Central nervous system (CNS) fatigue occurs after many types of exercise, but especially after strength training. While CNS fatigue also during exercise, the primary causes of CNS fatigue during and after exercise are probably quite different, and so we have to consider them separately.
#1. What is CNS fatigue, and how is it measured? Like fatigue in general, CNS fatigue can be defined as a reduction in strength that occurs after exercise. However, CNS fatigue differs from fatigue in general because the reductions in strength occur due to changes in the brain or spinal cord. Such changes cause reductions in strength by means of a decrease in the level of motor unit recruitment (or the firing rate) that is attained during the contraction. Importantly, these changes inside the brain and spinal cord do not affect the level of effort that is exerted during a strength test. In fact, CNS fatigue occurs when the expected level of motor unit recruitment is not attained, despite a maximal level of effort being exerted. For example, when performing a set of reps to muscular failure, the perceived level of effort is maximal at the point of task failure. However, this does not necessarily mean that the level of motor unit recruitment is also maximal at this point. . The gold standard measurement of CNS fatigue is voluntary activation, which is measured by comparing the amount of force that a muscle can produce through voluntary effort with the amount of force that the same muscle can produce during involuntary electrical stimulation. Yet, non-local changes in strength are also indicative of CNS fatigue. For example, if a workout is performed for a single leg and reductions in strength are observed in the other leg, then this would indicate the presence of (systemic) CNS fatigue. . #2. Does CNS fatigue really last for days after a strength training workout? Despite the fact that some strength training experts continue to repeat the idea that CNS fatigue dissipates within minutes after the final set of a strength training workout, the research is clear that CNS fatigue often lasts for days after a workout, even in strength-trained individuals. For example, this recent study in male athletes found that ten sets of a heavy load strength training exercise for the quadriceps caused CNS fatigue that lasted for 48 hours afterwards. Similarly, this even more recent study that was performed in both strength-trained and untrained males found that ten sets of a moderate load strength training exercise for the quadriceps caused CNS fatigue that lasted for 72 hours afterwards. And most recently of all, a study in untrained males found that a bout of calf raise exercise caused CNS fatigue for 48 hours. This study tested the effect of a single-leg workout on CNS fatigue in both the trained and untrained legs, and found that there was similar amount of CNS fatigue in both legs during the period of time following the workout. This indicates that the main mechanism through which CNS fatigue is active in the days after a workout is systemic (which means that it affects the whole body) rather than local (which would mean that it only affected the trained muscle). . #3. When does CNS fatigue take longer to dissipate? As a general rule, CNS fatigue seems to take longer to dissipate after a workout when the overall amount of fatigue is greater and more long-lasting. For example, it is well-known that the overall amount of fatigue is greater and more long-lasting when training with light loads to failure compared to when training with heavier loads. Similarly, the amount of CNS fatigue is greater and more long-lasting when training with light loads to failure compared to when training with heavier loads. Similarly, it is well-known that the overall amount of fatigue is greater and more long-lasting after eccentric training workouts compared to after normal strength training workouts. In the above studies, CNS fatigue lasted for between 48 – 72 hours after exercise. However, some studies measuring CNS fatigue after unaccustomed, eccentric training workouts have recorded CNS fatigue lasting for longer than this. Finally, it is also well-known that overall fatigue is greater and more long-lasting after unaccustomed workouts compared to after workouts to which the lifter is accustomed. Similarly, the amount of CNS fatigue is also greater and more long-lasting after unaccustomed workouts compared to after workouts to which a lifter is accustomed. . #4. Why does CNS fatigue take longer to dissipate? In the above section, we saw that CNS fatigue takes longer to dissipate after workouts that involve more fatigue in general. Workouts involving strength training with lighter loads to failure cause greater and more long-lasting fatigue (and CNS fatigue) compared to workouts involving heavier loads. Similarly, workouts involving eccentric training cause greater and more long-lasting fatigue (and CNS fatigue) compared to normal strength training workouts. And finally, there is more fatigue (and CNS fatigue) when lifters perform an unaccustomed workout compared to when they carry out a workout to which they are accustomed. This seems to imply a link between certain aspects of fatigue in general and the amount of CNS fatigue that occurs after a workout. This link may be the inflammatory response that occurs after exercise, which occurs primarily in order to repair muscle damage. . Indeed, research has often identified a strong link between inflammation and CNS fatigue. So it is logical that the inflammatory response after muscle-damaging workouts might also cause CNS fatigue. This then makes sense of the fact that greater fatigue overall is linked to greater CNS fatigue, because greater fatigue overall is likely caused by greater muscle damage. Greater muscle damage probably causes a more pronounced inflammatory response, and this in turn causes more CNS fatigue. . #5. Why might some experts assume that CNS fatigue dissipates quickly after exercise? There are two reasons why some experts might think that CNS fatigue dissipates after exercise. Firstly, the CNS fatigue that occurs during (and immediately after) exercise and the CNS fatigue that occurs in the days following exercise are probably caused by different mechanisms. The CNS fatigue that occurs during exercise is probably caused by spinal mechanisms (which probably dissipate quickly) and by supraspinal mechanisms involving afferent feedback (and subsequent increases in perceived effort). Thus, the CNS fatigue that is present during exercise does go away quite soon after the exercise is finished, as the causes of afferent feedback also dissipate. Nevertheless, the muscle damage that was triggered during the workout causes an inflammatory response, which rises quickly after approximately 3 hours. This inflammatory response is what is responsible for the CNS fatigue that occurs in the days after exercise, and it is unrelated to the CNS fatigue that occurs during exercise. Thus, if we focus on CNS fatigue in the couple of hours immediately after exercise, then we will likely observe a rapid removal of the CNS fatigue that was present during exercise. Yet, that does not mean that there will not be any CNS fatigue the following day, since that would be caused by a different mechanism. . Secondly, some other measurements of CNS activity do go back to normal quickly after a workout (and not change in the days afterwards). For example, the reductions in corticospinal excitability immediately after a strength training workout disappear very quickly afterwards. While reductions in corticospinal excitability likely do contribute to CNS fatigue, they are not a good measurement of CNS fatigue. The best measurement of CNS fatigue is a reduction in voluntary activation, and another good measurement is non-local fatigue in a muscle that did not exercise. . Yet, it is clear that changes in corticospinal excitability do not reflect changes in the ability to activate the muscle, because such changes are also even more pronounced after motor learning. For example, strength training at a slow tempo and motor skill training lead to similar long-term increases in corticospinal excitability, and these increases are greater than those that occur after normal strength training at a self-paced speed. It seems likely that the short-lived changes in corticospinal excitability after a workout are more closely related to adaptations in coordination than they are to temporary reductions in voluntary activation. . Conclusions Central nervous system (CNS) fatigue often lasts for a few days after a strength training workout, even in strength-trained individuals. The duration and extent of the CNS fatigue seems to be linked to the overall amount of fatigue. When workouts produce more long-lasting fatigue, they also produce more CNS fatigue. For example, workouts involving light load training to failure cause more fatigue (and CNS fatigue) than heavier load workouts, while eccentric training causes more fatigue (and CNS fatigue) than normal strength training. The CNS fatigue after a workout seems to be produced by the inflammatory response that is involved in muscle damage repair. When workouts cause more muscle damage, this leads to more long-lasting fatigue. However, it also leads to a larger inflammatory response, and this creates a larger amount of CNS fatigue.
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