Posted by Dave on July 26, 2011 | 8 Commentsfatigue and exhaustion, discussing why we get tired when we do. This week I’m finishing up Noakes’ chapter on energy systems and running performance.
I started running in the early 1980s, when runners carbed up before races, but even marathoners didn’t each much or ate nothing at all during races. Since I was running shorter cross-country and track events, this didn’t affect my running too much, but in the past few years, as I began training for half-marathons and marathons, I faced a dilemma: I had never eaten or had anything other than water to drink while running. Many runners these days fuel up with carbohydrate drinks or gels, and my running partners told me that fueling up during a long race made a big difference.
I tried eating gels and drinking Gatorade during long runs, but my stomach tended to get upset. Eventually I figured out a brand and flavor of gels that I could tolerate, and I’ve been using them on long runs ever since. I’m pretty sure they help, but it’s always been unclear to me exactly how much they help, and whether they’re truly helping or just acting as a placebo. PepsiCo and other companies make millions of dollars selling products like Gatorade, so it’s in their interest to convince runners and other athletes that buying and consuming these products really helps.
Tim Noakes points out that researchers as early as the 1920s and 30s knew that consuming sweets during long runs seemed to enhance endurance, but somehow that knowledge was lost during the 60s and 70s. It wasn’t until a landmark study was published in 1986 that researchers once again demonstrated that consuming carbohydrates during extended workouts actually helped.
A team led by Edward Coyle had trained cyclists ride stationary bikes at a set pace until they couldn’t continue. In one session the riders consumed around 60 g of carbs per hour in an energy drink, and in a separate session the same riders had a placebo. The sessions were separated by a week, so there was ample time for recovery. Normally we think of a placebo as a “sugar pill,” but in this case they drank artificially sweetened beverages flavored to match the carbohydrate drink. When they consumed carbs, the cyclists could sustain the pace for an average of 4 hours, compared to just 3 hours when consuming the placebo—a dramatic, statistically significant difference. In addition, there was a measurable difference in the amount of glucose in the riders’ blood:
Interestingly, there was no difference in muscle glycogen levels in the carb versus placebo. At that time, muscle glycogen was thought to be the key to endurance; endurance athletes were encouraged to carbo-load to increase their levels of muscle glycogen before a race because the muscles couldn’t readily use blood glucose to do work.
So why do athletes perform better when blood glucose levels are high? Noakes says the blood glucose is necessary to preserve brain function. Without sufficient blood glucose, athletes become hypoglycemic, and end up shutting down completely, regardless of whether their muscles are getting enough energy. You’ve probably seen videos like this from the end of the 1997 Hawaii Ironman Triathlon, where Sian Welch and Wendy Ingraham are reduced to crawling to the finish:
After you get over the heart-wrenching emotion of the scene, note that they seem to have muscle function, just not the ability to control their muscles. Noakes argues that normally the central governor in the brain works to prevent us from becoming hypoglycemic, which means we simply stop before we get to the desperate situation of Welch and Ingraham in the video. When athletes have enough blood glucose, they can then use other resources such as fat to power their muscles.
Noakes himself was one of the researchers that added another piece to the puzzle of how muscles fatigue: why we tend to slow down in long runs rather than just stop completely. In a 2001 study led by Alan St. Clair Gibson, the researchers had cyclists perform a 100K time trial on stationary bikes, but with intermittent all-out sprints. What they found is that the pace in the sprints slowed down almost from the start; each sprint was a little slower than the previous one. But most interestingly, they also measured the electrical signals coming from the brain to the leg muscles, and they found those signals were diminishing along with the pace of the sprint. This suggests that the brain slows the riders by activating fewer muscles as the body tires: It’s the central governor, once again telling the body to slow down before it hurts itself.
Noakes says this can also explain why the cyclists in the Coyle et al. study suddenly stopped: They reached the point of fatigue at which the central governor wouldn’t recruit enough muscle fibers to maintain the prescribed pace. The riders could have continued at a slower pace, of course, but that wasn’t the point of the study.
A whole cadre of studies since Coyle has confirmed that consuming carbs immediately before and during extended endurance events significantly improves performance, and that this effect is more reliable than carbo-loading. Carbs can improve race performance in events as short as one hour; at the end of marathon-length events, pace is improved by as much as 12 percent. For longer races like Ironman triathlons and ultramarathons, some sort of in-race nutrition can prevent more dramatic collapses.
What this means for you: Consuming carbs during long workouts and races (more than one hour) improves performance. While the exact quantities you need haven’t been established, most studies have examined doses between 40 and 60 grams of carbs per hour. You can consume carbs in the form of energy drinks or gel packs, but note that it’s difficult to know the exact amount of carbs you’re getting from an energy drink in a race situation, where the cups are filled unevenly and the drink may be diluted.