As any runner who's ever felt his legs turn into lead anvils at the end of a hard workout or race knows, running farther or faster all boils down to a battle against fatigue. And so you train to increase either the distance or the pace-or both-you can sustain without tiring. But hold on just a second: Fatigue might not be what you think it is. According to the latest exercise science, that dead-tired feeling could be all in your head. What's more, the way you've been training to prevent fatigue in your legs may not be the best way to train to prevent it where it really starts: in your brain.
Throughout most of the 20th century, the prevailing theory of exercise fatigue was the so-called "catastrophe" model. According to this theory, fatigue is an involuntary drop in performance caused by the loss of homeostasis (or balance) somewhere in the body. For example, due to lactic-acid buildup, the muscles lose pH balance and become too acidic to function properly, causing you to slow down. Or the muscles become depleted of glycogen (their primary fuel source), so there's no longer sufficient energy available to sustain performance.
But in the 1980s, a new generation of exercise scientists, led by Tim Noakes, M.D., of the University of Cape Town, South Africa, began to poke holes in the catastrophe model. First, they found that the common functional breakdowns-things like lactic-acid buildup and glycogen depletion-don't always occur as we tire. Studies, in fact, showed that fatigue often develops before the muscles reach a level of acidity that would cause direct muscle dysfunction, and that you can feel tired even when there is still muscle glycogen available in the working muscles.
What's more, these researchers also argued that the old catastrophe model couldn't account for the peculiar phenomenon known as the "end spurt" - you know, the guy who elbows past you in the last few yards of a marathon. If fatigue was always caused by direct physiological events within the muscles, then runners who began to slow down during the latter stages of a race couldn't possibly sprint the last 100 yards or so. "Athletes can often surge during the latter stages of a race, knowing that they won't have to continue once they cross the finish line," says Chris Abbiss, Ph.D., a biomedical and health-science researcher at Edith Cowan University in Western Australia.