Rethinking the Wall

Research provides a new definition of fatigue

Friday, April 1, 2005, 12:00 am

You're running a marathon. All is well until mile 23, when suddenly you hit the dreaded, metaphorical wall. You start to feel as though you’re carrying a lead-stuffed backpack and running in sand. Your pace slows inexorably. The devil on your shoulder tempts you insistently to stop. After 10 more minutes of exquisite suffering, you heed him.

Why did this happen? Based on what you’ve been taught, your first guess will probably be that you didn’t take in enough carbohydrate during the race, or that you didn’t do enough long, slow "fat-burning" runs in training. But, new research suggests it’s just as likely that you failed to take in enough protein during the race, or that you didn’t do enough plyometrics in training.


In other words, the wall might not be what you think it is.

Damage, Not Depletion?

For most runners, "hitting the wall" means running out of energy, specifically, carbohydrate fuel—either muscle glycogen or blood glucose—during the latter portion of a long race or workout. However, recent findings in the field of exercise physiology are painting a more complex and nuanced picture. In particular, it now appears that muscle damage may be an equally important cause of hitting the wall.

The scientists currently pursuing this line of research believe that a protective mechanism causes the brain to curtail muscle stimulation and produce feelings of discomfort and exhaustion when muscle damage approaches dangerous levels during prolonged exercise. That exercise causes muscle damage has been known for more than a century. What’s new is the idea that such damage is perhaps as potent a cause of hitting the wall as running out of carbohydrate-derived energy.

One of the leading proponents of this new understanding of endurance fatigue is Tim Noakes, M.D., an ultramarathon runner and author of Lore of Running. "I believe that the intense discomfort you feel near the end of ultramarathon races is not due to muscle glycogen depletion but is more likely due to muscle damage and your brain trying to tell you to please stop, as you are going to destroy your muscles if you continue," Noakes says frankly.

New evidence in support of this hypothesis comes from a study performed at the University of Cape Town, South Africa, where Noakes works. Researchers there discovered that the brain-signaling molecule interleukin-6 plays a role in telling the brain when muscle damage is approaching dangerous levels during exercise. As interleukin-6 levels rise, running performance plummets. Runners injected with interleukin-6 before a 10K time trial ran a full minute slower than they did when given a placebo.

Many questions remain about the mechanisms by which muscle damage causes fatigue, not least of which is whether these mechanisms have any practical implications for athletes. In other words, is there anything you should do to prevent fatigue caused by muscle damage that you aren’t already doing?

Several Ounces of Prevention

One measure that looks promising is consuming protein during exercise. Studies have shown that when athletes take in protein with carbohydrate and fluids during exercise, muscle damage is substantially lower and endurance significantly greater than when carbohydrate and fluids are consumed alone. In one new study, muscle damage was found to be 83 percent lower and endurance 29 percent greater when protein was added to a sports drink.

There are two theories about how the addition of protein to a sports drink might reduce exercise-related muscle damage. The protein in the sports drink may be used preferentially for energy during extended exercise, resulting in less breakdown of muscle protein. The protein may also raise amino acid levels in the blood, which is known to reduce muscle protein breakdown.

In addition to protein, vitamins C and E, both antioxidants, reduce muscle damage during running, but they have not been shown to enhance performance.

Caffeine, on the other hand, appears to enhance performance without reducing muscle damage by tricking the brain. According to the new understanding of endurance fatigue, it’s really your brain that causes fatigue by reducing muscle recruitment. Normally it does so in response to information received from the muscles, but it is possible to influence the brain’s response to muscle damage without actually affecting the muscles themselves, and caffeine appears to do just that.

Endurance fatigue is related to a shift in the balance of important neurotransmitters in the brain. Specifically, when dopamine levels decrease and serotonin levels increase, you start to feel miserable and the electrical signals sent from your brain to your muscles are suppressed. Caffeine is able to slow this shift and thereby delay fatigue.

Researchers are currently looking for other, more powerful drugs to prevent muscle fatigue by acting directly on the brain. Their stated goal is to treat diseases such as fibromyalgia, but they acknowledge that there will undoubtedly be a market for these drugs among unscrupulous athletes too. Here’s hoping that scientists also find safe, legal, nutritional means to reduce muscle damage and/or the brain’s response to it.

Training Solutions

It will be interesting to see whether modifying the conventional training habits of runners in certain ways can reduce muscle damage and push back the wall. Scientists have not yet plunged into this line of inquiry, but if I had a Ph.D. in exercise physiology, I know which methods I’d like to test first.

The primary cause of muscle damage during running is eccentric stress: the tendency of muscle fibers to rupture as they resist gravity’s efforts to make your joints buckle every time your foot lands. It seems safe to predict that any training measure that enhances the muscles’ resistance to eccentric stress will improve performance in long races, and Dr. Noakes agrees. "My understanding and observation of runners over the years is that eccentric muscle strengthening appears to improve performance in prolonged exercise," he says.

Plyometric training (i.e., jumping drills) has been shown to increase the muscles’ resistance to eccentric damage, so there’s a good chance it can help you run farther before muscle damage reaches a critical, bonk-inducing level. Plyometric training has already been proven to enhance running economy and performance in shorter races.

Downhill running is a more sport-specific way to "inoculate" your muscles against eccentric damage, which is exacerbated when you run in gravity’s direction. Again, the effect of downhill running on endurance fatigue has not yet been studied, but there is anecdotal evidence of its benefit. For example, in preparing for the 2001 Boston Marathon, Rod DeHaven propped up the back end of his treadmill and ran as far as 17 miles "downhill" on it, and was rewarded with a 2:12 PR.

Eccentric muscle damage is also closely related to impact forces. The harder you land, the more muscle damage you are likely to incur. A simple, non-technical way to train yourself to run softer is to try to reduce the sound your feet make when striking pavement. Australian Derek Clayton is a noteworthy example of a runner who successfully taught himself to land softer, and as a direct result, he believes, he developed from an unexceptional track runner into a marathon world-record breaker.

Stay Tuned

Carbohydrate fuel depletion is the cause of hitting the wall in some cases, but not in every case. Muscle damage is to blame just as often. We’re just beginning to explore nutritional and training-based ways to address this newly recognized cause of hitting the wall. Keep an eye out for the exciting new discoveries in this area that are sure to come. In the meantime, consider trying the strategies suggested here.