Once every four years, people from all nations watch the Olympic Games and marvel at the greatest athletes in the world. What is it that allows them to run, jump, and throw two to three times faster and farther than the average person? Is it good genes or sheer hard training?

Scientists have been looking into these questions for almost as long as the Olympians have been setting new records. Along the way, we have uncovered many answers, while much remains to be explained.

Some athletes, like USA cyclist Taylor Phinney, seem to have followed the suggestion of the Swedish physiologist Per Olaf Astrand, who once joked, “If you want to be an Olympic champion, then choose your parents wisely.” Phinney’s parents are cycling royalty. His mom, Connie Carpenter-Phinney, won a gold medal in the 1984 Summer Olympics in the women’s road race. His dad, Davis Phinney, was a bronze medalist in the 1984 Olympics and a stage winner in the Tour de France.

Among U.S. distance runners, Shalane Flanagan has a similarly unmatched pedigree. Her mother was the first female marathoner to break 2:50, her father a 2:18 marathoner and several times a member of the U.S. team that competed in the World Cross Country Championships.

However, it is not just genes that make a great athlete. Getting an early start in sports also seems to benefit future Olympians. A study conducted at the 1976 Summer Games in Montreal found that most of the Olympians surveyed had at least one older sibling. The scientists speculated that trying to keep up with an older brother or sister helped them develop essential motor skills for sport.

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Runners’ hearts are one of many factors that determine their success.  Not only do Olympic athletes have enlarged hearts, but echo cardiograph and MRI studies have shown that endurance athletes have hearts with larger chambers that allow them to pump more blood. In Peter Snell’s case, this lead to a finding that baffled scientists at first.

Snell, from New Zealand, was once the world’s most dominant middle-distance runner despite being built more like a muscular sprinter than a lean miler. He won gold in the 800 meters in 1960, and gold in both the 800 and 1500 meters in 1964. Exercise physiologists at San Diego State University found that, oddly enough, Snell’s lungs failed to saturate his blood with oxygen during maximal exercise. Normally this is a sign of pulmonary disease, which seemed unlikely in Snell’s case. Eventually the scientists determined that his large athletic heart propelled blood through his lungs so quickly that there wasn’t enough time to saturate the red blood cells with oxygen. Even so, Snell’s aerobic power was great enough to beat back his rivals.

In 1975, 20 top distance runners were evaluated at the Institute for Aerobics Research in Dallas. Treadmill tests revealed that their maximal oxygen uptakes were nearly twice as high as that of the average man. They also had low levels of body fat and high proportions of slow-twitch fibers. Steve Prefontaine had the highest maximal oxygen uptake of all the runners (84 milliliters of oxygen per kilogram per minute), while Frank Shorter had the lowest (71 milliliters).

Prefontaine’s high max and relatively high percentage of fast-twitch muscle fibers allowed him to unleash a 54-second last lap at the end of 5,000-meter races. This brought him 15 American records at distances from two miles to 10,000 meters, and a fourth place in the 1972 Olympic 5,000.

Despite his lower max, Shorter excelled at another important measure: running economy. He was more efficient than the other runners tested, meaning that he consumed less oxygen while running at a given speed. In addition, he could maintain a high percentage of his max for extended periods of time. These traits allowed him to cruise to a seemingly effortless win in the 1972 Munich Olympic Marathon.

Sprinters have a different kind of muscle fiber. Colin Jackson, a Welsh sprinter and hurdler who was a silver medalist in the 1988 Seoul Olympics, set a world record of 12.91 seconds in the 110-meter high hurdles in 1993. In 2013, a BBC film crew followed him to the Ball State University Human Performance Laboratory in Muncie, Indiana. There scientists removed small samples of his thigh muscles so they could study them in the laboratory. The results showed, as expected, that nearly three-quarters of Jackson’s muscle fibers were of the fast-twitch variety. However—and this was the surprising part—one third of his fast-twitch muscle fibers were super-fast, which are very rare. His super-fast fibers were six time as powerful as his regular fast-twitch fibers and 12 times as powerful as his slow-twitch fibers. No doubt about it: Jackson was born to sprint.

The external environment is as important as the body’s internal characteristics. When Jim Ryun began preparing for the 1968 Olympic Games in Mexico City, he had little respect for the altitude. “At that time most people, myself included, thought it was mostly a psychological problem,” he said. Then he ran his first time-trial at altitude. Result: a 4:29 mile—38 seconds slower than his best sea-level mile. That served as a wakeup call that he needed to do some serious altitude training before the Games. He did, and ran well in Mexico City (earning the silver medal), but had little chance against Kenya’s Kip Keino, born and raised at high altitude in the Rift Valley region of his country.

The 1984 Olympics, held in Los Angeles, posed a hot-weather challenge for athletes. U.S. marathon star Alberto Salazar was advised by a team of heat and hydration scientists, including U.S. Army experts. They found that he sweated in excess of 2.75 liters per hour, and advised him to train in hot conditions (he spent some time in Houston) and to practice drinking large amounts of water on the run. Salazar ran strongly under a hot sun in Los Angeles, but his 2:14:19 was only good for 15th place.

Getting ready for the historically important first Olympic Marathon for women, Joan Benoit trained at home in coastal Maine, with its own summer humidity. In Los Angeles, she pulled away from the pack early en route to an impressive win. Gabriela Anderson Schiess, from dry Sun Valley, Idaho, was not so fortunate. She succumbed to heat exhaustion and staggered around the final lap on the track, horrifying the L.A. Coliseum crowd and worldwide TV audience with her loss of body control. Fortunately, she recovered quickly. The episode sparked new interest in medical guidelines for hot weather marathon and in treatment of heat illnesses.

Olympic athletes seem to be not just spectacularly talented, but also healthier. In 2012, the British Medical Journal published the first large-scale study on the longevity of Olympic athletes. The authors examined more than 15,000 Olympians from nine country groups who won medals between  1896 and 2010. They concluded that Olympic medalists live about three years longer than men and women in the general population, with medalists from endurance sports having a greater survival advantage than those from the power sports. Of course, no one can be sure if the Olympians’ longevity is due toexercise, good genes, or overall healthier lifestyles; it’s probably a combination of all three.

Beyond teaching us about physiology, Olympic athletes impress us with their examples of the boundless human spirit. At the 1964 Olympics, Billy Mills improved his personal best by 50 seconds to win the 10,000. Bob Beamon produced the “leap of the century” in Mexico City, improving the world long-jump record by almost two feet, an effort that defies all attempts at scientific analysis. And how do you explain gymnast Kerri Strug completing her final vault at the 1996 Olympics on a badly injured ankle?

These and all Olympic performances remind us of the words of Baron Pierre de Coubertin, founder of the modern Olympic Games: “The important thing in life is not the triumph but the struggle; the essential thing is not to have conquered but to have fought well.”

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About The Authors:

David Bassett is professor and head of the Department of Kinesiology, Recreation, and Sport Studies at the University of Tennessee, Knoxville. Scott Conger is an assistant professor in the Department of Kinesiology at Boise State University. They have developed a course called Physiology of Athletes: Exploring the Limits of Human Performance.