Imagine crossing the finish line of a 10K running race—or a bike ride or any other activity that pushes you to your limits. You’re out of breath, and
your heart is thumping. Your legs are burning, you’re overheating and dripping sweat, and you feel as though your fuel gauge is on empty. All these
factors contribute to your sense of fatigue, but which was the one that actually prevented you from going faster or farther? Scientists have been
pursuing the answer to this question for the last century. But according to a radical theory that has been gaining momentum in the last few years,
there is no answer—because it’s the wrong question.
Researchers test the limits of endurance by putting athletes on a treadmill and gradually increasing the speed until they’re forced to stop (or fall
off the back of the treadmill). But compare this to what happens in real-life athletic contests. While running a race, you never reach a point where
you simply keel over (unless something goes badly wrong). Instead, you’re constantly adjusting your effort with the goal of running as fast as you can
while ensuring that you complete the distance. So whatever “failure” causes you to fall off the treadmill at the end of a maximal test can’t be the
same thing that prevents you from running faster over 10K.
What’s been missing here is the role of the brain. Instead of our limits being dictated by “peripheral” fatigue—a failure somewhere in the
muscles of your legs, the beating of your heart, or the pumping of your lungs—South African researcher Tim Noakes has proposed that a “central
governor” in the brain regulates our physical exertions. This governor integrates physiological information from throughout the body—core
temperature, blood oxygenation, muscle signals, and so on—along with other data based on previous experience and knowledge of how long you
expect to continue. Operating beyond conscious control, it regulates how much muscle you’re able to activate, with the goal of holding you back
before you reach a state that could damage your heart or other organs.
This doesn’t mean that fatigue is imaginary. Your body really does have physical limits—but, if the central governor theory is correct, your brain
rarely permits your body to actually reach them. The simplest example of this phenomenon is the finishing sprint that is a nearly universal
phenomenon across endurance sports, from novices to world-record holders. No matter how hard you thought you were going, you suddenly find as
you approach the finish that your legs can move faster after all. Nothing has changed physiologically—but your central governor allows you to speed
up now that the finish line is in sight.
In contrast, if you put subjects in a hot room and ask them to pedal an exercise bike as hard as they can, their power output will be lower than in
cool conditions—right from the first pedal stroke. The slowdown happens long before any of the physical effects of heat could be relevant—further
evidence that the brain is quietly enforcing a safe “maximal” effort.
This debate between peripheral and central models of fatigue is perhaps the most controversial topic in current exercise physiology. No
definitive conclusions are in sight, but there’s broad recognition that the brain plays a larger role than previously acknowledged. This role is
unconscious, so you can’t simply “decide” to push through to your true physical limits—which is probably a good thing. What you can do, though, is
gradually teach your brain what your body is capable of. For example, training at your goal race pace not only increases fitness, but also allows your
mind to become familiar with the accompanying physiological feedback. You can’t turn your central governor off—but with patience you can adjust
its settings.
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