Marathon man: How not to hit the wall

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CHICAGO: Marathon runners can train for months to condition for the big race, yet struggle to finish if they exhaust stores of carbohydrates too quickly, a phenomenon known as "hitting the wall."

A new formula by a marathon runner and student at Harvard and MIT gives elite runners and marathon enthusiasts a more exact way to calculate just how many carb calories they need to take to stay in the 26.2 mile race.

"About 40 percent of marathon runners hit the wall," said Benjamin Rapoport, a student in the Harvard-MIT Division of Health Sciences and Technology, whose study appears in the Public Library of Science journal PLoS Computational Biology.

Essentially, that means the runner has burned up all of the carbohydrates stored in his liver and leg muscles, forcing him to slow down dramatically as the body starts to burn fat.

"You feel like you're not going anywhere," Rapoport said in a statement. "You can't will yourself to run any faster."

He said many runners believe hitting the wall is inevitable, that it is just part of a marathon.

"That is not true at all," Rapoport, who has run 18 marathons, including a personal best of 2 hours 55 minutes at this year's Boston Marathon, said in a telephone interview.

"What I came up with was essentially a set of formulas," he said.

"People need to know really three things: how much they weigh, what their target marathon time is and their maximum oxygen intake capacity," he said. "That is a measure of a person's aerobic fitness."

Aerobic capacity, also known as VO2max, is a measure of how much oxygen the body can transport to the muscles and consume during aerobic exercise.

Measuring exact VO2max requires a treadmill stress test at maximum effort, but an informal way to estimate aerobic capacity is to divide your maximum heart rate by your resting heart rate and multiply by 15, Rapoport said. To find your maximum heart rate, simply subtract your age in years from 220 beats per minute.

The result, he said, is a number that tells runners how many excess carb calories they need to take in before a race.

He said many runners also supplement their stored carbohydrates by taking gels and sports beverages as they are running, but a runner can carry much more fuel in their legs and liver, if they know the right amount.

Until now, runners have had to guess at what that was. The new calculator will make that easier, Rapoport said.

Rapoport has built an online calculator to help runners estimate their aerobic fitness pace and race goals which can be found at Endurance Calculator website.

"It's my gift to my fellow runners," he said.