Fatigue - it's mostly in your head

Ultra runners will ingest only about 75% of the calories that they burn. The other 25% comes mostly from body fat. Is race nutrition really about keeping the brain happy rather than fueling the muscles?

from the Times 5/23/2006

A spate of recent studies has contributed to growing support for the notion that the origins and controls of fatigue lie partly, if not mostly, within the brain and the central nervous system. The new research puts fresh weight to the hoary coaching cliché: you only think you’re tired.

From the time of Hippocrates, the limits of human exertion were thought to reside in the muscles themselves, a hypothesis that was established in 1922 with the Nobel Prize-winning work of Dr. A.V. Hill. The theory went like this: working muscles, pushed to their limit, accumulated lactic acid. When concentrations of lactic acid reached a certain level, so the argument went, the muscles could no longer function. Muscles contained an ‘‘automatic brake,’’ Hill wrote, ‘‘carefully adjusted by nature.’’

Researchers, however, have long noted a link between neurological disorders and athletic potential. In the late 1800’s, the pioneering French doctor Philippe Tissié observed that phobias and epilepsy could be beneficial for athletic training. A few decades later, the German surgeon August Bier measured the spontaneous long jump of a mentally disturbed patient, noting that it compared favorably to the existing world record. These types of exertions seemed to defy the notion of built-in muscular limits and, Bier noted, were made possible by ‘‘powerful mental stimuli and the simultaneous elimination of inhibitions.’’

Questions about the muscle-centered model came up again in 1989 when Canadian researchers published the results of an experiment called Operation Everest II, in which athletes did heavy exercise in altitude chambers. The athletes reached exhaustion despite the fact that their lactic-acid concentrations remained comfortably low. Fatigue, it seemed, might be caused by something else.

In 1999, three physiologists from the University of Cape Town Medical School in South Africa took the next step. They worked a group of cyclists to exhaustion during a 62-mile laboratory ride and measured, via electrodes, the percentage of leg muscles they were using at the fatigue limit. If standard theories were true, they reasoned, the body should recruit more muscle fibers as it approached exhaustion — a natural compensation for tired, weakening muscles.

Instead, the researchers observed the opposite result. As the riders approached complete fatigue, the percentage of active muscle fibers decreased, until they were using only about 30 percent. Even as the athletes felt they were giving their all, the reality was that more of their muscles were at rest. Was the brain purposely holding back the body?

‘‘It was as if the brain was playing a trick on the body, to save it,’’ says Timothy Noakes, head of the Cape Town group. ‘‘Which makes a lot of sense, if you think about it. In fatigue, it only feels like we’re going to die. The actual physiological risks that fatigue represents are essentially trivial.’’

From this, Noakes and his colleagues concluded that A.V. Hill had been right about the automatic brake, but wrong about its location. They postulated the existence of what they called a central governor: a neural system that monitors carbohydrate stores, the levels of glucose and oxygen in the blood, the rates of heat gain and loss, and work rates. The governor’s job is to hold our bodies safely back from the brink of collapse by creating painful sensations that we interpret as unendurable muscle fatigue.

Fatigue, the researchers argue, is less an objective event than a subjective emotion — the brain’s clever, self-interested attempt to scare you into stopping. The way past fatigue, then, is to return the favor: to fool the brain by lying to it, distracting it or even provoking it. (That said, mental gamesmanship can never overcome a basic lack of fitness. As Noakes says, the body always holds veto power.)

‘‘Athletes and coaches already do a lot of this instinctively,’’ Noakes says. ‘‘What is a coach, after all, but a technique for overcoming the governor?’’
The governor theory is far from conclusive, but some scientists are focusing on a walnut-size area in the front portion of the brain called the anterior cingulate cortex. This has been linked to a host of core functions, including handling pain, creating emotion and playing a key role in what’s known loosely as willpower. Sir Francis Crick, the co-discoverer of DNA, thought the anterior cingulate cortex to be the seat of the soul. In the sports world, perhaps no soul relies on it more than Jure Robic’s.

Some people ‘‘have the ability to reprocess the pain signal,’’ says Daniel Galper, a senior researcher in the psychiatry department at the University of Texas Southwestern Medical Center at Dallas. ‘‘It’s not that they don’t feel the pain; they just shift their brain dynamics and alter their perception of reality so the pain matters less. It’s basically a purposeful hallucination.’’

Noakes and his colleagues speculate that the central governor theory holds the potential to explain not just feats of stamina but also their opposite: chronic fatigue syndrome (a malfunctioning, overactive governor, in this view). Moreover, the governor theory makes evolutionary sense. Animals whose brains safeguarded an emergency stash of physical reserves might well have survived at a higher rate than animals that could drain their fuel tanks at will.

The theory would also seem to explain a sports landscape in which ultra-endurance events have gone from being considered medically hazardous to something perilously close to routine. The Ironman triathlon in Hawaii — a 2.4-mile swim, 112-mile bike ride and marathon-length run — was the ne plus ultra in endurance in the 1980’s, but has now been topped by the Ultraman, which is more than twice as long. Once obscure, the genre known as adventure racing, which includes 500-plus-mile wilderness races like Primal Quest, has grown to more than 400 events each year. Ultramarathoners, defined as those who participate in running events exceeding the official marathon distance of 26.2 miles, now number some 15,000 in the United States alone. The underlying physics have not changed, but rather our sense of possibility. Athletic culture, like Robic, has discovered a way to tweak its collective governor.

When we try understanding Robic’s relationship to severe pain, however, our interest tends to be more visceral. Namely, how does it feel?
‘‘I feel like if I go on, I will die,’’ he says, struggling for words. ‘‘It is everything at the same moment, piled up over and over. Head, muscles, bones. Nobody can understand. You cannot
imagine it until you feel it.’’

A few moments later, he says: ‘‘The pain doesn’t exist for me. I know it is there because I feel it, but I don’t pay attention to it. I sometimes see myself from the other view, looking down at me riding the bike. It is strange, but it happens like that.’’ Robic veers like this when he discusses pain. He talks of incomprehensible suffering one moment and of dreamlike anesthesia the next. If pain is in fact both signal and emotion, perhaps that makes sense. Perhaps the closer we get to its dual nature, the more elusive any single truth becomes, and the better we understand what Emily Dickinson meant when she wrote that ‘‘pain has an element of blank.’’