Energy for Nothing, Carbs for Free

Wide Angle / by Evan Lerner /

When we push athletes to their limits in the lab, we’re learning that their brains give up before their muscles do. Is fatigue really just all in our heads?

The finish line is in sight, but you’re not going to make it. Your lungs are burning and deeply in oxygen debt. The muscle fibers in your legs have converted most of the carbohydrate supply in your body into lactic acid. This was a fine plan for generating the extra energy needed to keep you moving in the absence of adequate oxygen, but it is now making your quads and hamstrings feel like they are on fire — which is more or less the case. In short, your fuel tank is empty and you’re running on fumes. You are not going anywhere, except possibly to the ground in a heap. 

The above scenario sounds totally plausible but is almost totally wrong. Runners are often at their fastest at the end of a race, when they should be the most tired. And, according to a new study, we can push ourselves harder when we expect to get a boost of energy from sugar, but long before those carbohydrates are actually absorbed. The experiment is clear evidence that our physical limits are less in our muscles and more in our minds.

The idea that muscle fatigue involves more than a lack of chemical energy is in some ways self-evident. Athletes perform better after consuming a sugar-laden energy drink, though the amount of energy contained therein is tiny compared to the reserves in their muscles. And according to a paper published earlier this month in The Journal of Physiology, athletes don’t even have to swallow those sugary drinks to feel a boost. The study — led by Ed Chambers, Matt Bridge, and David Jones at the University of Birmingham — cuts to the core of why we get tired at all.

“The issue revolves around what is known as central fatigue,” says Jones. In this case “central” refers to the central nervous system, with the feeling of tiredness stemming from signaling in motor neurons or other parts of the brain. “This is opposed to peripheral fatigue, which is the failure of some part of the machinery of the muscles themselves,” he says. If fatigue is primarily a function of the brain, the boost we get from incoming calories could be a mental — not physiological — effect.

To test this idea the Birmingham team concocted three “energy drinks” and had subjects rinse their mouths with them while strenuously exercising on stationary bikes. The placebo was a non-caloric drink, essentially synthetic saliva sweetened by saccharine. The two experimental drinks contained actual carbohydrates, glycol or maltodextrin, but their tastes were masked by saccharine so subjects couldn’t guess which they were trying.

And even though the cyclists never actually swallowed the drinks, they worked harder and performed better when swishing with the energy-filled ones. Some yet-unknown mechanism tipped their brains off signaling the presence of carbohydrates in their mouths, and the brain consequently doled out extra energy to their muscles in anticipation. Such a finding suggests that our brains control performance and endurance more than do the physical limitations of our muscles, heart, and lungs.

In a second part of the study, the Birmingham researchers attempted to track this effect in the brain by having subjects rinse with the three drinks while in an fMRI scanner. Reflecting the finding with the cyclists, only the glucose and maltodextrin drinks activated subjects’ anterior cingulate cortex and striatum, parts of the brain thought to regulate to motor control and reward.   

Credit: woodleywonderworks

The study piqued the interest of Tim Noakes, a professor of exercise and sports science at the University of Cape Town. “Here the brain is interpreting that the liver is producing more glucose or is otherwise advantaged in that regard,” says Noakes. “But this is because the brain is protected. It’s not worried about the muscles, as they can burn fat or glycogen. But the brain needs its glucose,” he says.

Noakes calls this mental model of exertion the “central governor.” He coined the term for the theory in 1997, though it borrowed from concepts first proposed by exercise physiology pioneer Archibald Hill in the 1920s.

Noakes’s conception of the central governor is a kind of athletic homunculus, independently operating on a subconscious level. The governor carefully compares external conditions like temperature and the expected length of the exercise period, with the body’s internal chemistry. The governor then coordinates the release of energy to the muscles and the perception of fatigue, trying to reach a pace that’s optimal for both power and safety. According to Noakes, this could explain how cyclists expend the same amount of energy during the Tour of Italy, a two-week race, as they do in the three-week Tour de France. 

But Noakes, through his papers and book, The Lore of Running, has sparked controversy within the athletic community. The central governor theory challenges some precepts of high-level training, such as the relevance of an athlete’s maximal oxygen uptake, or VO2 max, leading some to reject it entirely. Noakes attributes this to an “emotional reaction” on their part stemming from athletes’ unwillingness to accept that they might not have total conscious control over their will to continue.

David Jones, however, attributes this reaction to Noakes’s overreaching on how the central governor might work. “There is no doubt that we do set our pace according to how far there is to go, but I would say that this is an entirely conscious process based on experience that is separate from the protective reflexes associated with central fatigue,” says Jones.  “If we consciously get the calculation wrong and start too fast, then the unconscious protective reflexes of central fatigue may well cut in to slow us down.”

The uncertainty surrounding the mechanisms of the central governor might be resolved by further neuroscientific study, but there are some practical hurdles to clear. While the fMRI results of the Birmingham experiment were promising, they did not come from subjects who were actually exercising. Because fMRI requires subjects to keep their heads perfectly still, it’s not clear how the technique can be effectively applied to bodies in motion. 

Noakes agrees that further study is necessary. “Until we can actually show what’s happening in the brain, we’re speculating,” says Noakes. “We’re showing the effects of a central governor, we’re not showing where it is and how it’s acting. And until we do that, people will say,  ‘Maybe it doesn’t exist.’”

But beyond the brain chemistry, Jones points out a more fundamental gap in our understanding of fatigue. All we can do to determine when exercising experiment subjects feel tired is to have them tell us. An objective test of how much further they could go beyond that point would be helpful, but designing such a study presents its own hurdles. “Offering the subjects a huge financial reward to keep going for another 10 minutes or releasing a hungry lion might do the trick,” jokes Jones, “but these types of experiments are either too expensive or are difficult to get past ethics committees.”

Originally published April 28, 2009

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