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Fatigue: Mind over muscle

New science suggests that signals of fatigue may reside in the brain, and that training may be as much about psychology as physiology.

Editor’s note: This article was originally published in the November 2015 issue of Velo magazine.

Australian cyclist Jack Bobridge is well acquainted with the physical pain of pushing his body to the limit. But even a career on the track and road didn’t prepare him for the pain of what was to be an unsuccessful attempt on the hour record earlier this year. (His pacing didn’t help.)

“This is the closest to death I will ever be, I think, before actually dying,” Bobridge told the assembled media. “I can’t even describe how much pain my glutes and quads are in. It’s unbelievable.”

But was his suffering — like his determination to continue pushing the pedals — limited more by his head than his legs? For all the talk of burning quads and glutes, some scientists now believe that fatigue may be more complicated a process than originally thought, one that is as much psychological as it is physiological.

“According to this model,” writes Timothy Noakes, a professor of exercise and sports science at the University of Cape Town, “the winning athlete is the one whose illusionary symptoms [of fatigue] interfere the least with the actual performance — in much the same way that the most successful golfer is the one who does not consciously think when playing any shot.”

That’s the idea behind the central-governor theory, a model Noakes first proposed in 2001 that suggests the brain has a safety switch that kicks in to prevent the body from getting too close to dangerous limits. And it does this before it needs to — the brain’s way of protecting us from ourselves.

The point at which the brain feels like enough is enough has to do with fitness, fuel, and natural ability. But regardless of when you start to feel it, the central-governor theory holds that it’s not so much that your legs have had enough as it is that your brain is telling your legs they’ve had enough.

So Jens Voigt had it wrong. The better tagline would be “shut up, brain.”

Noakes wasn’t the first to question the view that an individual’s physical performance has absolute, physiological limits. In the early 20th century, Italian researcher Angelo Mosso proposed the theory that physical limits may be controlled less by our body and more by our brain. While perhaps ahead of his time, Mosso’s concept of central fatigue soon gave way to Nobel laureate A.V. Hill’s idea that peripheral fatigue — that is, all the physiological stuff having to do with muscles and lactate and fuel and everything else we know — was the sole determinant of exercise fatigue.

Current research, led by Noakes and Samuele Marcora of the University of Kent’s Endurance Research Group, has revitalized the suggestion that the brain, acting in self-preservation mode, cuts off signals to working muscles when it senses that effort is too high.

The difficult part for scientists is that a central control switch can’t be measured or localized like other signs of exertion. We can test for blood lactate or oxygen consumption, but the central regulator has yet to be found.

But importantly for cyclists looking to adjust the central governor, training doesn’t just build bigger muscles and a better aerobic capacity; hard workouts also teach the brain that high levels of exertion aren’t life threatening. The heart didn’t explode after that hard interval, and muscles and tendons are still intact; ride on.

“The brain learns to tolerate heat, lack of oxygen, muscle pain — basically it becomes better at suffering,” says Allen Lim, a sports physiologist and founder of Skratch Labs.

Much evidence for a central regulation of fatigue is indirect, since it’s understood that peripheral mechanisms can’t explain everything we know about fatigue. One supporting factor comes from studies using mostly trained, male subjects, showing that cyclists can trick their bodies into riding faster simply by rinsing their mouths with a carbohydrate solution then spitting it out. Though they didn’t ingest the carbs and therefore none of the physiological benefits, on average riders in these studies improved power output by two to 11 percent. This suggests that simply letting the brain think there’s extra fuel could be enough to delay the kill switch.

The central governor also plays a protective role in hot conditions, reducing performance to limit a rise in core temperature. In a 2012 study conducted in England, researchers had competitive cyclists run two trials in temperatures of 72 and 90 degrees Fahrenheit. Core temperature for both rides was the same, yet the distance the cyclists achieved was four percent lower, and average power output was reduced on the hot rides. Interestingly, in a third trial, when the researchers deceived the cyclists into believing that the temperature was 79 degrees instead of the actual 90 degrees, the difference in performance disappeared. The researchers concluded that when the brain senses higher ambient temperatures, it reduces output to protect the body from heat injury.

But Lim emphasizes that, just as training teaches our brain to suffer, heat acclimatization can change the set point at which the brain raises the white flag. In a 2015 study, uninitiated riders saw a 16 percent drop in power between hot (97 degree Fahrenheit) and cool conditions (47 degrees Fahrenheit). After 14 days of heat acclimatization, the power difference lessened to only three percent between the two temperature scenarios.

Brain Games

Could Jack Bobridge have gone farther during his hour record attempt, despite the physical pain he was in? Were Jens Voigt’s legs what was holding him back? New science suggests the real problem was all in their heads.

Researchers have also shown that mental fatigue — the type induced by a long, difficult exam for instance — impairs subsequent endurance performance. In a 2009 study at Bangor University in Wales, cyclists were divided into two groups and asked to pedal at 80 percent of peak power for as long as they could. The only difference between the two groups was that one group had to undergo 90 minutes of cognitive testing requiring sustained attention and a memory challenge. The other group watched a documentary on trains. The mentally exhausted riders dropped out an average of 15 percent earlier.

So the day before a big race is not the time to close on a new house, buy a car, or do your taxes.

When it comes to cyclists, cadence may be another way to trick the brain into giving the body a longer leash. Training at both high and low cadences delivers similar improvements in physiological exercise capacity, but high-cadence training seems to increase measures of brain efficiency, according to a 2015 study conducted at the University of Basel in Switzerland. As compared to the low cadence group (90 rpm), cyclists that had trained using high cadence (120 rpm) were able to exercise using reduced EEG measured brain activation (a good thing) despite identical intensity.

In the end, training lets us go farther faster. That hasn’t changed. What is evolving, though, is our understanding of how that training helps us. Endurance sports are artificially induced struggles, and it’s during the struggle that systems adapt — not only heart and lungs but also the brain.

“Training the brain allows it to understand what’s possible,” Lim says. And with a little bit of trickery, you might convince your brain that you’re capable of even more.