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  The 2-hour marathon

Is there a limit to how fast, long someone can run?

By Judy Foreman, Globe Correspondent, 4/13/2004

Hardly anyone thought it was possible for a human being to run a mile in less than four minutes -- until Roger Bannister did it in 1954. Within three years, nine other men had done it, too.

Once women started running marathons, it was considered a given that no woman could do it in less than two hours and 20 minutes -- until Paula Radcliffe ran the (flatter-than-Boston) London Marathon exactly one year ago today in 2:15:25.

Athletic records are made to be broken: That's the fun of it.

But what are the limits of human performance?

"We still don't completely know," said Miriam Nelson, director of the John Hancock Center for Physical Activity and Nutrition at the Friedman School, Tufts University. "World records will be set for many years to come." And Nelson fully believes that someone, someday will complete a marathon in under two hours, besting Paul Tergat's current record by five full minutes.

In general, human performance in endurance events like the marathon depends heavily on genetics, in particular the genes that govern cardiac output, and on training, the physiological adaptations the body makes to respond to the stress of intense, prolonged exercise. Nutrition, motivation, equipment (like better running shoes) all count, too. So does the ruggedness of joints.

For endurance events, "the first limit is the ability of the heart to pump enough blood and to deliver oxygen to the peripheral, skeletal muscles," said geneticist and exercise physiologist Claude Bouchard, executive director of the Pennington Biomedical Research Center in Baton Rouge, La. "Cardiac output is extremely important," and good cardiac output (as well as bad) has a strong genetic component: It tends to run in families.

The second determining factor: How efficiently muscles can combine that oxygen with fuel (carbohydrates or fats) to make adenosine triphosphate, or ATP, the energy molecule that allows muscles to contract. The production of ATP takes place inside cells in the mitochondria; the more mitochondria a person has, and the more efficiently they work, the better the muscles contract and the faster the person can run.

In other words, the two most important factors, at least for endurance events, are getting enough oxygen into muscles and the ability of muscles to use this oxygen to contract. This combination is often referred to as VO2 max, or maximum volume of oxygen.

Training increases both the number and efficiency of mitochondria, Bouchard said. And like cardiac output, the ability to respond favorably to training -- "how trainable you are" -- also runs in families. "You can't be an elite athlete if you don't have both sets of conditions -- highly endowed and highly trainable."

Raw talent is easier to spot than trainability. "You can't tell until you train someone how well the mitochondria will respond," said David Costill, former director of the Human Performance Laboratory at Ball State University in Muncie, Ind. In genetically favored people, he said, "you see a large increase at the cellular level in mitochondrial number and all the enzymes in mitochondria."

Training also produces an increase in capillaries -- tiny blood vessels that bring oxygen to cells; and of course, an increase in muscle strength.

Fuel matters, too. For optimal endurance, athletes need to be able to burn both carbohydrate, which is stored in muscles in a form called glycogen, and fat, which is stored everywhere. "We can store a functionally infinite amount of energy in the form of fat," but not carbohydrate, said Russell Pate, a professor of exercise science at the University of South Carolina. That's why marathoners spend the last two or three days before a race eating carbohydrates and letting glycogen build up in their muscles.

When marathoners "hit the wall," it's usually because they are running out of glycogen. The way to avoid this is to "be well-adapted for fat metabolism," said Pate, which means teaching the body to burn fat to supplement waning carbohydrate stores, which can be done by endurance training. (Elite runners do this by training 120 to 140 miles a week.)

Genetically gifted marathoners also are endowed with ideal ratios of fast-twitch to slow-twitch muscles. The best endurance athletes have lots of slow twitch, or Type I, muscles, which look red and can use oxygen quite efficiently. (Ducks, which, like marathoners, travel long distances, also are loaded with slow-twitch muscles, which is why duck meat is red; chickens, not exactly endurance champs, are rich in white, fast-twitch, or Type II, muscles.) Toward the end of a marathon, when most racers are running out of glycogen in their slow-twitch muscles, the lucky ones can recruit fast-twitch muscles for the final push.

In the long run, and a marathon is clearly that, being an elite athlete takes a combination of good genes and grueling training. Dr. Lisa Callahan, a sports medicine physician at the Hospital for Special Surgery in New York, put it this way: "If you take two people with the same physique, the same running style, the same motivation and drive, who train exactly the same and one always beats the other, that may be the genetic edge."

But Costill begged to differ: "It's not genetics. Most winners will tell you it's having a killer instinct, and truly believing they are the best."

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