One way to understand "natural" is "biological," which is how it's used in cases that concern the fairness of competing with prosthetic limbs. In its initial disqualification of double-amputee sprinter Oscar Pistorius from the Olympics in 2008, the International Association of Athletics Federations' official statement cited research that showed Pistorius's prosthetic blades needed "less additional energy than running with natural limbs" and exhibited unique biomechanical characteristics that placed athletes with "natural legs" at a disadvantage.
Yet disqualifying all athletes with "non-biological" features would be virtually impossible, says Alena Grabowski, a professor of integrative physiology at the University of Colorado who specializes in the biomechanics of prostheses in sports.
"There aren't many elite athletes who haven't had some kind of surgery or medical intervention," she points out. To make sense of what is already allowed, natural talent must be understood as a healthy biological body or its technologically assisted equivalent.
Grabowski worked on the team that successfully helped Pistorius appeal the IAAF's decision and qualify for the 2012 Olympics. With the exception of gymnast George Eyser - who won six Olympic medals using a wooden leg in 1904 - Pistorius in 2012 was the first athlete to compete with a prosthesis. (He was subsequently convicted of shooting and killing his girlfriend in South Africa, for which he recently received a six-year sentence).
This year, Grabowski tried to help German long-jumper Markus Rehm, who lost his right leg at age 13 in a wakeboarding accident and, like Pistorius, uses a prosthetic blade to compete. The stakes were high: Rehm had a chance at medaling this summer in Rio, given that his personal-best time for 2015 would have qualified him for gold in the 2012 Olympics.
There is no official scientific standard for proving that a prosthesis is or is not equivalent to a biological leg. But that didn't stop Grabowski from trying. Along with two other scientists from Germany and Japan, she produced a study of prosthetics and long-jumpers. The researchers covered participants in the study with reflective markers and had them run past a battery of infrared cameras. They measured the downward force exerted on the track with force plates. They calculated metabolic rates.
The results of the study were ambiguous. On one hand, prosthetics limit a long-jumper's top speed, which is a disadvantage. On the other hand, the prosthetic limb appears to facilitate slightly the transfer of vertical force into vertical and horizontal force at takeoff, which is advantageous. Although Grabowski is convinced Rehm has no advantage, the IAAF ruled against him in June, saying he had failed to prove the blade didn't grant an unfair competitive advantage. As a result, Rehm withdrew his Rio Olympics bid.
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But natural advantages - however unfair they might be - are completely legitimate. American swimmer Michael Phelps, for example, has very large feet and double-jointed ankles that together create a flipper effect in the water. The less genetically fortunate are not allowed to level the playing field through "unnatural" means such as prosthetic flippers.
But there are ways to boost your natural talents that are harder to see - and to regulate. Take hematocrit levels, which measure the concentration of red blood cells that provide oxygen to muscles. Raising these levels increases stamina and performance, which is why such athletes as cyclist Lance Armstrong have engaged in illegal blood doping. Options include autologous blood transfusions (using your own blood), homologous blood transfusions (using someone else's blood) and dosing with EPO, a peptide hormone that stimulates production of red blood cells.
All three are potentially harmful. "Blood viscosity goes up, which increases the strain on your cardiovascular system, raising the risk of heart disease and blood clots in your brain and lung tissue," explains Nikolai Nordsborg of the University of Copenhagen, an expert in sports-related hematology. "If you use someone else's blood, there is also the possibility of anaphylactic shock and infection."
People's natural hematocrit levels depend on a host of factors, including genetics, geography and training strategies. If you are in the low-normal range, it's illegal to dope yourself even just into the high-normal range. But there's no rule against being naturally gifted, the most famous example of which is the Finnish cross-country skier Eero Antero Mantryranta, who won seven Olympic medals in 1960s with a hematocrit level 50 percent higher than an average person's, thanks to a genetic mutation called primary polycythemia.
In 1997, the International Cycling Union, or UCI, responded to concerns about blood doping by ruling that anyone above that 50 percent level had to take a two-week break from competition and immediately submit blood samples to further scrutiny.
"Some athletes got really sophisticated, testing their levels all day with hand-held machines," says Don Catlin, the founder of UCLA's Olympic Testing Laboratory. "They micro-dosed with EPO to get themselves right up to" the cutoff.
Competitors who weren't naturally close to the 50 percent level could improve themselves an enormous amount without triggering the rule, and they did.
To remedy the situation, the UCI and other organizations have turned to "biological passports," which record individual athletes' biomarkers over time. "The biological passport allows us to look at natural variation in a single individual," explains Jacob Morkeberg, an anti-doping specialist whose work revealed anomalies in Armstrong's blood samples from the 2009 Tour de France. "We can also consider confounding factors such as genetics or training at altitude, which can raise hematocrit levels."
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The existence of such confounding factors raises an important question. If living and training at altitude - or simulating it with high-altitude tents, as many athletes do - can increase hematocrit levels, why is that treated differently from blood doping?
Catlin, Nordsborg and Morkeberg all emphasize a huge disparity in health risks. Fair competition shouldn't require doping with EPO until the blood runs sludgy in your veins. Nevertheless, they also acknowledge that many permissible forms of training also can threaten an athlete's health. After all, subjecting the body to grueling training has serious side effects, not to mention the inherent danger of certain sports such as boxing.
Given the acceptance of natural biological advantages and natural training, the case of Indian sprinter Dutee Chand is particularly confusing.
Authorities agree that Chand, who is slated to compete in the Rio Olympics, has never taken illegal substances. The problem is her naturally high levels of testosterone, a condition called hyperandrogenism. On average, men have 10 times more testosterone than women, which many scientists believe accounts for differences in athletic ability between men and women. Exogenous testosterone - in the form of anabolic steroids - is routinely used to increase lean muscle mass.
In 2014, tests - which included a battery of mortifying physical exams usually employed to determine sex - revealed that Chand's testosterone level was in the typical male range. It was also above the limit set by the IAAF for female competitors.
The Sports Authority of India ruled in 2014 that "the athlete will still be able to compete in the female category in [the] future if she takes proper medical help and lowers her androgen [testosterone] level to the specified range."
Chand appealed her case to the Court of Arbitration for Sport, the equivalent of athletics' Supreme Court, where she argued that any advantage she enjoyed was a "natural genetic gift," like high hematocrit levels, and that in no other case do natural physiological characteristics disqualify an athlete .
"When a man has unusually high levels of testosterone, the next step is a carbon isotope test," says Stanford bioethicist Katrina Karkazis, who helped Chand with her appeal. "If it's deemed to be natural, the case is closed. But for women, if it's natural the case is not closed, and you get ushered into more tests."
The controversy over what is natural has led some experts to be wary of using the term. "I don't think the term 'natural' is very helpful for deciding rules," argues Patti Zettler, a legal scholar at Georgia State who focuses on bioethics. "Things that we accept for athletes - run stairs, use shoes, lift weights - you could say those are not 'natural.' . . . So much of what we think is natural depends on what we're used to seeing, not some kind of objective framework."
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But if the idea of "natural," however imperfect, is abandoned as a rough guide to what's allowed in sports, where would we be left? Anti-doping advocates warn about a possible future in which Olympic hopefuls regularly dope and take dangerous drugs, not unlike the current world of professional bodybuilding, where use of steroids is widely acknowledged to be rampant. As a consequence, it is impossible to win without them, and bodybuilders who want to be drug-free are forced to compete in the alternative (and less glamorous) "natural" events.
The dangerous and prohibited practices of the past that caused some to call for the disqualification of the entire Russian Olympic team in Rio would become the norm.
Even if that's an exaggeration, sporting authorities would still depend on other philosophically ambiguous terms - fair, healthy, normal - as a basis for making rules.
As biotechnology advances, problems with defining "natural" will only become more pronounced. If parents genetically engineer their children to be stronger or faster, will those children be allowed to compete in the Olympics? Or would it violate the "spirit of sport"?
The founder of the modern Olympics, Pierre de Coubertin, once said that the most important thing in life wasn't winning, but "fighting well." Nearly a century later, it remains an open question whether that also means fighting naturally.
