Is Central Nervous System Fatigue Slowing You Down?

Regardless of their athletic prowess, most people intuitively understand physical fatigue (or at least the basics of it). Simply put, it’s the inability to maintain power output—the point at which you can no longer pedal your bike fast enough, swing the kettlebell high enough, or find enough oomph to glide past other swimmers in the pool. You might start out strong, but after a while you… just… slow… down. That kettlebell might as well be a Fiat given the likelihood of you lifting it again.

When that happens, we usually blame our muscles for giving out, and that’s a distinct possibility. From glycogen depletion to acidification, plenty of things can contribute to localized muscle weakness. What a lot of people don’t realize is that physical fatigue can also be a symptom of a larger issue. It might feel like your legs quit after too many burpees, but the problem might not be your quads—something else might be going on that keeps your entire system from living up to its potential.

How Central Nervous System Fatigue Impacts Your Body

One possibility is a phenomenon called central nervous system fatigue (also known as CNS fatigue or plain ol’ central fatigue). The theory is that overtraining symptoms—including chronic fatigue, reduced athletic performance, and longer-than-usual recovery times—can stem from wear and tear on the complex of nerves in your brain and spinal cord (i.e., your CNS) that control the movements of your body.

You can think of your CNS as a city electric grid and your muscles as factories in the suburbs that demand a disproportionate amount of energy. Eventually, their demand overwhelms their power lines, causing local power failures and system-wide energy shortages. Even if your muscles wanted to sustain their output, they can’t because their power supply is compromised. Scientists demonstrate this phenomenon in the lab by stimulating a muscle’s motor nerve directly, producing induced contractions that are stronger than the subject’s voluntary ones. The takeaway: There’s more potential for muscular work than the brain can muster. In practice, such fatigue manifests as a lazy, sleepy, overall weakness that kills the motivation to lace up despite (in theory) having received enough rest.

Related: 7 Tips to Avoid Overtraining

Misconceptions About Central Nervous System Fatigue

While central fatigue is an extremely viable explanation for overtraining issues, it does have a checkered history. Experts spent much of the 1990s misidentifying and over-blaming it for poor athletic performance (sort of like the sports equivalent of fibromyalgia and chronic fatigue syndrome). But central fatigue is not just slang for feeling bad or having an off day at the gym. Numerous studies have confirmed that something, or more likely some things, potentially interfere at a cellular level with athletic performance following sustained exertion or excessive training.

Central fatigue should not be confused with peripheral fatigue. According to Emma Ross, head of physiology at the English Institute of Sport, when it comes to peripheral fatigue, “we’re talking about a reduction in the muscle’s ability to generate force.” This reduction can be due to any number of physical processes, such as the buildup of lactic acid or the depletion of fuel stores. The key is that it’s happening in the muscles themselves. Central fatigue, by contrast, is marked by impairments happening upstream of the neuromuscular junction. In other words, problems occur before the nerves even connect to the muscles in question.

Ross and her team have found that central fatigue can torpedo performance by 15 percent. In practical terms, that means your competitors have a 15 percent advantage if they avoid it and you don’t. “It is now really well established that central fatigue happens,” says Ross. “And it occurs because exercise—particularly continuous, prolonged, high-intensity, endurance-type exercise—elicits changes in the excitability of the motor cortex and in the brain’s ability to drive the muscle fully.”

Debates Around Central Nervous System Fatigue

Exactly how strenuous exercise causes those changes is a matter of debate. The most accepted theory was outlined in a study by Belgium researchers in the journal Sports Medicine. The short version is that it involves an uptick in levels of the sleep-related neurotransmitter serotonin (5-HT) brought on by a drop in blood levels of branched-chain amino acids (BCAAs), as happens during exercise. There’s a cascade of biochemical processes that explain why this could be so, but suffice it to say that while it may not be the whole story, the shifting BCAA-5HT ratio seems to be a key component.

There’s also some debate as to whether central fatigue is even a bad thing. After intense effort, the condition could simply be the body’s way of throwing up a caution sign and forcing us to cool our jets before we inflict serious (and perhaps permanent) damage on ourselves.

The situation also becomes a bit tricky in competitive sport, particularly in endurance activities like running, cycling, rowing, and swimming, all of which involve pushing physical limits to achieve a new, superior level of “normal.” What’s needed in endurance training is a controlled approach to such redlining so you don’t fry your circuits. Broadly stated, working at or near your max regularly but infrequently is a good thing, but doing so too often will take a toll.

“There is some central fatigue that occurs with almost every strenuous exercise session, but this usually goes away relatively quickly,” says Mark Davis, Ph.D., director of graduate programs in applied physiology at the University of South Carolina, Columbia. It’s more pronounced with extreme workouts/competitions, such as marathons, triathlons, or three-day combat missions, but the body usually does a decent job of dealing with it. You might take a little longer than usual to recover fully—especially if tissue injury is involved—but recover you will.

How Much is Too Much Fatigue?

For competitive athletes, this is an expected part of training, says Davis. But if that training isn’t managed properly, it can quickly shift from an acute issue to a chronic problem with the athlete stuck in a perpetual cycle of central fatigue. That not only makes training suck, but it’s also a real bummer on race day.

How do you avoid becoming a fatigue slave? Recovery. The first steps are creating an intelligent, periodized training plan, logging enough hours of sleep, and prioritizing. You can also mitigate the risk with a few key diet modifications.

If scientists are right about the serotonin connection, eating sufficient carbs and branched-chain amino acids (BCAAs) as you train should help keep your brain from taking on too much of the sleepy stuff. Good BCAA sources include whey and casein proteins, beans, beef, chicken, and soy.

A dash of caffeine and maintaining normal blood glucose levels also helps, as does tweaking your diet to fight inflammation (e.g., by increasing your consumption of fruits, vegetables, and omega-3-rich fish, and reducing your intake of refined carbs and trans fats). Some supplements can also help, including quercetin, which studies suggest is a potent anti-inflammatory. Beachbody Performance Recover helps combat exercise-induced muscle soreness after your workout.

Overall, David believes, central fatigue is for the most part inevitable. Nearly everyone who performs intense workouts with will experience it. But with appropriate training, rest, and nutrition, it doesn’t have to get in the way of achieving your goals.