The Great CNS Fatigue Myth

Chapters:

00:00 Intro

00:29 What is CNS fatigue?

02:15 Central fatigue

03:38 CNS vs Central fatigue

04:17 Mental fatigue

06:10 Neuromuscular vs Mental fatigue

06:30 Outro

Transcript:

You’ve probably heard claims like these: “You should be very careful with deadlifts because they fry the CNS”, or “Take a deload weak every couple of weeks because central fatigue accumulates”. Most of the people making these claims have no idea what CNS fatigue actually is.

Research finds that neuromuscular fatigue after deadlifts is actually no greater than after squats or bench presses. Even leg presses do not stimulate more neuromuscular fatigue than leg extensions.

Central nervous system fatigue is not what the name suggests at all. Central nervous system fatigue is not actually central in nature. Most of the fatigue from the CNS is actually local, peripheral. The popular idea about CNS fatigue is that the central nervous system, in particular – the brain, has some dysfunction and therefore it affects your entire body. In that case, if the fatigue is in the brain, in the CNS, the effect is central, meaning it affects all of your body parts, and therefore this type of central fatigue would affect your biceps just as much as your quads. However, most of CNS fatigue in exercise science actually takes place in the motor neuron.

Now, this is technically called CNS fatigue because the cell body of the motor neuron, which is what transmits the electrical activity from the brain to the muscle to activate it, lies in the CNS, specifically in the spinal cord. It’s not even in the brain, it’s in the spinal cord. But the axon, like a tentacle, extends outward into a muscle, and it is that specific part of the chain where the fatigue takes place. So it’s the motor neuron that is fatigued, and the effect of that is only the fibers that it innervates. So it’s a very specific part, and it’s just that muscle that is affected by the fatigue of the motor neuron. But technically we call this CNS fatigue in exercise science because the motor neuron is part of the CNS. This confuses a lot of people because they naturally assume that CNS fatigue, “Central Nervous System” fatigue is in the brain and therefore must be central in effect. It is not. It is local. The vast majority of CNS fatigue is related simply to muscle damage. It is entirely local in effect, and it only affects the muscle group that was trained.

In fact, the whole idea of central fatigue as a concept is highly questioned in exercise science, especially for strength training. For example, one study found that all of the CNS changes that occurred after ten sets of squats were local in effect. There was no evidence of central fatigue. They were seen as changes, but they were related to the motor neurons, not to the actual brain, the overall capacity to direct the body as a whole. The strongest refutation of the idea of central fatigue is probably a 2021 meta-analysis of the entire literature that concluded that for strength training central fatigue is generally trivial or absent, in contrast to endurance training where we do actually have evidence of central fatigue.

If you think about it, that makes sense, because with endurance training, you train the heart, the lungs and central systems a lot more. Strength training is, by and large, local. You train your biceps, the biceps gets fatigued, the biceps adapts. There is no other process that occurs in the quads, for example. When you train the quads, the quads adapt, there’s stress in the quads, fatigue in the quad, growth in the quads, nothing happens in the biceps. Endurance training is a more central training in general, so it makes sense that there could be central fatigue after endurance training, but for strength training there isn’t. In the words of the researchers, they concluded: “Overall, the findings do not support the existence of a general non-local muscle fatigue effect.”

What had happened on social media is that they take a concept from exercise science, which is real CNS fatigue, they don’t understand what it means, and they conflate it with central fatigue, and therefore they make it out to be some general boogeyman with all sorts of implications that do not actually make any sense. The absence of any central fatigue after resistance training is actually very easy to verify yourself. You can just do a lower body workout and then check your biceps curl performance. Your biceps curls should not be any weaker because you did a bunch of leg extensions, squats, or any other exercise for the lower body beforehand. Similarly, after an upper body workout, your leg extensions or your leg curls should not be any weaker just because you trained other muscle groups.

When I teach this to my students they often protest: “But I can clearly feel the fatigue.” Well, what you feel is mental fatigue. Mental fatigue is measured as a subjective sensation. In exercise science fatigue refers to loss of force production. So if your biceps curl 8 rep max is still the same, you can still do 8 reps with your 8 rep max then there is no neuromuscular fatigue, but there might still be mental fatigue. We see big discrepancies between mental and physical fatigue in many scenarios.

For example, music can make you a lot more energized and can get rid of mental fatigue, but it doesn’t do anything for neuromuscular fatigue, obviously. Mental fatigue and loss of willpower sometimes called ego depletion in psychological research, actually occurs even when you do any other type of activity that’s not physical at all, but it is a “have to” activity, something you don’t necessarily want to do. Whereas if you’re doing something that you really enjoy, that you’re passionate about, you don’t suffer a lot of willpower depletion. For example, you might experience a lot of fatigue when visiting family members that you don’t necessarily want to see, or when you’re in traffic and you’re commuting.

Truck drivers experience a lot of mental fatigue, typically, but they’re not actually doing anything physically, they’re just sitting in their truck. But they develop mental fatigue because it’s boring. In fact, boredom and mental fatigue go hand in hand very strongly. Both occur during “have to” activities instead of “want to” activities. Mental fatigue can be thought of as a system that tells us that there is a lack of instant gratification for what we are currently doing.

Going to the gym is the ultimate lack of instant gratification. You have to push yourself past your comfort zone and suffer immediate pain to get later future results. As a result, in the gym we develop mental fatigue and even though mental fatigue is just psychological, in large amounts it can actually impair performance. So some people argue that this is all semantics. It’s all just giving the fatigue a different name, but it is central fatigue. While it is central fatigue, it is physiologically a very different concept.

If there was indeed neuromuscular fatigue in the brain, that would warrant a deload or some other reduction in training stress. However, knowing that the fatigue is mental has a completely different implication, because mental fatigue you can get rid of with caffeine or music, or just pushing through knowing that it is safe and that it is mental fatigue. So I hope this video helps demystify a lot of the bro science about deadlifts, frying your CNS, or needing deloads because of central fatigue accumulation.

In reality, central nervous system fatigue, despite what the name suggests, is mostly local in effect, and it is nothing to worry about. It is a normal part of neuromuscular fatigue, and as long as your performance is okay, you are fine. So in practice you just have to measure your performance. As long as you are progressing in a gym, you are fine and you don’t have to worry about any CNS fatigue boogieman.

If you’re tired of slop and misinformation on social media, you will probably love my Online PT Course. The link is in the description. Or you can keep watching my own YouTube slop like this video about the theory that broke fat loss science.