major difference in overall training ability and recovery. I feel like it takes me twice as long to fully recover. I sleep for long periods of time already usually at least 10 hrs. a nite but lately I've been adding 3-4 hour daytime naps plus an extra 1-2 hrs at nite.

I believe this is due to CNS fatigue. Even though it seems like a phenomenom noone can put their finger on exactly I think it's the cause of my recent problems. Plus I've been having unexplained back pain but that could be from my crappy matress. Who the hell knows.


Do I understand you correctly? You were sleeping 10 hours per night but have added an additional 2 hours to that plus up to 4 hours in naps during the day?

That's 16 hours of sleep per day. There's only 8 hours left....

You might want to back off on your training intensity, I have never heard of anyone requiring this much sleep. Well maybe infants...

(Not trying to be a wise guy-)


Good God. Koala bears don't even sleep that much.


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.


I'm going to try to experiment more regularly with Power Drive post-workout to see if that helps, as some have suggested. I'll keep you posted. A good, looong night's sleep definately helps a bit (at least more than having to get up early and not getting sufficient sleep), but that in and of itself isn't enough. I can still feel fried for an entire day afterwards. Maybe I'm just getting old. :frowning: I don't think this happened when I was 21. (I'm 31 now).