Fatigue Is An Emotion

Have you used up all the energy stored in your muscles and brain? Probably not.

Why are you tired? Are you working too hard, thinking too much? Have you used up all the energy stored in your muscles and brain?

We assume, logically, that exhaustion is the result of overexertion and depletion. Used up, your body begins to shut itself down. I used to think, for example, that when my legs burned during sprints, it was because my muscles were out of oxygen or glycogen or that I’d built up too much lactic acid and my heart wasn’t pumping fast enough. In other words, I was “pushing myself to the limit,” which explained my urge to quit during those excruciating seconds before the timer went off. But I didn’t quit, or, when I did, not once did I collapse in a heap of quivering jelly, unable to restart. A sixty-second jog to recovery, and somehow, I could do it all over again. Maybe I was never as near to the “limit” as I felt.

What about a long day’s race toward a project deadline? You know the feeling. Ninety minutes before the 5 pm cutoff, and you’re desperate. You yearn to make a pillow of your forearms and pass out right there on top of your keyboard. Of course, you fight the urge and press forward. At 4:55, as you click FINISH on the uploaded grant application, that terminal heaviness in your head evaporates. Fresh and ready, it’s time for the next thing.

These past ten days— exhausting, am I right? I wish I had a nickel for every time I heard someone (including me) say the word, exhausted. We slogged so hard, for so long, for so little. How can we go on?

But we go on.

Consider this: If we were truly so often at the limit of our capacity, we would routinely expire like so many spent parking meters. Yet, that rarely happens. Runners sometimes die in finish chutes, but such examples are always shocking, newsworthy outliers. Most of the time, fatigue sets in long before our bodies are in any kind of real physical jeopardy.

In the late 19th century, Italian physiologist Angelo Mosso proposed that fatigue is not a physiological state but rather an emotion manufactured by the brain in response to signals from the body. The point of fatigue is to protect the body from depletion before a crisis. In his model, the brain and the skeletal muscles work together like “two telegraph offices”, sending messages back and forth.

Mosso hypothesized that fatigue, which might “at first sight might appear an imperfection of our body, is on the contrary one of its most marvelous perfections.” One’s sense of fatigue increases more rapidly than actual depletion, as a way of protecting the body from potential harm. Mosso wrote that fatigue, which might “at first sight might appear an imperfection of our body, is on the contrary one of its most marvelous perfections.”¹

Over the past 125 years, other scientists have refined Mosso’s idea of fatigue-as-emotion, postulating that fatigue is created not in the muscles but in the brain, as a way to regulate energy expenditure and protect the body from potential harm. Timothy Noakes, a South African sports physiologist, proposes the idea of the brain as a “Central Governor” that creates the emotion we experience as “fatigue” as a way to slow us down well far in advance of any actual harm to the body.

That’s why you can work or work out “to the point of exhaustion” and then get up, make a cup of tea, walk your dog, call your mom, and go on with the day. That’s why ordinary people are capable of extraordinary feats of strength in times of emergency, like lifting cars to save victims pinned underneath.

In my early twenties, when I was living in St. Louis, there was a predator on the loose, kidnapping and raping women in Forest Park, where my friends and did our marathon training. It’s hard to believe it as I type these words now, but I was young and stubborn, and I kept training.

And then, one cloudy winter day, running alone on a narrow part of the trail bounded by Highway 64 to my left and the deep woods to my right, a man stepped out from behind a tree. He was naked except for his shoes. I stopped running. We locked eyes.

Cars whooshed past me on the other side of the high chain-link fence. It was now or never. I felt my brain release my body—there is no better way to describe it. I pushed past him. The fastest miles of my life flowed beneath my legs. How I wish someone had timed me! That was the first thing I thought in my kitchen, a half-block from the city limit, breathing hard, as I dialed the Clayton, Missouri Police.

“I want to report the naked man in Forest Park.” (Remember: at this time, women were being kidnapped and raped.)

“Was he dead?” came the monotone response.

“He was chasing me.”

“Well, was he heading towards St. Louis or Clayton?”

Apparently, at times like these, what matters most is jurisdiction. I said I didn’t know where the man was headed. Probably back to the woods (to get his clothes.) The police declined to make a report.

A few months later, my husband and I were strolling together through a sunny street fair along Lindell Boulevard, on the other side of the park. Suddenly, the man (now fully clothed) appeared among the festive crowd. We locked eyes. The street fair seemed to fade away.

My legs turned to jelly as I struggled to explain to my husband what I’d just seen. When I looked up again, to show him, the man was gone. We stumbled to a bench—I was suddenly exhausted. At home, I sank into a long, deep, dreamless nap.

In that first encounter, my brain protected me by lifting the “governor” and allowing my body to tap into its reserve. In our second encounter, I was safe but also strangely powerless—what else could I have done in the moment?—and my brain filled my body with fatigue.

• For a terrific discussion of fatigue as an emotion and the potential for therapies to help people with Chronic Fatigue Syndrome, check out Chapter 4 of CURE: A Journey Into the Science of Mind Over Body by Jo Marchant

1. Noakes, Timothy David. “Fatigue is a Brain-Derived Emotion that Regulates the Exercise Behavior to Ensure the Protection of Whole Body Homeostasis.” Frontiers in physiology vol. 3 82. 11 Apr. 2012, doi:10.3389/fphys.2012.00082