Your CNS isn't "Fried"
It's often said throughout the powerlifting community, and the lifting community in general, that someone's Central Nervous System is fatigued, or that their CNS is "fried" or a certain movement is "hard on the CNS". It is often said and accepted without question. But I have questions. First, how do you define CNS fatigue and how do you measure it? Second, how do you know that the fatigue you're feeling is specifically your CNS? And finally, what does the research say?
Davis et al. (1997) define CNS fatigue as a subset of fatigue(failure to maintain the required or expected force or power output) associated with specific alterations in CNS function that cannot reasonably be explained by dysfunction within the muscle itself. The researchers purposely left this definition broad to account for possible psychological factors contributing to fatigue as well. So in this case, CNS fatigue is anything that can't be attributed to muscular fatigue, neuromuscular, or metabolic factors. This is quite vague and therefore leads to the challenge of measuring Central Fatigue. Most often, this is measured by examining Voluntary Activation. If voluntary activation decreases relative to baseline, it is evidence that there is sub-optimal central output (Paul et al. 2015). This is not the only way to measure Central Fatigue but is one of the more commonly tested markers.
The second question to address, how do you know the fatigue you are feeling is specifically your CNS. Truth be told, you don't. I don't, nobody does. Unless you were to go in and get some central fatigue markers measured. However, fatigue in general is very multifactoral, so to attribute it to the Central Nervous System alone would not be accurate. Now you may say, well, "I had heavy squats and deadlifts this week, and those are hard on your CNS". This is an understandable assertion, as it is often thought that since these movements use a large amount of musculature, they are inherently more fatiguing. Deadlifts in particular seem to get a bad rap for this. However, based on recent evidence, this may not be the case.
Belcher et al. (2019) measured the time it took to recover from Squat, Bench Press, and Deadlift. This study was performed across 3 weeks, with week 1 being squat, week 2 bench press, and week 3 deadlift. Each participant performed 4 sets to failure with 80% of their 1 rep max. Subjects in the study were well-trained males. Outcome measures included limb swelling, joint range of motion, delayed onset muscle soreness, average concentric velocity (ACV) at 70% of 1RM, creatine kinase, lactate dehydrogenase, and cell-free DNA (cfDNA). The researchers found that it took about 72 hours for all markers to be within an acceptable range to be considered recovered. Meaning, there was no difference between exercises and squat and deadlift did not seem to take any extra time to recover from. And, it only took around 3 days to get to those levels! It should certainly be noted that the total volume done by each participant was likely not particularly high, and therefore high volume protocols should certainly be examined to see if similar results are found.
Barnes et al (2019) similarly evaluated squat and deadlift to determine if deadlifts led to more central fatigue than squats. In this study, 10 resistance trained males completed 8 sets of 2 reps with 95% of their 1 rep max for squat and deadlift and measured Voluntary Activation to examine central fatigue as well as other markers to examine peripheral (muscular, neuromuscular, etc.) fatigue. The researchers examined these before, 5 minutes post exercise and 30 minutes post exercise. The researchers did find some peripheral fatigue and some change in voluntary activation, but found no difference between exercises. So, even though more load was lifted and for great volume on deadlifts, this did not lead to greater central fatigue. It should be noted that this study measured these variables acutely, and it is possible that these markers could change as days went by.
Okay, so that compares levels of central fatigue, but what does the research say about CNS fatigue in general and is there evidence to attribute it to the heavy resistance training we typically expect it from? Howatson et al. (2015) looked at the effects of heavy resistance training on elite track and field athletes. Ten athletes performed 4 sets of 5 on squats, split squats, and push press and several variables were measured, including activation. these sessions were performed on separate days and variables were measured pre, post, and 24 hours after the session. While again there was evidence of peripheral fatigue, specifically neuromuscular fatigue as counter-movement jump height decreased, there was no change in activation, indicating limited central fatigue caused by these sessions. As always, limitations apply in that there were only 10 athletes and that other variables of central fatigue that weren't measured could have changed. However, based on current evidence, it appears these movements did not lead to substantial CNS fatigue of any kind.
An important point to acknowledge, and a limitation of these studies and many studies regarding central fatigue, is they often measure acute fatigue. These studies are not evaluating fatigue after a difficult peaking phase or volume phase, but are evaluating these markers during usually a week at most. This is definitely an area that should be examined more thoroughly, however, a strong argument could be made that if these markers of fatigue already begin to decrease within 24 hours, or there is little, if any, effect to begin with, any longer term studies would be unlikely to find higher levels of CNS fatigue than those indicated here. However, you know what they say about assuming, and while there is some research that has been done, more replication needs to be done to ensure reliability of results.
Now, this doesn't mean you should go crazy and squat, bench, deadlift, overhead press, etc. all day every day. While these movements may not inherently lead to increased CNS fatigue, muscular fatigue, neuromuscular fatigue, and peripheral fatigue as a whole are still important variables to consider. Also, doing wild and crazy volumes of work is a recipe for an overuse injury, as you are likely to exceed your tissue's abilities to adapt to what you are presenting to it. You are stlll likely to be better off starting with the minimum effective dose that gives you a positive response, then gradually increasing volumes and intensities from there. With that said, based on the evidence, it seems to indicate that CNS fatigue may not be what is causing you to feel less than stellar. The fatigue is definitely real, but the causes of it may be different than what is widely accepted.
It is also important to remember that, even though the research indicates that recovery times are similar across movements, you should always account for your personal recovery response. Anecdotally, you may find that it seems to take you an extra day to recover from deadlifts or squats. There's nothing wrong with that, just use that information to structure training accordingly. That type of information is also important to consider leading up to a competition as you develop a peaking and tapering strategy. The research is there to serve as a guide. As you learn more about yourself as a lifter, your strategies will adjust accordingly.
To wrap it up, the fatigue you're feeling is certainly real. However, based on current evidence, your CNS is probably not fatigued, or fried, or shot, or any other adjective, simply form lifting weights. That said, it should still be addressed in order to optimize performance.
Barnes, Matthew J.1; Miller, Adam1; Reeve, Daniel1; Stewart, Robin J.C.2Acute Neuromuscular and Endocrine Responses to Two Different Compound Exercises: Squat vs. Deadlift, Journal of Strength and Conditioning Research: September 2019 - Volume 33 - Issue 9 - p 2381-2387 doi: 10.1519/JSC.0000000000002140
Belcher, D. J., Sousa, C. A., Carzoli, J. P., Johnson, T. K., Helms, E. R., Visavadiya, N. P., ... & Zourdos, M. C. (2019). Time course of recovery is similar for the back squat, bench press, and deadlift in well-trained males.Applied Physiology, Nutrition, and Metabolism,44(10), 1033-1042.
DAVIS, J. MARK; BAILEY, STEPHEN P. Possible mechanisms of central nervous system fatigue during exercise, Medicine & Science in Sports & Exercise: January 1997 - Volume 29 - Issue 1 - p 45-57
Howatson, G., Brandon, R., & Hunter, A. M. (2016). The Response to and Recovery From Maximum-Strength and -Power Training in Elite Track and Field Athletes.International journal of sports physiology and performance,11(3), 356–362. https://doi.org/10.1123/ijspp.2015-0235
Marshall, P. W., Finn, H. T., & Siegler, J. C. (2015). The Magnitude of Peripheral Muscle Fatigue Induced by High and Low Intensity Single-Joint Exercise Does Not Lead to Central Motor Output Reductions in Resistance Trained Men.PloS one,10(10), e0140108. https://doi.org/10.1371/journal.pone.0140108