Back when I was doing powerlifting there was a lot of talk about “central nervous system fatigue” and how it played a part in training modalities, and how we should have gone about avoiding it.
At the time, I made a lot of arguments about the fact that I didn’t believe that “CNS fatigue” was a real thing, because to me, the way it was explained over and over again by powerlifting blowhards, was that if you did a lift too often, you’d get “CNS burnout”. Or that really heavy training “takes a toll on the CNS”. Just really galactically stupid ideas that made absolutely no sense to me at that time, but I couldn’t quite put my finger on why.
Turns out, I was wrong. Turns out, I was right, too.
I was wrong in saying “CNS fatigue” isn’t real. It’s real. Ok, let me quantify that. You can’t “burn out” your CNS from lifting weights. And you don’t stall on a lift because you have CNS burnout, either. But central fatigue is absolutely a REAL thing. I was correct in my feelings and intuition that the way lunkhead powerlifters talked about it were absolutely wrong.
So, let’s break down what it is we’re talking about when we discuss central fatigue, what really causes it, what it really looks like in training, and how we can mitigate it.
What Central Fatigue Really Is
Central fatigue is something that happens when your nervous system cannot recruit the usual amount of high threshold motor units that it usually does (in an unfatigued state), so fewer muscle fibers will be activated when you’re doing a set, which means you won’t be able to mechanically load as many fibers, which means the hypertrophy response is now lessened.
A shorter way to say this is that central fatigue is a temporary state of performance reduction.
Central fatigue happens within the workout itself, and then for a period after that workout (could be a few hours or days all depending on what else it is you’re doing). But both central and peripheral fatigue begin to dissipate within hours after the workout.
Within the workout itself, if you do not allow for adequate recovery between sets, then you’ll not recover effectively from set to set, and there won’t be as many muscle fibers active within those sets. And then the sets performed in that fatigued state won’t be as effective for muscle growth as they could have been if you had simply rested longer.
Within the workout what you’re actually doing insofar as training goes will have an effect on central fatigue as well. What I mean by that is, doing heavy low rep work is not central nervous system taxing, as the lunkhead powerlifters always claimed that it was. It actually creates less central fatigue than stuff like metabolic stress style work and endurance training.
Anecdotally it can also be experienced when a weight moves slower than it usually does.
Metabolic stress style work tends to create a higher degree of central fatigue due to the afferent feedback the nervous system gets about the metabolite accumulation and muscle fibers being active for longer durations.
Endurance work that goes on for longer durations also has a high degree of central fatigue, while shorter sessions tend to tax the localized tissue itself (Goodall, 2015).
Movements that require a high degree of coordination create more central fatigue, as do cognitive tasks. Central fatigue will also impair the degree you can learn a highly coordinated motor skill and even will create a longer learning curve from that point on each subsequent time you engage in it (Branscheidt, 2019).
What “CNS fatigue” Is Not
Central fatigue isn’t “workout hangover.” This is also something I got wrong as well. You could be suffering from central fatigue on a day you feel tired and worn out, but that in and of itself is not a sign that you’re suffering from central fatigue.
Stalling on progress on a lift because you “did it too often” isn’t a sign of having “CNS burnout” either.
Generally, you still end up stalling on a lift because you’ve simply maximized your ability to produce more force at those particular joint angles, or you need better rate coding (less latency in motor neuron to unit firing) from a neural perspective.
You could be stalled on a lift due to central fatigue yes, but that doesn’t mean that changing up the lift was the answer to alleviating that.
Anytime you introduce a novel stimulus in the way of an exercise there will be a neural adaptation to it, which means there’s an upwards trajectory of strength at the inclusion of it, but this doesn’t mean that changing lifts “gave your CNS a break”.
That is galactically stupid.
Another bullshit theory was that your CNS gets taxed most heavily during “the heavy compounds for max loading+low reps.”
This is wrong on the premise that this style of training is the most central fatiguing. Central fatigue is seen in all forms of training but most especially in longer duration endurance work and metabolic style training. Not heavier training with lower reps. But leave it to lunkhead powerlifters to just continue to make shit up based on “feelings” and not science.
This also leads to a question of “so how do I know if I have some central fatigue that is going to make training less optimal than it could be?”
One really popular method is grip strength stuff. But this is a really poor way to assess central fatigue because by the time we’re seeing that, it means that central fatigue was already present for some time, and has continued to build.
Since central fatigue will keep us from recruiting the high threshold motor units that control the larger fast-twitch fibers, what is a better way to test for it would be with jumps or explosive throws. That is because this type of exercise requires the high threshold motor units to be active right away.
So theoretically, if a lifter were to have some benchmarks for his jumps or throws, then he could use those benchmarks to see just how recovered he is.
Why All of This Matters for Optimal Hypertrophy Training
All of the above is important for maximizing hypertrophy training because it’s kinda of super freaking important to make sure that the largest of the muscle fibers are activated during our training. Remember, based on the SIZE principle that motor units are recruited from smallest to largest.
So if we’re suffering from central fatigue then we will FAIL on our training sets before the body can actually recruit the largest fibers.
If what we want is to maximize our training so that we grow bigger muscles, then the following is what we’re really looking for.
We want to have a lot of localized fatigue at the muscular level, with as little central fatigue as possible within the training session. And to make sure that we are recovered between each training sessions as well and not walking into the training sessions with central fatigue.
The reason we want more fatigue at the muscular level with as little central fatigue as possible is because as the muscle fatigues, the nervous system will dial up the high threshold motor units faster in the subsequent sets, and then those big ol’ muscle fibers get tension early in the set. This is good.
This also means we don’t want to walk into training sessions already carrying some central fatigue because those sessions are going to be substandard in terms of muscle fiber activation, and will just accumulate more central fatigue. All bad things.
What are the things that impact this the most?
In no particular order:
Rest between sets
Let me also state that there’s actually a multitude of other factors that can induce central fatigue like the degree of coordination involved in a particular movement and life stress, but I can’t cover them all in this single article. What I can do is give you a few really good tools to avoid central fatigue in your training as much as possible. That way, your training sessions can be more result-producing.
This is easily the least sexy, but most important of all things related to reducing central fatigue.
Truth is there’s not been a ton of research really digging deep into this, but I think we’ve all known intuitively that a lack of sleep probably isn’t a good thing in terms of recovery.
The most irritating thing about bringing up this topic are the people who get less sleep but then peacock around about how they are still making gains in sleep-deprived states.
Ok, brainiac, let’s imagine for a minute that you were getting 8.5 hours instead of 5.5. Is it just POSSIBLE that you’d make better gains than what you are seeing if you were getting that extra sleep?
I think it is possible. That’s dripping heavily with sarcasm in case you’re not catching on.
For example, we’ve seen in rodent studies that sleep deprivation did, in fact, cause muscle loss (Monico-Neto, 2017) and muscle loss in hypocaloric states for people (Nedeltcheva, 2010).
Sleep loss is directly associated with a multitude of studies concerning performance connected to central fatigue (Skurvydas, 2021 is an example) so the fact is, if you want to hold on to lean mass when dieting and desire better body composition, and want your training sessions to be as productive as possible, then you need to prioritize sleep.
Naps can mitigate sleep loss, so yes, if you’re a shift worker or new parent(s), then getting in what you can will always be better than opting for less sleep.
Rest Longer Between Your Sets to Failure (or Close to Failure)
This is one of my favorite topics because for years, guys have said to have short rest periods between sets, in order to do shit like facilitate “more growth hormone from the training session”. That’s due to some acute spikes in growth hormone in a few studies that aren’t going to make any difference in the long run, in terms of more lean tissue.
But the fact is, short rest periods continuously shit the bed in terms of producing more muscle mass when compared to longer rest periods, eg three minutes or longer.
Shorter periods between sets to failure consistently show reductions in hypertrophy compared to longer rest periods. This is probably because the nervous system can’t recruit as many high threshold motor units when the CNS is still recovering from the previous set. But it’s also been shown that shorter rest periods attenuate mTOR and muscle protein synthesis (McKendry, 2016).
If you’re not incredibly verbose what that means is, when you have short rest periods between your hard sets, then you actually decrease the mechanical signals that kick off the biological signaling to make you grow bigger muscles.
The part that we’re absolutely sure of at this point is that short rest periods produce less hypertrophy outcomes than longer rest periods. That’s not even an argument anymore.
So if you want to have more productive training sessions, rest three minutes or longer between your working sets.
The last issue is that of your actual rep range. As I touched on before, when you train the fibers that are activated during a set, they stay active during the duration of that set. And the more reps you do, the more metabolites and pain sensory afferent feedback the nervous system gets, the more fatigued it becomes.
Afferent feedback is related to the signals the afferent nerves send the nervous system when we’re doing training sets that cause both muscular and central fatigue. This is also why that doing strength training after aerobic work is probably less effective due to the afferent nerve feedback related to such aerobic work.
This is also why we probably see methods like blood flow restriction causing mechanical tension in low loads with lower reps, because you literally cannot get due more reps due to the afferent nerve feedback given to the nervous system.
So staying within an efficient rep range like 6-10 reps to failure or close to failure, will also be far more efficient than doing very high rep sets, despite the fact that they produce similar hypertrophy outcomes.
To Wrap This Whole Thing Up…
You can mitigate central fatigue during and after a workout by doing the following –
Don’t use high rep sets. Sets of 6-10 reps (to failure or close to it) will be optimal.
Don’t take short rest periods between sets, eg 90 seconds or less. 3+ minutes will be best.
Don’t do concurrent training like marathon running, swimming, etc that are CNS fatiguing more than other forms of exercise if you’re really trying to maximize muscle growth.
Make sure that you’re getting at least 8 hours of sleep.