Ask any strength coach or sport scientist what fatigue is and you’re likely to get an answer like, “reduced force output”. It is generally understood that fatigue reduces our ability to produce force, but that definition hints more at a symptom of fatigue, rather than addressing the cause of the reduced output. Often we think of fatigue as a muscular problem: microtears in the muscle leading to delayed onset muscle soreness. This muscular fatigue is called peripheral fatigue. However, there is an equally, if not more influential system involved in fatigue, the central nervous system. When the CNS cannot carry an action potential to the muscles with adequate frequency and amplitude to meet the required force output of the motor unit, this is called central fatigue.
While it is easy to understand peripheral fatigue, we can feel it as sore muscles in the days after a heavy lift or training, it’s not so easy to feel the effects of central fatigue. The path from the brain to the muscle complex and influenced by many factors, the alteration of which can reduce neural drive and subsequent force output.
Serotonin, for instance, plays an important role in modulating neural drive during exercise. In sustained maximal voluntary contractions, serotonin is released to increase excitability of the motoneuron and aid in the sustained maximal contraction, but even still, the relative force is greatly diminished after only 15 seconds of MVC.
While evident during short duration MVC, this central fatigue plays significant role in reduced force output for longer duration exercise. As stated by Taylor, et al, “Supraspinal fatigue is relatively modest (accounting for about 25% of the loss of force) for short-duration sustained MVCs (about 2 min) and most of the fatigability encountered is located in the muscle. In contrast, for long-duration submaximal contractions (< 15% MVC), the reduced ability of the brain to drive the muscle develops incrementally during the effort and contributes to a greater extent (50-66%) to the whole level of fatigability than during a brief MVC. Similarly, greater supraspinal fatigue also occurs with lower intensity, but longer duration, whole body exercise.”
As we see, the CNS fatigues gradually, but ultimately to a great degree at the conclusion of long duration exercise, such as a hockey, soccer, basketball, football, or any other match where the athlete may have repeated bouts of lower intensity mixed with intermittent high intensity efforts.
CNS fatigue contributes to reduced force output in both reduced frequency and amplitude of action potentials. As one motor neuron can innervate up to 300 muscle fibers, a compromised neuron can greatly impact the muscles ability to produce force. Understanding central fatigue and its role in performance is critical for performance specialists to appropriately train and prepare athletes for competition.
This blog was written after reading the paper: “Neural Contributions to Muscle Fatigue: From the Brain to the Muscle and Back Again” by Janet L Taylor and colleagues. If interested, that article can be accessed on Google Scholar.