Performance: Bonk Bustin'

You know the feeling: You're dead tired at the end of a hike or run or ride. As your legs turn to lead, you consider the unthinkable--quitting. But then something happens. Despite the crisis in your muscles, you surge through the rest of your workout with ease.

What happened? Recent research suggests that feeling fatigued is not a true indication of muscle failure, but a sensation created by your nervous system to keep you from over-exercising. The problem, however, is that your brain often underestimates what your body is capable of and will even shut you down preemptively, if you let it.

Exercise scientists in Cape Town first detected the brain's tight control over performance in 2004 through an ingenious experiment in which experienced endurance athletes were asked to exercise as intensely as possible for two short bursts.

Both tests lasted 36 seconds, but the subjects were told that one would last just 30 seconds. In this "deception trial," the athletes watched a clock that had been adjusted to run more slowly than usual (so that 36 seconds would pass in 30 seconds according to the clock.)

The results were amazing: The subjects poured it on for 33 seconds (27.5 seconds on their clock,) but then slowed down over the last three seconds, indicating that their brains, not their bodies, had deemed the intensity excessive. Predictably, during the true 36-second trial the athletes maintained their pace over the last three seconds, with no indication of burnout.

"The study showed that fatigue is often a psychological construct rather than an absolute physical condition -- something that your brain creates when you start exercising, and then adjusts according to how long you train or compete," says Alan St. Clair Gibson, associate professor at the University of Cape Town. To get the most out of your body, you must reset how your nervous system perceives fatigue. Here's how to beat the bonk.

Two Ways to Combat Exhaustion

Fatigue Training. The two most effective methods for recalibrating your brain are to increase the intensity of your workouts and to mentally break down your efforts into smaller parts.

For the former: On a day when you are feeling great, warm up thoroughly and then run, cycle or swim as far as you can in six minutes. Determine your average speed during this six-minute effort, and then--once or twice a week--hit one-to-three-minute intervals at this velocity, being careful not to exceed 15 total minutes per session. Be sure to retest your average speed every six weeks; as your brain resets your acceptable intensity, you will cover more ground (or water) in six minutes.

For the latter: Think about a long race such as a 10K as two 5Ks run back-to-back. Your brain should allow a faster pace in each 5K segment.

Fatigue Psychology. The fact that fatigue is often only in your mind means that you can choose to ignore it.

For example, when you experience a significant bonk during a hike, climb or ride, try to put aside the typical defeatist reaction: My muscles are failing; there's no way I can continue at this pace. Instead, say to yourself, in a crisp, clear voice, "Thanks for the warning, but I'm going to relax and keep on going." When fatigue is treated as a normal sensation associated with exercise rather than as a performance blockade, the athlete is liberated to push onward.

Somewhat paradoxically, research also shows that it may be beneficial to speed up slightly when fatigue strikes. This increases neural input to your muscles, gives your conservative neural governor a sharp slap in the face and can quickly alleviate the fatigue.

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