Editor’s Note (12/5/17): Scientific American is re-posting the following article, originally published August 5, 2016, in light of the International Olympic Committee’s announcement on Tuesday that Russia has been suspended from competing in the 2018 Winter Games as penalty for doping.

At this month’s summer's Olympic Games in Rio, the world's fastest man, Usain Bolt—a six-foot-five Jamaican with six gold medals and the sinewy stride of a gazelle—will try to beat his own world record of 9.58 seconds in the 100-meter dash.

If he does, some scientists believe he may close the record books for good.

Whereas myriad training techniques and technologies continue to push the boundaries of athletics, and although strength, speed and other physical traits have steadily improved since humans began cataloguing such things, the slowing pace at which sporting records are now broken has researchers speculating that perhaps we’re approaching our collective physiological limit—that athletic achievement is hitting a biological brick wall.

Common sense tells us that of course there are limits to athletic achievement: Barring some drastic amendment to the laws of physics, no human will ever run at the speed of sound. And physiologically speaking there’s only so much calcium that can flood into a muscle cell causing it to contract; there’s only so much oxygen our red blood cells can shuttle around.

In this vein, in 2008 running enthusiast and Stanford University biologist Mark Denny published a study attempting to determine if there are absolute limits to the speeds animals can run. To do so he analyzed the records of three racing sports with long histories of documentation: track and field and horse racing in the U.S., along with English greyhound racing.

By plotting winning race times back to the turn of the 20th century and by controlling for population growth, Denny was able to conclude that there is indeed a predictable limit to the time it takes for a particular species to cover a certain distance. In fact, his data show that horse and dog racing as well as some human track and field events may already be there. “We’re definitely plateauing,” Denny says. “Just look at the horse racing data, which I think parallels what’s happening in humans. Winning times in the Triple Crown haven’t really [improved] since the 1970s—and this is despite all of the millions of dollars being poured into breeding faster horses.”

As Denny explains, horses can still be bred to improve on a particular attribute, however doing so comes with collateral physiological drawbacks. “You can breed a horse to go faster than ever before or to have stronger muscles but then its legs will break. It really looks like we’ve maxed out the gene pool for thoroughbreds.” And we could be next.

Genetically speaking, racing horses are an especially homogenous lot, as all thoroughbreds descend from just three stallions brought to England in the 17th and 18th centuries (and a slightly larger number of “foundation mares”). But Denny points out that in a number of women’s track events speeds have also leveled off, with many records going unbroken since the 1980s (when, as he puts it, many competitors were suspected of being “doped to the gills.”) Denny cites marathoner Paula Radcliffe’s 2003 world record time of 2:15:25 (purportedly unassisted by performance-enhancing drugs, despite an investigation) as being nearly at his predicted maximum speed for the women’s marathon. Male marathon runners may still have some wiggle room. Denny’s model predicts that the current record of 2:02:57 can be improved on by three or so minutes, in line with the much publicized pursuit of the two-hour men’s marathon.

Bolt hopes to beat the researcher’s fastest predicted 100-meter dash time of 9.48 seconds. Unfortunately, according to Denny, the now notably older sprinter may have missed his chance. The sprinter was a chasm ahead of the pack in a semifinals race at the 2008 Beijing Olympics when he slowed up before crossing the finish line. “I think had he kept going at full speed he would’ve set an all-time, unbeatable world record,” Denny speculates.

Bolt may be comforted to know that for Southern Methodist University physiology professor Peter Weyand, one of the leading experts on the biology of performance, we humans haven’t quite reached our athletic ceiling. Weyand explains that when considering endurance, for example, there are two paths to improvement: either increasing the amount of blood being pumped out of the heart or increasing the oxygen concentration in the blood itself, as is the case with blood doping. “I don’t think we’ve hit our limits yet,” he believes, “I think people will find ways to enhance oxygen delivery through the body and squeeze more performance out of humans. The only question is will these approaches be considered legal.”

The answer to improved athletic performance might be in our mitochondria, the so-called cellular “powerhouses” that generate energy using oxygen via the Krebs cycle. In a person of average aerobic fitness mitochondria make up about 2 percent of each cell’s volume; in well-trained athletes it is 4 percent. In the hyperkinetic hummingbird the number climbs to around 40 percent, giving hope that perhaps human cells could accommodate more mitochondria, thereby boosting athletic ability. “Of course there’s a limit at which point you just can’t cram any more mitochondria into a cell, but I think in humans there’s room left,” Weyand says. “Sports have become such a global, lucrative and professionalized endeavor that as long as there’s money to be made and fame to be won, we’ll continue to see improvements—both in terms of sports science and equipment—that cause records to fall, though maybe less frequently.”

Weyand acknowledges that any future biological tinkering may bring with it the same ethical and philosophical concerns that shroud performance-enhancing drugs. “It’s going to be increasingly hard to determine what should be legal and what shouldn’t,” he predicts. “Now we say, ‘okay, training is a good thing, and so is diet,’ but what about supplements?”

On top of that, the watchdog groups will likely never be able to keep up with new biological and chemical enhancements that could inch—or perhaps propel—records forward, Weyand says. “The anti-doping authorities first have to find out what new substances are being used; then they have to develop an assay to detect them. The identification and the list of what’s banned is always going to lag behind what people are trying,” he says.

Blood doping may not be going away but the future of record-breaking, for better or for worse, most likely lies in the human genome. Gene-editing technologies like CRISPR–Cas9 now allow specific genes to be turned on, off or introduced—granting modifications that could confer any number of athletic advantages and that, as Weyand warns, would be nearly impossible to detect. “I do think we’ll see people trying things like CRISPR to introduce certain genes in the interest of athleticism,” David Epstein, author of the 2013 book The Sports Gene: Inside the Science of Extraordinary Athletic Performance, says. “I think the main reason why people aren’t doing this yet is that so many forms of traditional doping are available and effective. They haven’t needed to move on yet.”

Epstein, whose book explores the limits of human performance, points out that current concerns over CRISPR are often dismissed, given the complexities of our genetic code and the fact that at the moment we don’t actually know what most genes do. Yet, as featured in his book, there are examples of specific gene variants that result in enhanced athletic performance.

One such case involved Finnish skiing legend and seven-time Olympic medal winner Eero Antero Mäntyranta, who had runaway success throughout the 1960s,and was widely assumed to be blood-doping. Years later a genetic study on Mäntyranta and his family revealed that he carried a gene that greatly increases red blood cell mass and hemoglobin levels, the molecule that carries oxygen in blood. Epstein also cites the so-called “super baby,” an alarmingly muscular boy born in Berlin in 1999. The now-teenager has a mutation that blocks the production of myostatin, a protein that limits excessive muscle growth.

Lucky individuals aside, what will become of the public’s interest in competition if we are reaching a plateau in performance, one in which records—perhaps assisted by ethically dubious genetic tampering—will continue to fall, but at a far slower rate? Will people watch when there are no more records to break?

Denny isn’t concerned. “When I published my paper, the feedback I got was that this was going to destroy the Olympics,” he recollects. “That’s like saying the 1962 Brazilian soccer team was the best ever so no one’s ever going to watch the World Cup again. But if Bolt can run the 100 in 9.47 seconds and beat my prediction, then hats off to him. I think there’s always going to be the lure of ‘maybe someone’s going to do better.’”

Both Denny and Epstein feel this is especially true for more complex sports in which any number of variables can contribute to success, and in which an objective “best” is hard to define. A lot of factors have to fall into place for a team to win a basketball championship or a Super Bowl. And sports leagues are continually changing the rules to capture public interest, creating new benchmarks for athletic ability. “Basketball didn’t have a three-point line until 1979,” says Denny, an absence that makes one wonder if in another era, the league’s current phenom, Stephen Curry—whose single season three-point record of 402 so staggeringly soars above the former mark of 286, also set by him—might not have enjoyed the acclaim that he, given the rule change, deservedly has.

“The NBA and all the leagues know what they’re doing,” Denny jokes. “People will be arguing about sports over beers at the bar for decades to come.”