Matt Fitzgerald—Runners seldom hit the proverbial wall—that is, fall dramatically off their original pace toward the end of a race—in races of the half-marathon distance and less. But it happens all the time in marathons. Why?
The prevailing belief has been that the wall occurs when a runner depletes his or her very limited reserves of glycogen, a carbohydrate-based fuel source for muscle contractions. The body stores plenty of glycogen to get through shorter races, but not always enough to deliver runners to the finish line of a marathon, especially if their pace is too aggressive.
This general explanation for the phenomenon has stood up fairly well to scientific scrutiny. However, some runners hit the wall earlier than others, and some don’t hit it at all. Also, among those runners who escape the wall, some are able to run at much faster paces than others. Obviously, then, glycogen depletion is a highly individual matter. Given this reality, what are the specific factors that determine the risk of glycogen depletion in marathons? And how can these factors be used to predict glycogen depletion for the individual runner and thereby help him or her choose a marathon pace that will avoid the dreaded wall?
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Benjamin Rapoport of the Massachusetts Institute of Technology asked himself these questions and answered them by creating a mathematical model. He found that the primary factors that determine how fast and how far a runner can go before glycogen depletion occurs are aerobic capacity (or VO2 max), the mass of the runner’s leg musculature relative to the mass of the rest of the body, and the concentration of glycogen stores in the leg muscles and liver.
The higher an athlete’s aerobic capacity, the faster he can cover 26.2 miles, provided he has adequate glycogen stores.
The larger the athlete’s leg muscles are relative to his full body mass, the higher the VO2 max he can sustain for 26.2 miles. This is because a lower body mass means a lower energy cost of running and bigger leg muscles mean more room to store glycogen.
In addition, more concentrated glycogen stores in the legs and liver increase the runner’s absolute endurance range. Training greatly increases carbohydrate storage capacity. Carbohydrate loading enables runners to exploit that full capacity.
The formulas that Rapoport made on the basis of these rules yields some interesting insights. For example, they help explain why an even pacing strategy is the best way to avoid the wall and complete a marathon in the shortest time. It turns out you use up your glycogen stores faster if your pace fluctuates above and below a certain average than if your pace holds steady at that average. Another interesting finding is that, theoretically, some runners do not need to carbo-load to avoid the wall in marathons. They are able to store enough glycogen to go the full 26.2 miles at their maximum sustainable speed on any given day. Carbo-loading will only give them extra reserves that they will never use. Rapoport’s model can also be used to determine how much supplemental carbohydrate an individual runner must consume during a marathon to “push back the wall” to the finish line at a desired average pace.
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But carbo-loading also may not be necessary for runners who theoretically can’t store enough glycogen to fuel an entire marathon. One reason is the taper effect. The amount of glycogen stored in a runner’s body is not only a function of how much carbohydrate he eats, but also a function of how much he runs relative to what is normal for that person. If a runner sharply reduces his mileage, as he normally would in a pre-marathon taper, then the amount of carbohydrate his muscles burn daily will also sharply decrease. This will cause glycogen stores to increase without any change in the diet.
A 1992 study at McMaster University reported that a seven-day taper increased glycogen stores in middle-distance runners significantly and also resulted in a 22 percent increase in running time to exhaustion. Diet was not manipulated in this study.
A second reason carbo-loading may be unnecessary is that carbs consumed during races tend to minimize the importance of initial muscle and liver glycogen levels. In a 1993 study, researchers at Ball State University had athletes perform a long time trial with either high or low initial glycogen levels and either with or without carbohydrate intake during the time trial. They found that the athletes performed just as well when they started with low glycogen levels and consumed carbs during it than when they started with high glycogen levels and did not consume carbs. What’s more, the athletes didn’t perform any better when they started with high glycogen levels and consumed carbs during the time trial.
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For certain athletes in certain races, carbo-loading certainly can make a difference even with a pre-race taper and carbohydrate consumption during the event. Therefore, I think it’s a good idea for runners to go ahead and carbo-load before longer races as a kind of insurance. If there’s a five percent chance it can help and a 0 percent chance it will hurt, then it’s worth it.