Lance Armstrong, fourth from right, and teammates celebrate after he won the Tour de France in 2003. (Pascal Rondeau/AP)

In his admission to Oprah Winfrey last week that he had taken performance-enhancing drugs, cycling legend Lance Armstrong argued that it was not possible to win the Tour de France seven times in a row without doping. Indeed, since 1995, all but two of the winners of the race have been implicated in doping controversies. Among the questions this depressing fact raises is: Why is doping so effective in cycling?

To appreciate how hard the competitors work during a multi-stage cycling race, you have to understand an exercise physiology concept known as VO2 max. Also known as maximal oxygen uptake, VO2 max represents your ability to transport and utilize oxygen during periods of high exertion.

Doctors measure VO2 max by putting a patient on an exercise apparatus — usually a treadmill or stationary bike — and raising the intensity of exercise every two or three minutes until the subject can no longer sustain the activity. VO2 max is typically expressed in milliliters of oxygen used per kilogram of body weight per hour of activity.

The average person has a VO2 max of around 45. Professional athletes in sports such as soccer and hockey usually register in the 50s or low 60s. Elite endurance athletes typically have high VO2 max values because they’re able to absorb the large amount of oxygen needed to sustain them over hours of hard work. The numbers for cyclists are particularly high. Armstrong’s VO2 max is 85. Miguel Indurain, who ruled the cycling world before the ascendance of Armstrong, was rumored to have a VO2 max of 95, although people who worked with him say it was more like 78.

Since the 1990s, participants in the Tour de France have worn heart rate monitors, enabling researchers to examine their level of exertion (which can then be expressed as a percentage of the VO2 max). Over the long, flat stages, the monitors suggest that riders hover between 50 and 70 percent of their VO2 max. That may sound like a light workout, but keep in mind that when a Tour de France rider is “resting” at 60 percent of his maximum capacity, he’s working about as hard as an average person at full exertion.

In an exclusive interview with Oprah Winfrey, Lance Armstrong explains why is now admitting to doping after 13 years of vehemently denying he ever used performance-enhancing drugs. (Courtesy of OWN)

The time trials and mountain stages are entirely different. The long time trials last more than an hour, during which the cyclists remain above 90 percent of VO2 max. (As a crude comparison, for the average person that would be like sprinting for an entire hour.) In the mountains, thinning oxygen supply becomes an issue as riders traverse terrain above 8,000 feet, all the while staying in the vicinity of 90 percent of full exertion. Researchers have identified pulmonary edema — an accumulation of fluid in the lungs caused by the effort to supply enough oxygen to the body — in Tour riders after mountain stages.

Studies have also shown that, during the course of a multi-stage race, professional riders experience a steady decrease in levels of testosterone and cortisol as the body struggles to rebuild itself after each day. This decline seems to be unique to cycling, as professional marathon runners have to train hard for six months before experiencing the kind of hormonal deficit that cyclists suffer in three weeks.

Against this backdrop, it’s perhaps not surprising that cyclists have managed to get steadily faster not only through superior training techniques but also through improved pharmaceuticals. For example, erythropoietin, or EPO, one of the substances Armstrong said he used during his run of Tour wins, increases the concentration of red blood cells. That enables cyclists to transport more oxygen to their muscles during training, which improves their VO2 max prior to race day. During the race, it makes them more efficient at using oxygen and speeds recovery.

Doping is so effective in cycling because the sport is almost entirely a test of endurance. A basketball player can increase his endurance and strength through cheating, but it’s difficult to improve shooting technique with drugs. The same goes for such other high-technique sports as tennis and soccer.

Drugs have been part of cycling since the beginning. Long before EPO came on the scene, riders used alcohol, cocaine, strychnine and amphetamines. Cycling legend Jacques Anquetil has been widely quoted as saying, “I dope myself. Everyone dopes himself. Those who claim they don’t are liars. For 50 years bike racers have been taking stimulants.”

Given the extensive coverage of Armstrong and his disgraced colleagues, you might think that doping is far worse in cycling than in other sports. In fact, many sports have as bad a doping problem as cycling.

Cyclists failed 1.19 percent of the doping tests administered by the World Anti-Doping Agency in 2010, which places cycling near the middle of the fail rate. Athletes involved in weightlifting (2.42 percent), boxing (1.94 percent) and archery (1.47 percent) failed at significantly higher rates than cyclists. (They use different drugs. Archers, for example, use drugs to calm their nerves and steady their hands.) Many inside the testing community believe other professional sports would have higher failure rates if their athletes were subjected to the rigorous testing regimes that have been imposed on cyclists.

None of this excuses Armstrong, of course. But his story is just one example of how humans respond to the incredible physical demands of sport, combined with money and public scrutiny.