They were caught doping. Now they're clean. Should athletes who have used performance-enhancing drugs be given a second chance?

Leading up to his widely anticipated faceoff against Usain Bolt in the men's 100-metre final on Sunday, U.S. sprinter Justin Gatlin, who has twice been caught using banned substances, defended his participation in the Rio Olympics.

"At the end of the day, I've served the time," Gatlin, who was suspended for four years for having elevated testosterone levels in 2006, told the Associated Press. "I've dealt with that punishment. I've moved forward."

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But even when athletes quit cheating, the effects of doping can have long-lasting consequences that extend beyond the damage to their reputations. According to experts who study muscle physiology, the use of anabolic steroids may lead to changes in the body that last for the duration of one's athletic career, and potentially even a lifetime.

The critical issue is how anabolic steroids (of which the most commonly used by athletes is a synthetic form of testosterone) affect what's known as muscle memory. Muscle memory is believed to be the reason why people who work out seem to bounce back quickly in fitness, regaining muscle strength and size, even after taking a break. There are a few possible explanations for how it works.

Unlike other cells in the body, muscle fibres don't divide, explains Stuart Phillips, a professor in the department of kinesiology at McMaster University in Hamilton. Rather, they rely on a population of cells, called satellite cells, that help regenerate and repair muscle fibres.

Anabolic steroids spark a dramatic surge in these satellite cells, says Phillips, who is the Tier 1 Canada Research Chair in skeletal muscle health. "It really switches them on," he says. "You become very, very efficient at repairing your muscles. So people who take steroids are able to tolerate higher volumes of work, they're able to put muscles under greater stress, and then recover much faster."

Once they increase in number, these satellite cells remain for a very long time, effectively for the rest of an athlete's career, Phillips says. Thus, he says, unlike other banned substances such as stimulants, which provide a boost that lasts only as long as the drug stays in the body, anabolic steroids continue to give athletes an advantage over their competitors, even years after they've stopped doping.

"If you're caught taking anabolic steroids, I think it should be a lifetime ban," he says.

At the University of Olso, physiology professor Kristian Gundersen also believes doping has an enduring effect on muscle memory, but he suggests another mechanism is at work. Most cells in the body have only one nucleus, which can be thought of as the command centre of a cell. But muscle fibre cells have multiple nuclei, Gundersen explains. Each nucleus can only serve a certain volume of muscle, so when you're trying to build more muscle, your satellite cells help deliver more nuclei to the muscle fibre, he says. Thus, having more nuclei means having a greater ability to grow bigger and stronger muscles.

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Once they're there, Gundersen says, these extra nuclei remain in the muscle. And that's where anabolic steroids provide a long-term advantage.

In a 2013 study on female mice, Gundersen and his team found even brief exposure to the anabolic steroid testosterone propionate allowed the rodents to regain muscle mass far more quickly months afterward, compared with those that never received the steroids.

Even though the rodents' muscles shrank back to normal when the researchers stopped administering testosterone, three months later, a period Gundersen suggests may be roughly equivalent to 15 years in human terms, the mice that received testosterone showed 30-per-cent growth in muscle after six days of strength exercise. Those that weren't given testosterone had 6-per-cent growth.

Although this research was done on lab animals and with a "highly artificial procedure," Gundersen says, "the mechanisms are very basic, and I would be very surprised if it wasn't similar in humans."

He notes other research on humans indicates the half-life of muscle cell nuclei is at least 15 years.

Gundersen adds this effect on muscle memory may not only have implications for banned substances in sports, but also for transgender athletes. The International Olympic Committee adopted new guidelines early this year to allow transgender athletes to compete without undergoing sex reassignment surgery, which they were required to have had under previous guidelines.

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"When it comes to strength-related types of competitions, I think one has to be very careful," Gundersen says. Even if trans female athletes no longer produce testosterone, he suggests, they may still have muscle memory. "I think it's not unlikely that they will have an advantage."