By now just about everyone with even a passing interest in current events has heard that Lance Armstrong admitted in an interview with Oprah Winfrey that he took performance enhancing substances on the way to winning seven Tour de France titles. What you didn’t hear is how he did it.Cycling athletes are one of the most heavily tested groups in sports, so how could the most noted cyclists in the world get away with it for so long? With a combination of science and cleverness.Armstrong admitted to using several banned substances, but the one that may have made the biggest difference is EPO, or Erythropoietin. EPO is a glycoprotein that is produced naturally in the body and stimulates the production of red blood cells in bone marrow. If you’ll recall from biology class, red blood cells carry oxygen throughout the body. For an endurance athlete, this is the key to victory.Taking EPO basically increases the oxygen processing capacity of the circulatory system beyond what it would naturally be. There was no test for laboratory-created EPO until the early 2000s, but even then it was not terribly sensitive. An athlete as smart as Armstrong knew better than to take large quantities of synthetic EPO in recent years. Instead, he likely resorted to one of several drugs that boost natural EPO production. Although, high levels of natural EPO, while not conclusive, are treated with a lot of suspicion.The issue of blood doping goes hand-in-hand with EPO. Blood doping is all about timing and, again, being clever. The risk of detection is much lower when EPO is used long before a race, but you want those high levels of red blood cells. The solution? When the blood is chock full of red cells, some of it can be taken and stored until it’s time to race.Why go to all that trouble? EPO, like all proteins, breaks down naturally over time. Many serum proteins are short-lived, and in the case of EPO its half life is only 5 hours. That means that after a few hours a blood sample will have lower concentrations of detectable EPO. In a few days, EPO is completely undetectable. Put the blood back in, and you get the benefits of the additional red blood cells with lower levels of EPO.Blood doping is often used in conjunction with saline IVs, which Armstrong is alleged to have used as well. When Armstrong knew he was not going to be able to simply avoid the testers, and felt his blood levels might give him away, he would flush a liter of saline through his system. This has the advantage of diluting the banned substances to more normal levels. You have to remember almost all the substances Armstrong has admitted to using are naturally produced in the body. You don’t have to remove all traces, just make things look passable.Testosterone was apparently another big part of Armstrong’s training regiment. Testosterone is the male steroid hormone responsible for the development of male secondary sexual characteristics during puberty. This hormone is also essential in the building of muscles, so you can probably see why some athletes would abuse it. Lance Armstrong in particular would have benefited after his bout with testicular cancer, which left him with lowered levels of the compound.Evading tests for testosterone is actually easier than you’d think. The test is really just looking at the testosterone-epitestosterone ratio (T/E) to make sure it’s not seriously out of control (the allowable maximum is 4-to-1). A very high testosterone imbalance indicates a serious metabolic issue or, more likely, doping. By taking low doses of very pure human-derived testosterone over time, Armstrong was able to reap the benefits without much risk of being caught.I imagine he learned a serious lesson from the downfall of fellow cyclist Floyd Landis in 2006. Landis won the Tour de France, but his T/E ratio was found to be 12-to-1. He claimed this was a natural occurrence, but the high levels allowed the anti-doping agencies to isolate the testosterone in his blood. Mass spectrometry confirmed that the carbon isotopes making up some of that testosterone indicated a non-human laboratory source. This is the kind of advanced science Armstrong went up against for over a decade, and he was only recently backed into a corner.Lance Armstrong spent years avoiding anti-doping agency testers when he knew he would be caught. He learned from the mistakes of his colleagues, and counted on his celebrity to keep him above suspicion. In the end, it was his staggering accomplishments that convinced many people of his shady dealings. Armstrong was very clever to leverage biology and chemistry to win those seven Tours, but now he’s paying the price for what was — quite clearly — cheating.