How A Tired Mind Limits the Body

When it comes to our self-understanding, we have been held back by an extraordinary philosophical mistake. It’s a forgivable error, since it reflects our most basic intuitions. The mistake I’m talking about is dualism, which holds that the mind and body are fundamentally separate things.

To borrow the famous framework of Rene Descartes, the human mind is a “thinking thing,” composed of an immaterial substance. (Our thoughts are airy nothings, etc.) The body, in contrast, is a “thing that exists,” just a mortal machine that bleeds. For Descartes, dualism was a defining feature of humanity. Every animal has a body. Only we have a mind.

The dualist faith continues to shape our lives. Like Descartes, we tend to assume that mental events have mental causes—you are sad because your brain is sad—and that physical events have physical causes. (If your back is in pain, there’s something wrong with your back.) Dualism is why we treat depression with pills (rather than exercise, which is often just as effective) and undergo so many spinal surgeries (which are often ineffective.)

Dualism seems obviously true. But it’s mostly false. In recent years, modern neuroscience has demolished these old Cartesian distinctions. It has done this mostly by showing how the body is not a mere power plant to the brain, but rather shapes every aspect of conscious experience. The bacteria in your intestines, for instance, seem to influence your mood, while that feeling of fear probably began as a slightly elevated heart rate. Our memory is improved when it’s connected to physical movement and the sweat glands in your palm can anticipate your gambling mistakes long before the cortex catches up. As the neuroscientist Antonio Damasio has written, “The body contributes more than life support. It contributes a content that is part and parcel of the workings of the normal mind.”

These studies are convincing. And yet, even if one acknowledges the subtle powers of the body— the soul is surprisingly carnal—there is still one realm in which dualism is taken for granted: athletic performance. When we look at our best athletes, we appreciate them as physical specimens, blessed with better flesh than the rest of us. They must have bigger hearts and more fast-twitch muscle fibers; highly efficient lungs and lower resting pulses. We ignore their “thinking thing” and focus instead on their body, “the thing that exists.”

But even here the body/mind distinction proves illusory. Consider a new paper by Daniel Longman, Jay Stock and Jonathan Wells. Their subjects were sixty-two male rowers from the University of Cambridge. They were all in excellent shape. On their first visit to the lab, the men rowed as intensely as possible for three minutes as the scientists tracked their total power output. On their second visit, the men were given an arduous mental task. Seventy-five words were briefly flashed on a screen; their job was to remember as many of them as possible.

The last visit to the lab combined these two measures. While the men worked up a sweat on the rowing machine, they were simultaneously shown a new set of words and asked to remember them. As expected, combining the tasks led to a dropoff in performance: the men remembered fewer words and generated less power on the rowing machine.

But here’s the interesting part: the decline was asymmetric, with physical performance suffering a dropoff that was roughly 25 percent greater than mental performance.

What accounts for this asymmetry? The scientists suggest that it’s rooted in the scarcity of blood sugar and oxygen, as the brain and body compete for the same finite resources. And since we are creatures of cogito—thinking is our competitive advantage—it only makes sense that we’d privilege the cortex over our quadriceps.

The larger lesson is that our thoughts and body are not separate systems—they are deeply intertwined, engaged in a constant dialectic. Those rowers didn’t perform worse because their muscles were run down. Rather, they had less physical power because their selfish brain decided to feed itself first. This means that the best athletes don’t just have better bodies – they also have minds that don’t hold them back.      

Such research adds to the evidence for the so-called Central Governor theory of physical endurance. (I wrote about this recently in Men’s Health.) Most closely associated with Timothy Noakes, now an emeritus professor at the University of Cape Town, the Central Governor theory argues that the feeling of bodily fatigue is primarily caused by the brain, and not the body. As Noakes points out, in the final stages of a race, up to 65 percent of muscle fibers in the leg remain inactive. In addition, levels of ATP—the molecule used to transport energy within our cells—almost never fall below 60 percent of their resting value. This suggests that we still have plenty of energy left, even when the body feels exhausted. The Central Governor is just too scared to use it.

It’s a simple idea with radical implications. After all, we’ve assumed for nearly a century that our physical limits were largely reducible to the laws of muscular chemistry. (In the 1920s, the British physiologist and Nobel laureate Archibald Hill began writing about the effect of “oxygen debt” and the accumulation of lactic acid during intense exercise.) Noakes, however, argues that the reality is far more complicated, and that our sense of fatigue is a subjective mental construct, based on countless variables, from the temperature of the skin to the cheers of the crowd. “I am not saying that what takes place in the muscles is irrelevant,” Noakes writes in his autobiography, Challenging Beliefs. “What I am saying is that what takes place physiologically in the muscles in not what causes fatigue.”

And this brings us back to dualism. After all, unless you admit the enormous mental component of physical performance then won’t be able to train effectively. You’ll be focused on VO2 max and lactate concentrations—highly imperfect measures at best—when you should be building up the threshold of your Central Governor.

So how does one train the Central Governor? In my Men’s Health piece, I profiled Holden Macrae, professor of Sports Medicine at Pepperdine. As part of the Red Bull High Performance research project, he gave endurance athletes a tedious mental chore for 30 minutes. Once their brain was sufficiently run down, Macrae then had them perform a difficult cycling workout. “We found that the power output of the mentally pre-fatigued athletes was way lower than the non-fatigued,” he told me. “It didn’t matter that their bodies were fresh. Their brains were tired, and that shaped their performance.”

Macrae argues that these findings have practical implications for training. If elite athletes are looking to push the boundaries of their endurance, then they should begin their physical training after a brain workout. “Because you are stressing the mind and the body at the same time, you are forcing yourself to write a new software program,” he says. “It’s the same logic as high-altitude training, only you don’t have to go anywhere. You just have to do something boring first.”

The appeal of dualism is inseparable from the fact that it feels true; the body and mind seem like such separate entities. But one of the profound potentials of modern neuroscience is the way it can falsify our longstanding assumptions about human nature. You are not your brain, and your body is not just a body; the soul and the flesh have a very porous relationship. Once we understand that, we can find ways to get more out of both.

Or at least get in a better workout.

Longman, Daniel, Jay T. Stock, and Jonathan CK Wells. "A trade-off between cognitive and physical performance, with relative preservation of brain function." Scientific Reports 7.1 (2017): 13709.