CNS fatigue is a normal response to exercise

Some strength training experts have suggested that central nervous system (CNS) fatigue is an uncommon response to strength training workouts, and claim that we can largely ignore it when programming. Yet, CNS fatigue is common both during and after exercise (so we should not confuse it with overtraining).

Even so, CNS fatigue involves a temporary reduction in our ability to access all of our high-threshold motor units (which is how it impairs force). This is important, because it prevents us from triggering adaptations in the muscle fibers that are controlled by those motor units. Therefore, training while CNS fatigue is present is detrimental for hypertrophy.

Ideally, we want to adopt training approaches that reduce the amount of CNS fatigue during workouts, and we want to avoid training a muscle while CNS fatigue is still present from a previous workout.

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#1. CNS fatigue occurs during exercise

Many studies have shown that CNS fatigue builds up during exercise, and can be measured immediately upon resting. This can be seen most easily during aerobic exercise, when CNS fatigue accumulates gradually over the duration of a long run or bike ride.

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Even so, there is also research showing that CNS fatigue is apparent after a single strength training workout. This makes it very important to consider CNS fatigue when writing a program. 

The study below is particularly useful for our understanding, because it shows (1) that attaining greater total fatigue by training closer to task failure make the presence of CNS fatigue more likely, and (2) that greater cardiovascular demand during strength training is strongly associated with a greater level of CNS fatigue.

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The causes of CNS fatigue during exercise are not clear, but likely involve afferent signals. Such afferent signaling could arise from the working muscles (due to receptors detecting the presence of metabolites or inflammation) or from the cardiovascular system. 

The above study makes a strong case that afferent signaling from the cardiovascular system is a major factor determining CNS fatigue during strength training. Even so, the exact causes of CNS fatigue may differ between different types of exercise, since longer duration aerobic exercise actually causes proportionally far more CNS fatigue than high-intensity exercise!

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Even so, there are still indications that afferent signaling from metabolites or inflammation may play a role in triggering CNS fatigue during strength training, because CNS fatigue is progressively increased when using progressively lighter loads during strength training (it is a myth that heavy multi-joint exercises like deadlifts cause the most CNS fatigue – they almost certainly do not).

Conclusions and practical implications

Based on our current understanding of how CNS fatigue works during exercise, it is likely to be increased by training closer to failure, training in higher rep ranges, using short rest periods, and implementing advanced training techniques that trigger a lot of local fatigue (drop sets, rest pause, pre-exhaustion, etc.). We can therefore minimize CNS fatigue during a workout by training using heavier loads (5 – 7 reps per set), working with 1 – 2 reps in reserve, taking longer rest periods between sets (2 – 3 minutes), and avoiding fatiguing techniques.

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#2. CNS fatigue occurs after exercise

Several studies have shown that prolonged CNS fatigue occurs after strength training workouts, and many other studies have reported CNS fatigue that lasts for days after other forms of muscle-damaging exercise. The amount of CNS fatigue seems to be linked to the amount of muscle damage that is caused by the workout. The muscle damage likely triggers a repair process that involves inflammation, which then produces a signal that causes CNS fatigue. 

Although low volume workouts do not seem to cause any CNS fatigue, moderate volume workouts (such as 10 sets of 5 reps for a muscle group) tend to cause CNS fatigue for a couple of days (48 hours) in strength-trained athletes.

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Higher volume workouts (such as 10 sets of 10 reps for a muscle group) tend to cause longer-lived CNS fatigue (at least 72 hours) in both trained and untrained subjects.

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Eccentric training workouts, which cause much more muscle damage than normal strength training workouts, tend to produce CNS fatigue that lasts for a longer period of time. As is shown below, CNS fatigue for as long as a week after a workout has been observed in some studies.

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Conclusions and practical implications

Based on our current understanding of how CNS fatigue works after exercise, it is likely to be decreased by training using methods that minimize muscle damage. Although there isn't space here to do a detailed examination of the factors that cause muscle damage, it is worth noting that, during normal strength training, the major factor that leads to muscle damage is actually peripheral fatigue and not high levels of mechanical tension. Thus, many of the approaches that work for reducing CNS fatigue during a workout also work for reducing CNS fatigue that lasts for days after a workout.

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Conclusions

Fatigue is a temporary reduction in muscular strength resulting from exercise. CNS fatigue is therefore a temporary reduction in muscular strength resulting from exercise, which is caused by an inability to recruit all of the high-threshold motor units. CNS fatigue is not an unusual phenomenon, nor does it imply overtraining. It occurs normally during and after both aerobic exercise and strength training. Even so, since it reduces recruitment of the high-threshold motor units, CNS fatigue impairs muscle adaptations. 

During exercise, CNS fatigue seems to be greater when exercise is longer in duration, when it involves a larger cardiovascular demand, and when it involves higher levels of fatiguing sensations. After exercise, CNS fatigue is greater when the muscle damage caused by the previous workout is greater. We can likely reduce CNS fatigue both during and after exercise by training with heavier weights, longer rests, and avoiding failure. 

For a much more detailed examination of the various factors that influence peripheral and central nervous system fatigue, see my earlier article: fatigue

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