3 myths about central nervous system fatigue

Do you know how tiring you are in squats and deadlifts?

Why does it take days to recover from a heavy squat or deadlift workout?

How do you have trouble sleeping after a strenuous workout?

How more tiring are polyarticular, high intensity exercises than isolation exercises and sets with more repetitions?

It's central nervous system (CNS) fatigue… so-called. CNS fatigue is a subject riddled with broscience. Many people citing CNS fatigue in their arguments cannot even explain what it is.

So let's start at the beginning.

What is CNS fatigue?

As the name suggests, CNS fatigue occurs in the central nervous system: the brain and spinal cord. If your CNS is tired, it is having a hard time activating your muscles. So even though your muscles are capable of producing a lot of force, they may not reach this potential because the CNS is not giving them the proper instructions.

More formally, central fatigue occurs when the excitement provided by the activity of the motor cortex and / or the motor neuron decreases. In other words, CNS fatigue is a decrease in voluntary muscle activation.

CNS fatigue is distinguished from peripheral fatigue, which occurs outside the CNS. Muscle damage and metabolic stress in your muscles is an example of peripheral fatigue. Their effects are local and specific to the muscle in which they occur. If you tear an hamstring, it won't inherently affect your quads. In contrast, CNS fatigue can affect your entire body.

Central fatigue vs peripheral fatigue

Myth 1: The higher the intensity of the exercise, the more CNS fatigue you induce

It is commonly said that CNS fatigue occurs as a result of exercise with high neural stress, namely high intensity exercise. So, the theory is that sets with low reps induce more CNS fatigue than sets with high reps. It sounds very plausible. The higher the training intensity, the more CNS activation is needed, the more fatigued the CNS is, right?

False. It is completely the reverse. Long-lasting, low-intensity exercise causes much greater central fatigue than high-intensity short exercise [ 2 , 3 , 4 ].

CNS fatigue is easily seen after endurance exercise, like marathons, but scientists often have to really go out of their way to reliably induce central nervous system fatigue with strength training. As an example of a "strength training" study that revealed significant central fatigue, Smith et al. (2007) studied a 70-minute bicep contraction. I don't know about you, but that's not how I train my biceps! A similar study found central fatigue after a 4-minute dorsiflexion contraction .

A more realistic training design compared 3 sets of 12 with 1 minute rest between sets versus 5 sets of 3 with 3 minutes rest between sets. What caused more CNS fatigue? Trick question. Neither workout caused CNS fatigue . Other research has also failed to find CNS fatigue during resistance training, regardless of the intensity used .

In fact, in both of these studies there was upregulation of central motive power, presumably to compensate for peripheral fatigue. So not only was all the fatigue peripheral, but the CNS was actually working overtime to compensate for local fatigue.

You might object that most of this research has focused on weak individuals performing isolation exercises. How about heavy lifting athletes? We have the perfect study on this. Howatson et al. (2016)studied the neuromuscular recovery of high performance athletes. The guys squatted over 190kg and ran the 100m in 10.44 seconds. For reference, the world record is 9.58 seconds, set by Usain Bolt in 2009. The ladies squat over 108 kg and covered the 100m in 11.73 seconds. The world record is 10.49 seconds, set by Florence Griffith-Joyner in 1988 (ridiculously ahead of its time, by the way). These high performance athletes then performed one of their typical workouts, consisting of 4 sets of 5 reps for the back squat, split squat, and push press. : a total of 12 series of heavy polyarticular work. Split squats are a serious contender for the place of the most brutal exercise there is in weight training. Push presses involve the entire human kinetic chain from the feet to the fingertips and involve more musculature than squats or deadlifts. Even then, there was no central fatigue. Voluntary central nervous system activation did not decrease from pre- to post-workout and remained stable 24 hours later. Of course, there was significant neuromuscular fatigue, as evidenced by reduced muscle contraction power (MVIC) and an insignificant tendency for lower jump height (CMJ). There was also metabolic stress, as measured by an increase in blood lactate. But the nervous system had no trouble activating the muscles. The muscles were just fatigued themselves, possibly from training damage and metabolic stress.

If you think about it, it makes sense that the central nervous system doesn't get tired easily. Muscle fatigue is easy to imagine: it can occur mechanically. Muscle fibers can literally tear under the strain of hard contractions. For the CNS, many people speak of "neural fatigue". How it works ? The CNS is more like a computer than a muscle. A computer does not tire with use. Of course, it can overheat and over the years it can get slower, but it does not tire intensely. It doesn't get slower and slower if you use it for a long time in one sitting. So how would the CNS get tired?

Some researchers have wondered if there is CNS fatigue at all . The vast majority of what was previously considered central fatigue can actually be explained by local fatigue . However, as we have shown above, central nervous system fatigue is real. CNS "fatigue" probably occurs through other mechanisms. For example, it can be neurochemical: due to the effects of neurotransmitters. Or it can be metabolic: muscle ammonia production during exercise can seep into the blood and cross the blood-brain barrier, causing neurotoxicity [ 2 , 3]. In any case, high activation of the brain's motor cortex per se does not cause CNS fatigue, so low reps do not cause more CNS fatigue than higher reps.

Myth 2: The more polyarticular the exercise, the more CNS fatigue

The conventional wisdom of the bros is that the deadlift is the bane of the CNS. Heavy deadlifts cause so much CNS fatigue that you can only do them once in a while or you will overtrain yourself. Squats are next, followed by most other polyarticular exercises. Isolation exercises do not cause CNS fatigue.

What science says:

Les push press lourds, le squat et le split squat lourd de l’entraînement ci-dessus n’a pas induit de fatigue du SNC. Pourtant, plusieurs des études qui ont révélé une fatigue du SNC utilisaient du leg extension ou des curls biceps. Il est donc clair que les exercices d’isolation peuvent provoquer une fatigue du SNC et que les exercices polyarticulaires ne la provoquent pas nécessairement. Que diriez-vous d’une comparaison directe dans la même étude ?

Barnes et al. (2017) directly investigated the claim that the deadlift causes more CNS fatigue than the squat. Trained men performed 8 sets of 2 reps at 95% 1RM with 5 minutes of rest between sets on the squat and deadlift on different occasions. These heavy weight training sessions did indeed lead to central fatigue, but not that much: a 5 to 10% reduction in central neuronal production. Despite the higher weights used, more muscle mass involved, and more total work done during deadlifts, deadlifts did not result in more central fatigue than squats.. There was also no significant difference in the production of testosterone or cortisol.

In conclusion, research shows no relationship between the amount of muscle involved in exercise and the amount of CNS fatigue it induces. Isolation exercises can cause CNS fatigue, and compound exercises do not always. If there is a relationship, it is definitely not as strong as it is commonly claimed. This is to say that the CNS is more like a computer than a muscle: more difficult tasks do not necessarily tire it more.

Myth 3: CNS fatigue takes longer to recover than muscle fatigue

We often hear the saying that while your muscles recover between workouts, your CNS may not be recovering. Over time, this build-up of fatigue could lead to overtraining. Cool theory, but let's take a look at some data.

Latella et al. (2016) studied the evolution of CNS recovery after strength training. They were successful in inducing a huge 46% decrease in corticospinal excitability (measured by mechanically evoked potential, or MEP). This means major CNS fatigue. How many days do you think it took the CNS to recover?

It took 20 minutes for the CNS to recover. There was already no significant loss of MEP after 10 minutes. Other research confirms that CNS fatigue is only evident directly after training, even when muscle pain and peripheral neuromuscular fatigue took more than 3 days to recover. This probably explains the lack of CNS fatigue in the high performance athlete study we discussed previously: Howatson et al. measured CNS fatigue 10 minutes after training. Perhaps it was already too late. Interestingly, Latella et al. also found evidence that there was upregulation of the CNS rather than fatigue in the days after training: see graph below. MEP = mechanically evoked potential, which is roughly the strength of the signal sent by the motor cortex to the exercised muscle. A decrease suggests that the CNS can no longer fully activate the muscle, i.e. CNS fatigue.

 CNS Fatigue Over Time

All other measures of central fatigue in the study by Latella et al. (ICF, LICI and SICI) showed no alteration at any time during the 72 hour recovery period studied. Even directly after training, they were not affected. Thus, only certain aspects of the functioning of the CNS seem susceptible to fatigue.

In conclusion, CNS fatigue is largely momentary. Normally, it doesn't take days for the CNS to recover. The CNS can recover in just a few minutes.

Related research and references:

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