COMPARED WITH FEAR AND ANXIETY trip wears onand ongradually depleting your energy stores. You feel sluggish, loopy, maybe even disoriented. Crank up the intensity by climbing a big hill, and you expend the dregs of your stored energy, causing blood sugar to plummet and your muscles to lock down. Now you feel lousy, maybe even panicky, depending on the pickle you've gotten yourself into.
The body uses different fuels for different levels and durations of exertionrunning a half-marathon (moderate duration, moderate intensity), for example, mainly taps the glycogen stored in the muscles and liver, while a 25-mile hike (long duration, low intensity) would deplete those stores and start burning fat for energy. Sprinting from a chargin' grizzly sow, on the other hand, would tap anaerobic systems to combust the tiny amount of energy stored inside the muscle cells. Understanding these processes can help you manage energy output, but fuel burning isn't entirely mechanical. "I started off focusing on muscles, like most exercise physiologists," says J. Mark Davis, director of the University of South Carolina's exercise biochemistry laboratory. "But they didn't tell the whole story." Instead, Davis found himself drawn to the brain's role in fatigue.
During endurance events, depleted energy stores can wreak havoc on your mental state. As fatigue sets in you'll lose concentration (increasing your chances of injury) and become susceptible to negative thoughts (worrying about your condition and focusing on the distance left to travel). Because mood can drive metabolism, the more stressed out you are, the higher your metabolic rate, causing your body to vaporize fuel even faster. Fortunately, athletes who learn to recognize the emotional results of fatigue can address mood swings with simple mind exercises. Focusing on breath to stay present in the moment, say, or mentally breaking down long distances into more manageable segments, can damp the mentaland therefore physicaleffects of fatigue.
Mental games have their limits, of course. If you haven't conditioned your heart and lungs to move more blood and oxygen or built glycogen reserves through endurance training and ample rest (see tips below), you can forget thinking your way to a first-place finish. But by feeding the mind with a combination of nutrition, training, and mental exercise, you'll be on your way to optimal performance. "The impulses come from the brain," Davis says. "When it's not working well, the flesh doesn't work well."
- DRINK PLENTY. Quaffing eight to 12 ounces of sports drink every 15 minutes from the start of a 90-minute-plus workout will divert (or at least delay) fatigue's hormonal tidal wave. "Stay ahead," Davis says. "You can't catch up when you're a liter down."
- HOP ON THE LONG, SLOW TRAIN. Don't neglect "overdistance" training (workouts at mileage longer than your race). Hour-plus sessions at 60 to 70 percent of your max heart rate condition your body to burn fat more efficiently and go longer before tiring.
- CONSIDER CAFFEINE. Java- or green tea-fueled training isn't for everyone, but Davis says moderate caffeine intake (16 ounces of coffee or the equivalent) may fend off fatigue during long outings.
- EXERCISE YOUR BRAIN. Fight fatigue the same way you combat anxiety. During distance training, develop mental exercises (again, focusing on your breath or breaking down long distances) that help you maintain a positive mood even when fatigue sets in.