MARATHON PHYSIOLOGY
Fine Tuned for Distance, Fuel Efficiency

When you find yourself struggling through the last few miles on Sunday, think about this: Even the slowest runners in the field will complete the course faster than Olympic sprint champion Michael Johnson could.

That's because the marathon is about endurance, not speed, and endurance comes only by training to go long. In turn, that training and a few race-day details determine whether you scale the marathon's infamous "wall" or crash headlong into it.

Training: It Does a Body Good

Say your favorite charity is sponsoring a 5K a month from now. With some gumption and a few workouts a week, even a current non-exerciser could comfortably complete that race. Not so with the marathon.

Why?

Any race involves a combination of speed and distance.

"The longer the race, the more important that pure endurance -- the ability to keep running without having to slow to a crawl -- becomes," said Pete Pfitzinger, manager of the UniSports Center for Sport Performance in Auckland, New Zealand, and the first American finisher in the 1984 and 1988 Olympic marathons.

That is, almost anybody can run 100 meters while almost nobody can run 100 miles.

At 26.2 miles, the marathon, even for the top finishers, is more about covering the distance than running fast. (The winners can run a 10K about 30 seconds per mile faster than their marathon pace.)

For most, it is a matter of battling glycogen depletion.

Glycogen is your muscles' preferred fuel for aerobic exercise. When runners "hit the wall," they have depleted their glycogen stores and have to rely increasingly on fat as a fuel source.

"When you have to burn more fat, you slow. In some cases, you stop," said Pfitzinger, who has a master's degree in exercise science. And even when you don't stop, you may run abnormally, causing increased muscle soreness.

Most people can store only enough glycogen in their muscles for 15 to 20 miles of running, thereby placing a potentially insurmountable wall between themselves and the finish line.

That's where training comes in.

"Your body adapts specifically to the stress you place on it," Pfitzinger said. "You should train by simulating the demands of the race."

For the marathon, this means one thing: Run long. Nearly every finisher on Sunday will have completed at least one 20-mile run in the weeks before. The fastest runners likely will have completed more than a half dozen such runs.

What do these long runs achieve? First, endurance training conditions your body to use more fat relative to carbohydrate at a given pace; this adaptation allows you to run farther before you run out of glycogen. You become more fuel efficient.

Second, long runs teach your muscles to store more glycogen. "When you do a long run and deplete your glycogen stores," Pfitzinger said, "your muscles are stimulated to restock to a higher level. This is a simple survival mechanism to ensure that you won't run out of glycogen again." In other words, your gas tank becomes bigger.

Finally, your fuel-delivery system improves, moving nutrients in and waste products out more efficiently.

Unless Michael Johnson has secretly been doing long runs for the last few months, his legs, no matter how powerful and speedy, don't have these attributes, and wouldn't be able to go 26.2 miles.

Because these adaptations take time to occur, training for a marathon isn't like cramming for a test. Most runners do their last long run two or three weeks before the marathon, then dramatically reduce their training. During this time, their muscles retain the ability to store a lot of glycogen but aren't being drained. In the few days before the race, a high-carbohydrate diet (or "carbo-loading," some runners' favorite aspect of marathon preparation) tops off the tank.

      Don't Lose Your Pace

One of the biggest obstacles to marathon success for properly trained runners is improper pacing.

Combine race-day nerves and legs rested from tapered training and many runners have a hard time not running the first several miles faster than they should.

The problem with this is that the faster you run, the more glycogen you burn. Even with the right training, your glycogen stores will probably be emptied by the finish, so prematurely burning them is asking for trouble.

"For every minute you gain through exuberance in the first half of the race, you usually hand back several during the second," Pfitzinger said. "The most efficient way to use your body's energy supply is to run close to an even pace. By holding back in the early miles, you'll conserve your glycogen reserves for later in the race."

An irony of marathoning is that many first-timers, who may be frightened by the distance and who will not try to run faster than they normally train, pace themselves more intelligently than veteran racers.

Drink and Be Merry

Even the most conservative runners have another race-day obstacle to contend with: dehydration.

David Martin, a physiologist at Georgia State University and co-author of Training Distance Runners, says that although sweating is good because it helps cool the body, a large proportion of the fluid in sweat comes from blood volume. If you sweat out much more than you drink on the run, your blood volume drops, reducing the amount of blood that is pumped with each heart beat. When this occurs, your heart rate increases; eventually and your pace will suffer. Dehydration also spurs muscle cramps.

Drinking enough fluid to prevent dehydration can be tough, even in cool weather. Most runners' stomachs can absorb only about 7 ounces of liquid every 15 minutes. That's why the smart runners on Sunday will drink at every fluid stop. As is the case at most marathons, the fluid stops at Marine Corps offer water and sports drink. When properly mixed, sports drinks are as effective as water in rehydrating and they provide an extra boost of carbohydrates. Research has shown that, when consumed on the run, these drinks enter he bloodstream almost immediately; this helps spare your glycogen stores. The same is true of energy gels. These pudding-like substances, which come in containers about the size of a ketchup packet, provide roughly 100 calories.

"You can't eat and drink yourself to a medal," said Pfitzinger, "but with the right training, combined with good pace judgment and a plan for taking in fluid and carbohydrates during the marathon, you can push the wall past the finish line."

Technically speaking:

Glycogen: A stored form of carbohydrate that is the muscles' preferred fuel for aerobic exercise.

Most people can store 1,500 to 2,000 calories of glycogen in their muscles. Running burns roughly 100 calories per mile, so a marathon requires more than 2,600 calories.

Fat as fuel: The breakdown of fat requires more oxygen per calorie released than the breakdown of carbohydrate, says coach and two-time Olympic marathoner Pete Pfitzinger. That means fat is a less efficient energy source, and the more a body has to rely on fat, the worse it performs.

Fuel delivery: Long runs increase the number of capillaries per muscle cell, which improves the efficiency of delivery of oxygen and other nutrients and the removal of carbon dioxide and other waste products," says Pfitzinger.

Sweat: During a marathon, sweat production routinely ranges from 1 to 1.5 liters per hour, and can exceed 2 liters per hour for some runners. If you sweat much more than you drink on the run, your blood volume drops, reducing the amount of blood that is pumped with each heart beat. When this occurs, your heart rate increases and eventually your pace will suffer.

-- Scott Douglas is a contributing editor for Running Times and co-author of Road Racing for Serious Runners, published this month by Human Kinetics.

Sources: Pete Pfitzinger, David Martin
Washington Post -- Friday, October 23, 1998; Page E06

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