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 3 years, nine other men had done it, too.
In the dark old days, some people thought women shouldn’t compete at all for a variety of silly reasons, including the belief that too much exercise might dislodge the uterus, rendering a woman infertile.
Once women finally did start running marathons, it was considered a given that none could run one in less than 2 hours and 20 minutes – until Paula Radcliffe ran the (flatter-than-Boston) London Marathon exactly one year ago today (April 13th) in 2:15:25.
Athletic records are made to be broken – that’s the fun of it. But there must be some limits to human performance, right? After all, as Steven Blair, president and CEO of the Cooper Institute, an exercise research center in Dallas, put it, “A time of zero seconds will never be achieved in the Boston Marathon, to state the ridiculous.”
So what are the factors that limit performance, especially in endurance events like the marathon? There are many, though some exercise physiologists nonetheless believe someone someday just might run a marathon in under two hours.
“We still don’t completely know” what all the limits of human performance are, 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.”
But in general, human performance 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 determinant is the efficacy of the skeletal muscle machinery” to use that oxygen, to combine it with fuel (carbohydrates or fats) to make ATP, adenosine triphosphate, the energy molecule that allows muscle filaments to contract. The production of ATP takes place inside cells in an organelle called the mitochondrion; the more mitochondria a person has, and the more efficiently they work, the better the ATP production.
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 make ATP. 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 – [being] highly endowed and highly trainable.”
And while two athlete wannabes might look the same on some physiological measures, “you can’t tell until you train someone how well the mitochondria will respond,” said David Costill, now partially retired but formerly the director of the Human Performance Laboratory at Ball State University in Muncie, IN. 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 we are limited in the form of energy as carbohydrate in the muscle itself,” 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 are also endowed with an ideal ratiosof fast-twitch to slow-twitch muscles. The best endurance athletes have lots of slow twitch, or Type I, muscles, which look red and have a high oxidative capacity – a good ability to use oxygen efficiently. (Ducks, which, like marathoners, travel long distances, are also 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 a final kick.
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 David Costill of Ball State 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.”